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ISLE labs for Physics 21X
General Information:
There are 3 types of Investigative Science Learning Environment (ISLE) labs: observation experiments,
testing experiments, and application experiments.
Observation experiments are intended for students to learn skills such as changing one variable at a
time, clearly recording and representing observations, and making accurate observations without mixing them
with explanations. This is the first step of the experimental cycle, and allows you to observe phenomena, look
for patterns in data, and start to devise explanations (once observations are carefully completed).
The next step, once explanations have been devised, is to do testing experiments, which is an
independent test students design that will test a hypothesis based on a specific explanation or rule. This helps
you practice the skill of making predictions about the outcome of an experiment based on an
explanation/rule/relationship. For a testing experiment, you can’t just do the experiment and record what
happens – they must have a predicted outcome based on some explanation – and if the outcome of the
experiment agrees with the prediction it gives confidence that the explanation may be correct, but if it disagrees,
then you know the explanation is incorrect. In order to make this judgment, you also need to apply basic
uncertainty calculations. (A guide for understanding uncertainty is included in this packet.)
The third type of experiment is the application experiment, where you apply some
explanation/rule/relationship that you have tested enough that you think it is ‘good’, and you apply it to
understand a new situation. Some application experiments require that you determine some unknown quantity
multiple ways – in order to determine if the methods are consistent, it is necessary to apply basic uncertainty
analysis.
By performing this sequence of experiments, it is possible to explore and devise a physics relationship,
test it, and when you are convinced it is good, apply it to understand a new situation – providing you with a
complete understanding of the basic physics relationships (equations). By designing your own experiments, it
gives you creative control, and assures that you understand the steps that you perform, as they are done by your
conscious choice, and not by following instructions or ‘playing around.’
Lab write-ups:
There is one lab write-up per group, each group member must clearly put their name and student ID at
the top of the write-up in order to obtain credit. Since the ISLE labs are labs you design yourself, and are
intended to help you learn specific skills such as justifying your conclusions, comparing results, and
understanding how uncertainty comes into play, it is important that you explain your work carefully in the form
of a lab write-up. The lab write-up is done in the lab, there is nothing for you to do before or after the lab
period. The write-up is not formal, but does need to be clear. Your lab may have specific questions for you to
answer, or specific statements of tasks to complete, in which case you need to answer and document those things
in your write-up. The lab will include a general set of bullet points that always need to be addressed within your
write-up. Those general bullet points are specific to the type of experiment: observation, testing, or application.
Do not make the mistake of writing too much in your lab reports. Your information does not need to be
presented in paragraph form, or often, even in complete sentences. You can use equations instead of trying to
write the math out in words. Your reports should have a few sentences or bullet points, equations and/or
diagrams where appropriate. You should address the required points succinctly and clearly. Your reports
should not be lengthy or wordy. This is good practice in science writing and will also save both you and the
grader valuable time.
Lab grade:
There are two requirements for passing the lab. If you do not pass the lab, you do not pass the course.
The lab points are also all or nothing. In order to pass the lab, you must attend and conduct all of the labs (you
can make up to two labs during week 10, the reserved lab make-up week, in the event you have an excused
absence during the term). You must also obtain an average of at least 2/3’s of the possible points for your lab
write-ups over the course of the term.
Grading:
Your TA will grade your lab write-up based on some subset of the items you are asked to include. Each
item will be graded out of a possible of 3 points. 0 points means the item is not included in the report. 1 point
means the item is included, but incompletely, or incorrectly. 2 points means the item is included, but with some
small mistake, or it isn’t completely explained. 3 points means it is correct and complete. Roughly 5-9 items
will be chosen for grading each week. The general items that are specific to the three types of ISLE experiments
(observation, testing, and application), there is a set of “rubrics” for you and for the TA to use to evaluate your
work. You are strongly encouraged to check your write-up with the rubrics while you are doing the lab. That
will allow you to have a good gauge for how complete your work is and allow you to improve it before the TA
uses those rubrics to grade.
Rubrics:
The observation, testing, and application rubrics specific to ISLE experiments are given on the next 3
pages. You are encouraged to bring these to lab and use them for self-evaluation as you do the write-up. They
are not just used for grading, but are also intended to help you fully understand the skills you are to develop and
demonstrate, and to help you assess and improve the quality of your work.
Other information:
Lab is not for testing your knowledge, but is a place for developing it. Take advantage of the time to
explore the physics relationships to your own satisfaction, make sure your work makes sense to you, and use
your TA as a resource to make sure you leave the lab with a good understanding of the material. In order to be
counted as present for the lab, you must arrive no more than 15 minutes late, and stay until your group has
turned in its lab write-up. During the first and the last lab, there may be a conceptual understanding or a similar
type of assessment test given to all students. These tests must be completed in order for your lab attendance to
count, but they are not ‘graded’ until after the final grades are submitted. Your specific responses are used only
to assess how much students understand from the course, and do not at all factor into your course grade. Your
lab TA reserves the right to deduct points from your lab write-up for marginal participation or other reasonable
reasons.
Ability to design and conduct an observational experiment
Scientific Ability
Missing
Inadequate
Needs some improvement
The phenomenon to be investigated is described but there are
minor omissions or vague details.
The experiment investigates the
phenomenon and it is likely the
data will contain interesting
patterns, but due to the nature of
the design some features of the
patterns will not be observable.
The chosen measurements will
produce data that can be used to
achieve the goals of the experiment. However, independent and
dependent variables are not
clearly distinguished.
All chosen measurements can be
made, but the details of how it is
done are vague or incomplete.
A description exists, but it is
mixed up with explanations or
other elements of the experiment.
A labeled picture is present. Or
some observations are mentioned
in the context of prior knowledge.
Some shortcomings are identified
and some improvements are
suggested, but not all aspects of
the design are considered.
Is able to identify the
1 phenomenon to be
investigated
Is able to design a reliable
experiment that
investigates the
2
phenomenon
No mention is made of the
phenomenon to be
investigated.
The experiment does not
investigate the phenomenon.
An attempt is made to identify the
phenomenon to be investigated but
is described in a confusing manner.
The experiment involves the
phenomenon but due to the nature
of the design it is likely the data
will not contain any interesting
patterns.
Is able to decide what is to
be measured and identify
independent and
3
dependent variables
The chosen measurements will
not produce data that can be
used to achieve the goals of
the experiment.
The chosen measurements will
produce data that can be used at
best to partially achieve the goals
of the experiment.
Is able to use available
4 equipment to make
measurements
Is able to describe what is
observed without trying to
explain, both in words and
5
by means of a picture of
the experimental set-up.
At least one of the chosen
measurements cannot be made
with the available equipment.
No description is mentioned.
All chosen measurements can be
made, but no details are given
about how it is done.
A description is mentioned but it is
incomplete. No picture is present.
Or, most of the observations are
mentioned in the context of prior
knowledge.
Is able to identify the
shortcomings in an
6 experimental design and
suggest improvements
No attempt is made to identify
any shortcomings of the
experimental design.
An attempt is made to identify
shortcomings, but they are described vaguely and no suggestions for
improvements are made.
Adequate
The phenomenon to be
investigated is clearly stated.
The experiment investigates the
phenomenon and there is a high
likelihood the data will contain
interesting patterns. All features
of the patterns have a high
likelihood of being observable.
The chosen measurements will
produce data that can be used to
achieve the goals of the
experiment. Independent and
dependent variables are clearly
distinguished.
All chosen measurements can be
made and all details of how it is
done are clearly provided.
Clearly describes what happens
in the experiments both verbally
and by means of a labeled
picture.
All major shortcomings of the
experiment are identified and
specific suggestions for
improvement are made.
Ability to construct, modify, and apply relationships or explanations
Is able to construct a
mathematical (if
7 applicable) relationship
that represents a trend in
data
Is able to devise an
8 explanation for an
observed relationship
Is able to identify the
9 assumptions made in
devising the explanation
No attempt is made to
construct a relationship that
represents a trend in the data.
An attempt is made, but the
relationship does not represent the
trend.
No attempt is made to explain
the observed relationship.
An explanation is made but it is
vague, not testable, or contradicts
the observations.
An attempt is made to identify
assumptions, but most are missing,
described vaguely, or incorrect.
No attempt is made to identify
any assumptions.
The relationship represents the
trend but no analysis of how well
it agrees with the data is included
(if applicable), or some features of
the relationship are missing.
An explanation is made and is
based on simplifying the phenomenon but uses flawed reasoning.
Most assumptions are correctly
identified.
The relationship represents the
trend accurately and completely
and an analysis of how well it
agrees with the data is included
(if applicable).
A reasonable explanation is
made and is based on
simplifying the phenomenon.
All assumptions are correctly
identified.
Rubric B
Ability to design and conduct a testing experiment (testing an idea/hypothesis/explanation or mathematical relation)
Scientific Ability
Missing
Inadequate
Needs some improvement
Adequate
1
Is able to identify the
hypothesis to be tested
No mention is made of a
hypothesis.
The experiment does not test
the hypothesis.
2
Is able to design a reliable
experiment that tests the
hypothesis
Is able to distinguish
between a hypothesis and
a prediction
No prediction is made. The
experiment is not treated as a
testing experiment.
A prediction is made but it is identical
to the hypothesis.
Is able to make a
reasonable prediction
based on a hypothesis
Is able to identify the
assumptions made in
making the prediction
No attempt to make a prediction
is made.
Is able to determine
specifically the way in
which assumptions might
affect the prediction
Is able to decide whether
the prediction and the
outcome agree/disagree
No attempt is made to
determine the effects of
assumptions.
A prediction is made that is distinct
from the hypothesis but is not based on
it.
An attempt is made to identify
assumptions, but the assumptions are
irrelevant or are confused with the
hypothesis.
The effects of assumptions are
mentioned but are described vaguely.
Is able to make a
reasonable judgment
about the hypothesis
No judgment is made about the
hypothesis.
Is able to revise the
hypothesis when
necessary
A revision is necessary but
none is made.
3
4
5
6
7
8
9
No attempt is made to identify
any assumptions.
No mention of whether the
prediction and outcome
agree/disagree.
An attempt is made to identify the
hypothesis to be tested but is described
in a confusing manner.
The experiment tests the hypothesis,
but due to the nature of the design it is
likely the data will lead to an incorrect
judgment.
The hypothesis to be tested is
described but there are minor
omissions or vague details.
The experiment tests the hypothesis,
but due to the nature of the design
there is a moderate chance the data
will lead to an inconclusive
judgment.
A prediction is made and is distinct
from the hypothesis but does not
describe the outcome of the
designed experiment.
A prediction is made that follows
from the hypothesis but does not
incorporate assumptions
Relevant assumptions are identified
but are not significant for making
the prediction.
The hypothesis is clearly stated.
The experiment tests the
hypothesis and has a high
likelihood of producing data that
will lead to a conclusive
judgment.
A prediction is made, is distinct
from the hypothesis, and
describes the outcome of the
designed experiment
A prediction is made that follows
from the hypothesis and
incorporates assumptions.
All assumptions are correctly
identified.
The effects of assumptions are
determined, but no attempt is made
to validate them.
The effects of the assumptions are
determined and the assumptions
are validated.
A decision about the
agreement/disagreement is made but is
not consistent with the outcome of the
experiment.
A judgment is made but is not
consistent with the outcome of the
experiment.
A reasonable decision about the
agreement/disagreement is made
but experimental uncertainty is not
taken into account.
A judgment is made and is
consistent with the outcome of the
experiment but assumptions are not
taken into account.
A reasonable decision about the
agreement/disagreement is made
and experimental uncertainty is
taken into account.
A reasonable judgment is made
and assumptions are taken into
account.
A revision is made but the new
hypothesis is not consistent with the
results of the experiment.
A revision is made and is
consistent with the results of the
experiment but other relevant
evidence is not taken into account.
A revision is made and is
consistent with all relevant
evidence.
Rubric C
Scientific Ability
Ability to design and conduct an application experiment
Missing
Inadequate
Needs some improvement
1
Is able to identify the
problem to be solved
No mention is made of the
problem to be solved.
The experiment does not
solve the problem.
2
Is able to design a reliable
experiment that solves the
problem
Is able to use available
equipment to make
measurements
At least one of the chosen
measurements cannot be
made with the available
equipment.
No discussion is presented
about the results of the
experiment
All of the chosen measurements
can be made, but no details are
given about how it is done.
Is able to evaluate the results
by means of an independent
method
No attempt is made to
evaluate the consistency of
the result using an
independent method.
A second independent method is
used to evaluate the results.
However there is little or no
discussion about the differences in
the results due to the two methods.
Is able to identify the
shortcomings in an
experimental design and
suggest specific
improvements
No attempt is made to
identify any shortcomings
of the experimental design.
An attempt is made to identify
shortcomings, but they are described vaguely and no specific
suggestions for improvements are
made.
Is able to choose a
productive mathematical
procedure for solving the
experimental problem
Is able to identify the
assumptions made in using
the mathematical procedure
Is able to determine
specifically the way in which
assumptions might affect the
results
Mathematical procedure is
either missing, or the
equations written down are
irrelevant to the design.
No attempt is made to
identify any assumptions.
3
4
Is able to make a judgment
about the results of the
experiment
5
6
An attempt is made to identify the
problem to be solved but it is
described in a confusing manner.
The experiment attempts to solve
the problem but due to the nature of
the design the data will not lead to
a reliable solution.
A judgment is made about the
results, but it is not reasonable or
coherent.
The problem to be solved is
described but there are minor
omissions or vague details.
The experiment attempts to
solve the problem but due to the
nature of the design there is a
moderate chance the data will
not lead to a reliable solution.
All of the chosen measurements
can be made, but the details
about how they are done are
vague or incomplete.
An acceptable judgment is
made about the result, but the
reasoning is flawed or
incomplete.
A second independent method
is used to evaluate the results.
Some discussion about the
differences in the results is
present, but there is little or no
discussion of the possible
reasons for the differences.
Some shortcomings are
identified and some improvements are suggested, but not all
aspects of the design are
considered.
Adequate
The problem to be solved is clearly
stated.
The experiment solves the problem
and has a high likelihood of
producing data that will lead to a
reliable solution.
All of the chosen measurements
can be made and all details about
how they are done are provided
and clear.
An acceptable judgment is made
about the result, with clear
reasoning. The effects of
assumptions and experimental
uncertainties are considered.
A second independent method is
used to evaluate the results. The
discrepancy between the results of
the two methods, and possible
reasons are discussed. A
percentage difference is calculated
in quantitative problems.
All major shortcomings of the
experiment are identified and
specific suggestions for
improvement are made.
Ability to construct, modify, and apply relationships or explanations
7
8
9
No attempt is made to
determine the effects of
assumptions.
A mathematical procedure is
described, but it is incomplete, due
to which the final answer cannot be
calculated.
An attempt is made to identify
assumptions, but most are missing,
described vaguely, or incorrect.
An attempt is made to determine
the effects of some assumptions,
but most are missing, described
vaguely, or incorrect.
Correct and complete
mathematical procedure is
described but an error is made
in the calculations.
Most assumptions are correctly
identified.
Mathematical procedure is fully
consistent with the design. All
quantities are calculated correctly.
Final answer is meaningful.
All assumptions are correctly
identified.
The effects of most assumptions
are determined correctly,
though a few contain errors,
inconsistencies, or omissions.
The effects of all assumptions are
correctly determined.
Rubric D
Physics 213 Lab 1: Electric Force
I. Observation Experiment
It is known that an acrylic rod rubbed with cloth is positively charged, and a foam board
rubbed with the cloth results in a negative charge on the foam board. Using this information,
design experiments to determine if the following objects are uncharged, positively charged or
negatively charged. When you conduct your experiments, be careful that the objects you are
touching are isolated from other objects (for example, don’t hold a metal can with your hand
while rubbing it with cloth, instead use some insulation between your hand and the can.)
i. a foam board
ii. a foam board rubbed with fur or cloth
iii. a piece of scotch tape that is first stuck to the top of a table and then pulled off.
iv. a piece of scotch tape placed on top of a second piece stuck to the top of the table,
and the top piece of tape pulled off
v.
any other objects at your table
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All lettered bullet points must be
addressed in your lab write-up, and a random subset will be graded. (No piece of the
observation experiment rubric not explicitly addressed in the bullets will be graded.)
1. (O4) Briefly describe how you will make use of the available equipment to make your
observations.
Perform the experiments.
2. (O5) Describe what is observed (without trying to explain) by means of a data table.
3. (O7) Using your data, state any trends you found in your observations.
4. (O8) Devise an explanation for your observed trends.
5. (O9) Identify any assumptions made in devising the explanation.
6. How could you test the observations you found? Give a brief description in your lab
report.
7. Use this test to check some of your observations and report the findings of your test: did
they confirm your previous observations or did you need to make corrections?
8. (O6) Identify shortcomings of your experimental design by listing the sources of
experimental uncertainty. Describe improvements you could and/or did make to
minimize them.
II. Testing Experiment
Design an experiment to show that the interaction between charged objects can not be
explained solely by gravitational interaction.
Bullets marked, for example, T3 refer to the testing experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in
your lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (T1) Identify the hypothesis (rule) to be tested.
b. (T2) Design a reliable experiment that tests the hypothesis including a brief description
of your procedure.
c. Draw a labeled sketch of the experimental set-up.
d. (T4) Make a prediction about the outcome of the experiment based on the hypothesis.
e. (T5) Identify the assumptions made in making the prediction. What assumptions about
the objects, interactions, and processes you need to make to solve the problem?
f. (T6) Determine specifically which assumptions might affect the prediction.
Perform the experiment
g. Clearly record the outcome of your experiment.
h. (T7) Decide whether the prediction and the outcome agree/disagree.
i. Decide whether your assumptions and experimental uncertainties can account for any
discrepancy between the predicted and measured value.
j. (T8) Make a reasonable judgment about the hypothesis based on your experimental
outcomes, the assumptions you made, and the estimated uncertainty.
k. Create a table like the following with the arguments, prediction and evidence for testing
your hypothesis. Look at the example provided.
Explanation/
Hypothesis
Experiment
design
Predicted
outcome
Observed
outcome
Conclusion
(hypothesis
supported or not)
If …
and I do this…
then …
And I saw/
But I saw …
Therefore …
III. Testing Experiment
A textbook says that metallic objects such as aluminum cans have electrically charged
particles that can freely move inside, and that insulating objects such as plastic bottles do not
have freely moving charged particles. Design an experiment to test this using any equipment
at your table. (cans, plastic bottles, tape, foil, and/or any objects from experiment 1)
Bullets marked, for example, T3 refer to the testing experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in
your lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (T1) Identify the hypothesis (rule) to be tested.
b. (T2) Design a reliable experiment that tests the hypothesis including a brief description of
your procedure.
c. Draw a labeled sketch of the experimental set-up.
d. (T4) Make a prediction about the outcome of the experiment based on the hypothesis.
e. (T5) Identify the assumptions made in making the prediction. What assumptions about
the objects, interactions, and processes you need to make to solve the problem?
f. (T6) Determine specifically which assumptions might affect the prediction.
Perform the experiment
g. Clearly record the outcome of your experiment.
h. (T7) Decide whether the prediction and the outcome agree/disagree.
i. (T8) Make a reasonable judgment about the hypothesis based on your experimental
outcomes and the assumptions you made.
j. Fill out a table with the arguments and evidence for testing your hypothesis as you did in
part II of this lab.
IV. Observation Experiment
Determine if paper is a conductor or an insulator. Tear a piece of paper into small bits and
lay them on the lab bench. Charge the foam board and bring it near the paper.
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All lettered bullet points must be
addressed in your lab write-up, and a random subset will be graded. (No piece of the
observation experiment rubric not explicitly addressed in the bullets will be graded.)
1. (O4) Briefly describe how you will make use of the available equipment to make your
observations.
Perform the experiments.
2. (O5) Describe what is observed (without trying to explain).
3. (O8) Devise an explanation for your observations.
4. What evidence do you have to support if there are or aren’t freely-moving charges inside
the paper?
5. (O9) Identify any assumptions made in devising the explanation.
6. (O6) Identify shortcomings of your experimental design by listing the sources of
experimental uncertainty. Describe improvements you could and/or did make to
minimize them.
V. Observation Experiment
Hang a very small wad of tin foil (using string and a ring stand) within 1 cm of a smooth,
charged, metal object (the side of a soda can should work). (Make sure the metal object is
isolated.) Put your finger roughly 1 cm on the other side of the wad. Let the wad touch the
metal, then try to get it oscillating between the metal and your finger (without you physically
swinging it).
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All lettered bullet points must be
addressed in your lab write-up, and a random subset will be graded. (No piece of the
observation experiment rubric not explicitly addressed in the bullets will be graded.)
1. (O4) Briefly describe how you will make use of the available equipment to make your
observations.
Perform the experiments.
2. (O5) Describe what is observed (without trying to explain).
3. (O8) Devise an explanation for your observations.
4. What evidence do you have to support whether or not your finger is a conductor?
5. (O9) Identify any assumptions made in devising the explanation.
Physics 213 Lab 2: Electric Field
I.
Observation Experiment:
At your lab table you have two nail-boards with differing numbers of nails in each, and a wire
loop. The nails represent electric field lines. Flux can be visualized as the number of nails
(electric field lines) which pass through the loop. Using the nail-boards and the loop, determine
the parameters on which flux is dependent.
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in your
lab write-up, and a random subset will be graded.
a. (O2) Design a reliable experiment that will investigate the phenomenon.
b. Draw a clearly labeled diagram of your experimental set-up. Make sure you represent the
important aspects of your experiment.
c. (O3) Decide what is to be measured and identify independent and dependent variables.
d. (O4) Briefly describe how you will make use of the available equipment to make your
measurements.
Perform the experiment.
e. (O5) Describe what is observed without trying to explain, both in words and by means of a
data table.
f. Explain specifically how you are finding proportionality relationships from your data.
g. (O8) Devise an explanation for your observed relationships.
h. What parameters does flux depend on, and how did you arrive at this conclusion?
i. (O9) Identify any assumptions made in devising the explanation.
j. (O6) Identify shortcomings of your experimental design by listing the sources of
experimental uncertainty. Describe improvements you could and/or did make to minimize
them.
II.
Application Experiment:
In this experiment, you will use a cathode-ray tube (CRT) to accelerate and deflect a beam of
electrons. The CRT is a glass vessel which contains: (1) an electron gun that emits electrons into
a beam that travels along the tube; (2) a deflection system to change the direction of the beam;
and (3) a viewing screen. CRTs are used in oscilloscopes, and the same principle is used in old
TV sets and computer monitors.
There are 3 ‘stages’ for the electrons moving along the CRT.
1. The electrons leave a heated filament (approximately from rest) and are accelerated
horizontally down the tube a distance a, due to a voltage Va applied across this section of
the tube. We assume that the electrons have reached a maximum constant speed just at
the end of this distance d, and that the entire acceleration is due to the applied voltage Va.
2. The electrons then enter the deflection region of length b, where they are bent from their
horizontal path by a deflection voltage Vb. This deflection is perpendicular to their
horizontal motion (either up, down, left or right, when facing the tube).
3. Finally, the electrons enter region ‘c’ where they travel some distance c at constant
velocity with no voltage applied, until they hit the screen.
A disassembled tube is available for inspection. The voltages are established by parallel plate
capacitors, and therefore have nearly constant electric fields between their plates (E = V/x, where
x is the distance between the two plates).
In this experiment, you will investigate the mathematical relationship between the deflection of
the electron, and the two voltages, Va and Vb (separately). You will need to take enough data to
plot the relationship, and also work out the theoretical prediction based on your knowledge of
electric force and field, and compare your plotted data to your derived equations.
PROCEDURE for collecting data: Before turning on the power supply to the CRT, verify, or
have your instructor verify, proper connections. A wiring diagram will be provided, and all
devices should be correctly wired up ahead of time – check with the diagram though, as an
incorrectly wired set-up can burn out the tube. The power supply has a standby position that
turns on the filament before the high voltage. Leave it in standby for 30 seconds or so before
turning on full power. Make sure that the meter switch is set correctly to read the B+ voltage and
set the voltage to around 350 V. Adjust the focus knob for the best spot size. It sometimes
happens at start up that electrons collect on the phosphor screen and don't migrate back to
ground. These electrons can repel the beam from the area and make the screen look like it has a
burned spot on it. Time will usually create a conducting path and correct the problem, but
sweeping the electron beam back and forth across the spot might speed up the process.
The deflection voltage VD is obtained from a voltage divider that takes its power from the table
outlets. Make sure that the middle terminal (GND) from the table outlet connects to the red
terminal on the CRT table power input terminal (GND). Positive and negative table power
voltages connect to the black CRT table power input through a reversing switch. This allows
positive and negative voltages to be put across the CRT's deflection plates, thereby reversing the
direction of the deflecting electric field Ex. The actual deflection voltage is measured with a
voltmeter connected across the output of the voltage divider and in parallel with the CRT's
horizontal deflection plates.
Bullets marked, for example, A3 refer to the application experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in your
lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (A1) Identify the problem to be solved.
b. Draw a sketch of your experimental apparatus, making sure each variable you will use is
clearly defined in terms of the actual apparatus.
c. Draw physical representations for appropriate portions of the electron’s motion.
d. (A7) Choose a productive mathematical procedure to derive the relationship between the
deflection of the electrons and the voltages in the tube using your physical representation.
e. Complete your derivation and from it, predict the mathematical relationship between the
deflection of the electrons and each of the two voltages.
f. Explain qualitatively why the relationships you found make sense (for example, if you throw
the baseball with more force it will have a higher initial velocity and travel further)
g. (A3) Discuss how you will use the available experiment to make the measurements.
h. (A8) Identify the assumptions made in using the mathematical procedure.
i. For one particular assumption, describe in detail how this assumption would affect the
equation and the prediction for the deflection of the electron (for example, will it be deflected
more than we predict because we have chosen a particular assumption in deriving the
equation)?
j. (A9) Determine specifically the way in which assumptions might affect the results.
k. What are the possible sources of experimental uncertainty? How could you minimize them?
Perform the experiment to test your relationship
l. Record the outcome of your experiment in an appropriate format, including graphing the
deflection vs. each of the two voltages (separately).
m. How does the outcome of your experiment compare with your mathematical prediction?
Resolve, or suggest and explain physical reasons, for any discrepancies.
n. What are the possible sources of experimental uncertainty when taking your data? How could
you minimize them?
o. (A4) Make a judgment about the results of your experiments.
p. (A6) Identify the shortcomings in the experiment and suggest specific improvements.
Physics 213 Lab 3: Electric Potential
Experimental Apparatus: A tray is filled to a depth of about 1/4" with tap water and two
electrodes are placed in the tray. When a potential difference V is placed across the
electrodes, an electric field is established in the water. Since the water is conducting and air
is not, and the electric current is always parallel to the electric field, virtually all the electric
field lines in the water are parallel to the surface of the water. This allows us to treat the
water and electrodes as a purely two-dimensional system. Make sure the water is very level
before proceeding (any tilt in the tray/table will be a source of error).
I.
Observation Experiment
Mapping electric fields for a button electrode inside a ring-shaped electrode:
Experimental Method: There is a voltage divider attached to the electrodes in the water, and
an amplifier-detector. The voltage divider allows us to tap off voltages from 0 up to the
maximum applied voltage V in steps of 0.1V. One lead of the detector is connected to the tap
of the voltage divider and the other goes to a probe placed in the water tank. By moving the
probe around until the output from the detector-amplifier is a minimum, we can find a point
in the water tank where the potential is equal to the potential of the voltage divider tap.
Objective: Move the probe in the water and observe that the voltage varies from point to
point. Find several points in the tray which measure the same voltage. Try to find a curved
path that has the same voltage everywhere. Using your probe, trace out locations (over an
arc of at least 90 degrees) where the potential is .2V, .4V, .6V, .8V and 1V. Use a sheet of
graph paper to mark the positions and shapes of the electrodes and the voltages you probe.
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in
your lab write-up, and a random subset will be graded.
a. Draw a clearly labeled diagram of your experimental set-up. Make sure you represent the
important aspects of your experiment.
b. (O3) Decide what is to be measured and identify independent and dependent variables.
Perform the experiment.
c. (O5) Describe what is observed without trying to explain, both in words and by means of
a graph of your data.
d. Explain specifically how you are finding the relationship from your data.
e. (O7) Using your data and graph, what can you conclude about the relationship between
distance from the central electrode and the potential around the electrode?
f. (O9) Identify any assumptions made in devising the relationship.
If necessary, review the relationship between equipotentials and electric field lines before
doing the following:
a. Draw a few representative field lines on your graph paper with a different color pencil
(or pen) than you used for the equipotential lines.
b. Describe the shape of the electric field around the central electrode, and explain why
it should have this shape.
c. Where is the electric field the strongest, and where is it the weakest? How do you
know?
d. Choose a single electric field line. Using Cartesian graph paper, plot the electric
potential along this line as a function of distance from the center of the tray. 5-8 well
chosen data points are sufficient for this plot.
e. Comment on the shape of the graph and write a general statement describing the
potential around the central electrode.
f. From your data, write a mathematical relationship for the potential around the
electrode and compare it to your earlier observation. Is the mathematical relationship
what you expected? If so, why? If not, reconcile any differences.
II.
Observation experiments
Electric potential and field around different geometries: Use the following geometries
and make the associated observations:
(1)
.
.
Point electrodes in water, separated
by about 6 inches.
(2)
Long straight electrodes, separated by
about 4 inches.
(3)
Geometry of part 2 but with a hollow
conducting cylinder between the two
electrodes.
For each of the above geometries:
a. Use the probe to map out the equipotentials.
b. Represent them on your graph paper.
c. Use the equipotential lines to map out the electric field.
d. Identify regions of higher and lower electric field, and explain why the electric field
has the distribution that it does.
e. For the ‘parallel plate’ geometry, devise a mathematical relationship for both the
potential and the electric field between the plates (and not near the edges).
f. For the ‘parallel plate’ geometry, what are your sources of error and how do they
impact your results?
Physics 213 Lab 4: DC Circuits I
I.
Observation Experiment:
At your table, you have a sheet of resistive paper, and several strips of the paper cut to different sizes.
You also have a multi-meter, several metal blocks, and wires. By placing the metal blocks on the ends of
the strips of paper, you can measure the resistance of the paper with the multi-meter. Devise an
experiment(s) to determine the relationship between the resistance of the paper and the geometry of the
paper. (Hint: the paper is essentially 2 dimensional, so how many parameters will you need to test?)
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3. Bullets without
such designation are given as additional guidance. All bullets must be addressed in your lab write-up,
and a random subset will be graded.
a. (O2) Design a reliable experiment that will investigate the phenomenon.
b. Draw a clearly labeled diagram of your experimental set-up.
c. (O3) Decide what is to be measured and identify independent and dependent variables.
Perform the experiment.
d. (O5) Describe what is observed without trying to explain, both in words and by means of a data table.
You will need to make several measurements in order to find a mathematical relationship between the
parameters.
e. Represent the data graphically (you will need one graph for each parameter you varied).
f. (O7) Using your graph(s), determine the mathematical relationship between all of the parameters.
What parameters does the resistance depend on, and how did you arrive at this conclusion?
g. (O8) Devise an explanation for your observed relationship(s).
h. (O9) Identify any assumptions made about the objects or measurements and describe how these could
affect the findings.
i. (O6) Identify shortcomings of your experimental design by listing the sources of experimental
uncertainty. Describe improvements you could and/or did make to minimize them.
II.
Application Experiment:
Using the relationship you found in the previous experiment, devise an experiment to determine how
voltage and resistance are related. In order to do this, you will need to keep any other parameters fixed,
which in this case is current. You are encouraged to check with your TA to make sure you have
accomplished this with your setup before you begin taking data. You have the same equipment as before,
plus a 3V battery source. Use the following guidelines:
a. Write a brief description of the procedure you will use and make a sketch of your experimental
design.
b. (A8) Identify the assumptions made in using the mathematical procedure.
c. (A9) Determine specifically the way in which assumptions might affect the results.
d. (A3) Discuss how you will use the available experiment to make the measurements.
Perform the experiment
e. Record the outcome of your experiment in the form of a table and a graph. Also include a brief
description of your observations
f. What are the possible sources of experimental uncertainty? How could you minimize them?
A quantity known as conductance, which is simply the inverse of resistance is sometimes more
illuminating when used in the place of resistance. Conductance is defined as: G = 1/R
a. Re-write your mathematical relationship from the first experiment in terms of conductance.
Your TA has an experiment to do with you – there is only one device for the whole class, so let your TA
know you have reached this point so they can coordinate doing the experiment with you after you have
done the following:
It is always important to test the limits of a prediction. These include when things get very cold, very hot,
very large, or in the case we will be exploring, very small. We will be testing the nanoconductance of
platinum. The way that we will accomplish this is to slowly pull apart two lengths of platinum wire. If this
is done slowly enough, small nanowires will form between the two lengths of platinum. Below is a
cartoon of this situation, note that the size of the nanowire has been greatly exaggerated.
Since this happens very quickly we will need different tools to explore conductance on this scale. To this
end we will be using an oscilloscope and a second device specifically created for this purpose. An
oscilloscope is used to measure electric potentials on a very fast time scale. Before we can make any
measurements, we must first discuss how our devices operate. The box used in this experiment has a knob
on the front. This knob controls the voltage supplied by the box that will be utilized to measure
conductance. This voltage (Vsource) is what is displayed by the multimeter hooked to the box. The
voltage out of the box (Vsignal) is measured by the oscilloscope. To read an oscilloscope multiply the
number of squares from the zero line the signal is and multiply it by the displayed units per division. There
are two knobs for each channel (we are using Channel 1 in this experiment). One knob controls the units
of voltage per division. The second knob controls the offset of the channel. This changes where the zero
voltage line is. There are two similar knobs for the time axis. Varying these knobs can make
measurements more convenient or illuminate details that would otherwise be unobservable.
Using these tools, the conductance or resistance of an object can be measured. The relationship used is:
-5
-1
G = k (Vsignal/Vsource), where k ≈ 10 Ω .
Experiment
The goal of this experiment is to observe a platinum nanowire. To accomplish this, the break in
connection must happen slowly enough to be recorded on the oscilloscope. The breaks you are looking for
will have a constantly decreasing potential and take 30 μs or more to completely go to zero. To most
easily observe nanowires, the following settings should be used:
Vsource= 10 -20 mV
Time Scale = 50 -100 μs
Voltage Scale = 100 mV/division
Voltage Offset = 2-3 divisions below zero
Trigger Level = -200 mV
When you think you've observed a slow enough break, press the stop button. Once your TA is satisfied
that you are observing the correct phenomena, accurately reproduce your data below, making sure to
include units on your axis.
b. Is this qualitatively consistent with your previous mathematical model for conductance?
c. Are the steps in your data consistent with your mathematical model? If so, why? If not, can you
come up with any ways you might resolve the differences between your data and your model? (It
may help to calculate the conductance at various steps.)
d. Using your graph, calculate the conductivity and resistance where the nanowire is the thinnest.
Before we can apply your mathematical model, you'll need to know a little bit more about the
nanowire. The steps in your graph correspond to atoms. The first step is a nanowire has a cross section
of a single atom, the second equal step has a cross section of two atoms, etc. The approximate length
of a typical atomic width wire is 1 nm, while the cross section is about 0.3 nm by 0.3 nm. The
-7
resistivity of platinum is 1.06 x 10 Ω*m.
e. Using your mathematical model for resistance, calculate the expected value of this resistance.
f. How does the expected value from your previous model compare to the actual measured value?
g. Can you account for any differences based on your sources of error?
h. Can you account for any differences based on the assumptions of your previous model? (Does
your large scale model for resistance apply on the nanoscale?)
Physics 213 Lab 5: DC Circuits II
I.
Observation Experiment
Given one piece of wire, a light bulb and a battery, find ways to make the bulb light in at
least two different ways. Also find two circuits in which the bulb will NOT light.
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3.
Bullets without such designation are given as additional guidance. All bullets must be
addressed in your lab write-up, and a random subset will be graded.
a. (O2) Design a reliable experiment that will investigate the phenomenon.
b. Draw a clearly labeled circuit diagram of each experimental set-up. Make sure you
represent the important aspects of your experiment.
Perform the experiment.
c. (O5) Describe what is observed without trying to explain.
d. (O8) Using your observations from each experiment, construct an explanation for
what types of circuit(s) will light the bulb and what type will not.
e. (O9) Identify any assumptions made in devising the explanation.
II.
Testing Experiment
Design an experiment to test whether Kirchoff‘s junction rule and loop rule work. The
junction rule states that the sum of the currents entering any junction must be equal to the
sum of the currents leaving that junction. The loop rule states that the algebraic sum of
the changes in potential encountered in a complete traversal of any loop of a circuit must
be zero. You must have more than one junction and more than one battery in your
circuit. Use at least 4 circuit elements besides batteries and wires. Equipment:
Batteries, resistors, light bulbs of different ratings, an ammeter, and a voltmeter.
Bullets marked, for example, T3 refer to the testing experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed
in your lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (T1) Identify the hypothesis (rule) to be tested.
b. (T2) Design a reliable experiment that tests the hypothesis including a brief
description of your procedure.
c. Draw a labeled sketch of the experimental set-up including a circuit diagram.
d. Devise the mathematical procedure that you will use to make your prediction.
e. (T4) Make a prediction about the outcome of the experiment (including both current
at various points along the circuit and voltages across various circuit elements) based
on the hypothesis.
f. (T5) Identify the assumptions made in making the prediction. What assumptions
about the objects, interactions, and processes do you need to make to solve the
problem?
g. (T6) Determine specifically in which assumptions might affect the prediction.
h. What are experimental uncertainties in this experiment?
Perform the experiment
i. Clearly record the outcome of your experiment.
j. (T7) Decide whether the prediction and the outcome agree/disagree.
k. Decide whether your assumptions and experimental uncertainties can account for any
discrepancy between the predicted and measured value.
l. (T8) Make a reasonable judgment about the hypothesis based on your experimental
outcomes, the assumptions you made, and the estimated uncertainty.
III.
Observation Experiment
Devise an experiment for determining the direction and strength (qualitative) of current
flow in a circuit while a capacitor is charging and discharging. (To discharge, you simply
remove the battery from your circuit.) You have wires, batteries, capacitors, light bulbs,
and a compass at your table. Hint: make a simple circuit with one capacitor and some
light bulbs. Where do you need to place the light bulbs to test current flow in each part of
your circuit?
Bullets marked, for example, O3 refer to the observation experiment rubric ability 3.
Bullets without such designation are given as additional guidance. All bullets must be
addressed in your lab write-up, and a random subset will be graded.
a. Draw a clearly labeled circuit diagram of your experimental set-up. Be sure to
indicate any differences in your diagram between when the capacitor is charging and
when it is discharging.
b. (O3) Decide what is to be measured and identify independent and dependent
variables.
c. (O4) Briefly describe how you will make use of the available equipment to make your
measurements.
Perform the experiment.
d. (O5) Describe what is observed without trying to explain.
e. Explain specifically how you are finding the direction of the current from your data.
f. (O7) Using your data, construct a relationship that describes the direction of the
current flow.
g. (O8) Devise an explanation for your observed relationship.
h. (O6) Identify shortcomings of your experimental design by listing the sources of
experimental uncertainty. Describe improvements you could and/or did make to
minimize them.
IV.
Testing Experiment
Using the information you found in the previous experiment, test the direction and
strength (qualitative) of current flow through all wires of a series circuit with more than
one capacitor. Do this for both charging and discharging the capacitors.
Bullets marked, for example, T3 refer to the testing experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed
in your lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (T1) Identify the hypothesis (rule) to be tested.
b. (T2) Design a reliable experiment that tests the hypothesis including a brief
description of your procedure.
c. Draw a labeled sketch of the experimental set-up including a circuit diagram.
d. Devise the mathematical procedure that you will use to make your prediction.
e. (T4) Make a prediction about the outcome of the experiment (including the current at
various points in the circuit during charging and discharging) based on the
hypothesis.
f. (T5) Identify the assumptions made in making the prediction. What assumptions
about the objects, interactions, and processes do you need to make to solve the
problem?
g. What are experimental uncertainties in this experiment?
Perform the experiment
h. Clearly record the outcome of your experiment.
i. (T7) Decide whether the prediction and the outcome agree/disagree.
j. Decide whether your assumptions and experimental uncertainties can account for any
discrepancy between the predicted and measured value.
k. (T8) Make a reasonable judgment about the hypothesis based on your experimental
outcomes, the assumptions you made, and the estimated uncertainty.
Physics 213 Lab 6: Magnetic Force
I.
Testing Experiment
Design three experiments to test the right hand rule as used to find the direction of the
force that a magnetic field exerts on a moving charge/current. The equation describing this
is F = qvxB. No more than two of the experiments can use the cathode ray oscilloscope.
(The cathode ray oscilloscope contains a beam of electrons which produce a green dot
when they hit the screen). Equipment: Bar magnet, cathode ray oscilloscope, horseshoe
magnet, battery, long wire. (If you are not sure about the magnetic field surrounding any
of the objects in this lab, use the magnetic field probe first to find this out before using the
object in your experiment).
Bullets marked, for example, T3 refer to the testing experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed
in your lab write-up, and a random subset will be graded.
Follow these steps and write in your report:
a. (T1) Identify the hypothesis (rule) to be tested.
b. (T2) Design a reliable experiment that tests the hypothesis including a brief description
of your procedure.
c. Draw a labeled sketch of EACH experimental set-up.
d. (T4) Make a prediction about the outcome of EACH experiment based on the
hypothesis.
e. (T5) Identify the assumptions made in making the prediction. What assumptions about
the objects, interactions, and processes you need to make to solve the problem?
f. (T6) Determine specifically in which assumptions might affect the prediction.
g. What are experimental uncertainties in this experiment?
Perform the experiment
h. Record the outcome of your experiments.
i. (T7) Decide whether the prediction and the outcome agree/disagree.
j. Decide whether your assumptions and experimental uncertainties can account for any
discrepancy between the predicted and measured value.
k. Which of your experiments best minimizes assumptions and errors? Briefly discuss
the merits and limitations of each experiment.
l. (T8) Make a reasonable judgment about the hypothesis based on your experimental
outcomes.
II.
Application Experiment:
In this activity we will use the force exerted by a uniform magnetic field to bend a beam of
electrons into a circular path. In the apparatus, the electrons are first accelerated through a
potential difference V. After acceleration, the beam enters a region in which there is a
magnetic field B perpendicular to the velocity of the electrons.
Apparatus Notes: The electron beam is produced by an electron gun consisting of a heated
cathode (which emits electrons), a focusing grid, and an anode held at a potential V with
respect to the cathode. The electrons are accelerated through this potential difference as
they move from the cathode to the anode. The electron beam emerges from a small hole in
the anode and then enters the magnetic field region. The magnetic field is produced by
two circular coils of wire whose separation distance is equal to their radius. This
arrangement is called a Helmholtz pair and produces a very uniform magnetic field in the
region between the coils. The field is much larger than the Earth’s magnetic field, which
can be neglected in this experiment. The electrons move in an evacuated tube into which a
small amount of inert gas has been introduced; when the electrons collide with the gas
atoms, the atoms emit blue light, which makes the path of the electron beam visible.
CAUTION: THERE IS A SHOCK HAZARD FROM THE POWER SUPPLIES USED IN
THIS EXPERIMENT.
The magnetic field due to the Helmholtz coils is
B
8 0 NI
125 R
(4)
where N is the number of turns in each coil, I is the current through the coils, and R is the
coil radius.
Experimental tips: The circuit has been pre-wired. Set the power supply to “standby.”
This will cause the filament to heat, and its glow should be visible in subdued light. After
a couple of minutes, turn on the high voltages and raise the anode supply voltage to about
80 volts. The blue trace from the beam should now be visible.
Your goal is to determine the magnetic field of the Helmholtz coil and compare it to the
theoretical equation given above. There are considerable sources of error in this
experiment (for instance, measuring the radius of the electron path within the magnetic
field), so it will be necessary to make measurements with different accelerating voltages
rather than relying on just one measurement for comparison. (The mass and charge of the
electron are given in your textbook.)
Bullets marked, for example, A3 refer to the application experiment rubric ability 3.
Bullets without such designation are given as additional guidance. All bullets must be
addressed in your lab write-up, and a random subset will be graded.
Include the following in your report:
a. (A1) Identify the problem to be solved.
b. (A2) Design a reliable experiment that solves the problem.
c. Draw a sketch of your experimental design.
d. Draw appropriate physical representations for the experiment.
e. (A7) Choose a productive mathematical procedure for solving the problem. Use the
physical representation to devise the mathematical procedure to solve the problem.
f. What is your derived mathematical relationship between the accelerating voltage and
the magnetic field of the coils?
g. (A3) Discuss how you will use the available experiment to make the measurements.
h. (A8) Identify the assumptions made in using the mathematical procedure.
i. (A9) Determine specifically the way in which assumptions might affect the results.
j. What are the possible sources of experimental uncertainty? How could you minimize
them?
Perform the experiment.
k. Record the outcome of your experiment. (Be sure to record the radius of the circle
made by the electron beam, the current in the Helmholtz coils, and the accelerating
voltage of the electrons.) To make an accurate determination of the vertical diameter
of the circle, use the mirror to line up the bottom of the beam with its reflection and
then the top.
l. Using your derived relationship and experimental measurements, determine the
magnetic field of the coil.
m. Compare your result to the theoretical value you can calculate for the magnetic field of
Helmholtz coils (your TA has the value of N).
n. How does the result of your experiment compare to the theoretical value? What are
possible reasons for the difference?
o. (A4) Make a judgment about the results of your experiment.
p. (A6) Identify the shortcomings in the experiment and suggest specific improvements.
Physics 213 Lab 7: Magnetism and Electric Current
I.
Application experiment
Apply your understanding of magnetic force on a current carrying wire and the right hand rule
you tested last lab to explain the operation of the simple motor at your table. This motor consists
of a coil of current-carrying wire (suspended in such a way that it makes electrical contact with
batteries but can still rotate freely) and a strong magnet held near the coil. In order to get your
motor started, you may need to first balance the coil so it rotates easily and give it a gentle initial
spin.
Follow these steps and write in your report:
a. Draw a diagram of the motor and a description of it while it is in operation.
b. Determine what positions of the coil with respect to the magnetic field are important for
understanding the rotation (what parts of the motion do you need to consider more carefully
to explain what is happening)?
c. Draw diagrams for each of these positions and apply the right hand rule to find the direction
of the force on the coil (will the force be the same everywhere along the coil? How many
places along the coil do you need to consider?)
d. Using your diagrams and a brief summary, explain the operation of the motor in detail. What
causes the coil rotate in the magnetic field?
II.
Application experiment
In this experiment, you will use the magnetic field produced by a circular loop of current to
determine the magnitude of the horizontal component of the Earth’s magnetic field. At a point
along the axis and a distance x from the plane of the current loop, the magnetic field pointing
along the axis of the loop is
 0 NIR 2
B
(1)
2( R 2  x 2 )3/ 2
where I is the current in the loop of radius R and N is the number of turns in the loop. At the
center of the loop (x = 0), this reduces to the form you are familiar with:
 NI
B 0
(2)
2R
We will measure the magnetic field of the loop by comparing it with the Earth’s magnetic field.
When a compass is placed near the loop, the direction of the compass needle will indicate the
direction of the total magnetic field (the vector sum of the field of the loop and the field of the
Earth). We first align the loop so that the Earth’s magnetic field is perpendicular to the axis of
the loop as shown in Figure 1. Figure 1 shows a top view looking down on the loop from above.
Only the horizontal component of the Earth’s field is observed.
Bloop
Loop
Btotal
θ
BEarth
Figure 1
Experimental tips: Connect the circuit needed to send current into the coils, but do not connect
the power terminals until your instructor has checked your circuit. CAUTION: To keep the
loop from overheating, place the switch in the neutral (center) position when you are not making
a measurement. Place the compass at the center of the loop when there is no current flowing.
Turn the loop until its axis is perpendicular to the direction of the Earth’s magnetic field, as
shown in Figure 1. Use this as your initial reference position.
Bullets marked, for example, A3 refer to the application experiment rubric ability 3. Bullets
without such designation are given as additional guidance. All bullets must be addressed in your
lab write-up, and a random subset will be graded.
Include the following in your report:
a. (A1) Identify the problem to be solved.
b. (A2) Design a reliable experiment that solve the problem.
c. Draw a sketch of your experimental design.
d. Draw an appropriate physical representation for the experiment.
e. (A7) Choose a productive mathematical procedure for solving the problem. Use the physical
representation to devise the mathematical procedure to solve the problem.
f. (A3) Discuss how you will use the available experiment to make the measurements.
g. (A8) Identify the assumptions made in using the mathematical procedure.
h. (A9) Determine specifically the way in which assumptions might affect the results.
i. What are the possible sources of experimental uncertainty? How could you minimize them?
Perform the experiments.
j. Record the outcome of your experiment.
k. Vary the current in the loop and observe the variation in the angle of the compass needle.
Record several currents and the respective compass angles. Try reversing the direction of the
current as well, and taking more data.
l. Use the equations given for the magnetic field of the loop and the data taken to calculate the
average value of the Earth’s magnetic field.
m. Compare this to the known value of the earth’s magnetic field, do your values agree within
the range of experimental uncertainty? If not, explain any discrepancies.
n. (A4) Make a judgment about the results of your experiment.
III.
Observation experiment
In this experiment we will use a compass to map the magnetic field of a small magnet. Place the
magnet so that its axis is horizontal. Place the compass about 10 cm (measured from the center
of the compass to the center of the magnet) from the N pole of the magnet along the axis of the
disk. Be sure the axis of the magnet is in the same horizontal plane as the compass. Assuming
the compass needle represents the direction of the field of the magnet, draw a vector on Figure 2
to indicate the direction of the field at that location. Keeping the distance between the center of
the magnet and the center of the compass the same, make similar observations of the direction of
the field at intervals of 30o around the circle. At each location draw a vector that indicates the
direction of the field.
N
Figure 2
(top view looking
down on magnet)
S
10 cm
a. Does the Earth’s field have a significant effect on your observations? Explain your
answer.
b. Suppose the large circle in the above figure represents the surface of the Earth. Mark the
approximate location of Corvallis. Assuming that the field directions you have drawn
can be taken to represent the direction of the Earth’s magnetic field at its surface,
estimate the angle of the magnetic field direction above or below the horizontal at
Corvallis.
IV.
Observation experiment
The magnetic field of the magnet can be approximated as that of a magnetic dipole. One
expectation for a dipole field is that it falls off along the axis of the magnet like 1/r3. Another
expectation based on a dipole field is that the field at a distance r along a line through the center
of the magnet perpendicular to its axis is half as large as the field along the axis at the same
distance r from the center.
Devise experiments to test these two predictions, and explain your experiment in your lab write
up. Use the observation experiment rubric sheet to guide what you include in the write-up. Do
your results agree with the predictions? Justify your conclusions.
Physics 213 Lab 8: Induction
These experiments purposefully give less instruction. You will need to decide what information
you need to include in your report to show a clear scientific process and justified conclusions.
Make use of the observation, testing, and application rubrics to guide your write-up.
I.
Testing experiments
Equipment: Magnets, galvanometer, coils of wire, connecting wires, batteries, a resistive light
bulb, and other miscellaneous objects.
a. Devise a method to test the following statement:
The only way to induce a current in a coil is to move a magnet near the coil. (Think of what
you should try to do to test a statement that says “The only way …”).
Write a brief outline of your design. Make a prediction, perform your experiment, record the
outcome and make a judgment about the statement.
b. Your friend thinks that the only way to induce a smaller current in the same coil is by moving
the magnet slower. Devise an experiment to test your friend’s statement. (Again, think of
what you need to do to test a statement that says “… will always induce …”). Write a brief
outline of your design. Make a prediction, perform your experiment, record the outcome and
make a judgment about the statement.
II.
Observation experiment
Your task in this part is to determine the relationship between the direction in which the
galvanometer needle deflects, and the direction in which the magnet is moved.
First, you will have to determine the direction in which the galvanometer needle deflects for
a certain direction of current. Construct a circuit with the galvanometer, a battery and a light
bulb acting as a resistor (no magnet). Draw the circuit diagram. Record the direction of
current flow in the coil, and the direction in which the galvanometer needle deflects.
Next, use Lenz’s law to predict what should be the direction of current in the coil when the
magnet is moved towards the coil with the north pole facing the coil. Then use your results
from the previous experiment to predict the direction in which the galvanometer needle
should deflect. Explain how you reached this conclusion. Draw pictures that show the
orientation of the magnet with respect to the coil, the direction of motion of the magnet and
the direction of magnetic field. Perform the experiment, record the outcome and revise your
reasoning if necessary.
Hint: Think of how the flux through the coil changes when the magnet moves, what should
be the direction of the induced magnetic field according to Lenz’s law, and what should be
the direction of the induced current which will lead to that induced field (use right hand rule).
Come up another way to move the magnet so that the galvanometer needle deflects in the
same direction. Explain your procedure. Perform the experiment. Record the orientation of
the magnet, the way you moved the magnet and the direction in which the needle deflects
(use pictures or words to record these).
III.
Invent your own investigation
a) Write a question that you want to investigate using the equipment you had in this lab.
You can use additional equipment that is commonly available in the lab.
b) Write a brief summary of the experiment you would like to perform to answer the
question. Check with your TA if you need additional equipment, and try to perform the
experiment. What is the outcome?
IV.
Informal Observation experiments
Play with the other objects at your table and explain briefly to your TA either how the device (or
effect) works (or can be explained).
EXPERIMENTAL UNCERTAINTIES
No physical quantity can be measured exactly. One can only know its value with a certain range of
uncertainty. This fact can be expressed in the standard form X ± ΔX. This expresses the experimenter's
judgment that the "true" value of X lies between X - ΔX and X + ΔX.
Uncertainties of measurements
1. Instrumental uncertainties
Every measuring instrument has an inherent uncertainty that is determined by the precision of the instrument.
Usually this value is taken as a half of the smallest increment of the instrument scale. That is 0.5 millimeter is
precision of a ruler, 0.5 sec is precision of a watches etc.
Instrumental uncertainties are the easiest one to estimate, but
unfortunately they are not the only source of the uncertainty in your
measured value.
You must be a very skillful and lucky experimentalist to get rid off all
other sources and to have the measurement uncertainty equal to the
instrumental one.
2. Random uncertainties
Sometimes when you measure the same quantity you can get different results each time you measure it. That
happens because different incontrollable factors affect your results randomly. This type of uncertainty,
random uncertainty, can be estimated only by repeating the same measurement several times. For example if
you measure the distance at which cannonball hits the ground, you could get different distances every time
you repeat the same experiment. For example, say you took three measurements and obtained 50 m, 51 m, and
49 meters. To estimate the absolute value of the random uncertainty you first find the average of your
measurements:
X = (50m + 51m + 49m) / 3 = 50 m.
You then estimate approximately how much the values are spread with respect to this average – in this case
we have a spread of about ΔX = 1 m. That is, our measurement of the distance was
X = 50 m ± 1 m.
2ΔX
Most cannonballs will get into the
range from X-ΔX to X+ΔX.
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X
Thus multiple trials allow you to find the average value and to estimate the uncertainty range.
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3. Effect of assumptions
Assumptions inherent in your model may also contribute to the uncertainty of the desired quantity. For
example, you wish to find the speed of the ball moving on the floor. You are assuming that a ball moves along
a straight line while in fact the surface of the floor in bumpy and the bumps contribute significantly to the
distance that the ball covers and thus decrease the speed that that you calculate. Repeating the measurement
will not let you get rid off the bumpiness of the floor and will contribute to the uncertainty with which you
will determine the value of the speed. This type of uncertainties is not so easy to recognize and to evaluate.
First of all, you have to determine the sign of the effect, i.e. whether the assumption increases the value of the
quantity, decreases it or affects it randomly. Then try to estimate the size of the effect.
For example, you measure the diameter of the baseball
assuming it is perfect sphere. However, the real size of the
ball may differ by 1÷2mm if you measure in different
dimensions. This difference will determine the uncertainty
of your measurement.
It is difficult to give strict rules and instructions on how to estimate uncertainties in general. Each case is
unique and requires thoughtful approach. Be ingenious and reasonable.
Comparison uncertainties
If you are comparing the uncertainties in the values of two quantities then by analyzing the absolute
uncertainty ranges you cannot tell which of the measurements is more accurate because the units of the
measured quantities are different. How can we decide then which quantity has a larger uncertainty? Here we
need to compare relative uncertainties, the ratio of the absolute uncertainty and the quantity itself ΔX / X.
This may be expressed as a fraction or as a percentage by multiplying the ratio by 100%.
The sizes of the circles at the picture are determined
with different accuracy.
The absolute size of the soft edge (9 units) is the
same for both blue circles. However the large blue
circle (90 units) looks sharper than the small one
(30units). That happens because we compare
relative uncertainties, which are 10% for the large
circle and 30% for the small one.
Note: Common sense and good judgment must be used in representing the uncertainties when stating a result. Consider
a temperature measurement with a thermometer known to be reliable to ± 0.5 degree Celsius. Would it make sense to
say that this causes a 0.5% uncertainty in measuring the boiling point of water (100 degrees) but a whopping 10% in
the measurement of cold water at a temperature of 5 degrees? Of course not! (And what if the temperatures were
expressed in degrees Kelvin? That would seem to reduce the relative uncertainty to insignificance!). However in most
calorimetry tasks value of interest is not temperature itself but only the change of the temperature or the temperature
difference.
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Reducing uncertainties
The example with the circles shows a way to reduce the relative uncertainty in your measurement. The same
absolute uncertainty yields the smaller relative uncertainty if the measured value is larger.
Suppose you have a bob attached to a spring and want to measure time for it to oscillate up and down, back to
its starting position. If you are using a watch to measure some time interval, the absolute uncertainty of the
measurement will be 0.5 s. If you now measure the time needed for the bob to go up and down one, you get 5
s. This means that you have a relative uncertainty of 10% in the time measurement. What if you measured the
time for 5 oscillations instead? Say you measure the time for 5 oscillations and get 25 s. The instrumental
uncertainty is still 0.5 s! The relative uncertainty in your measurement of the time interval is now: time
relative uncertainty = (0.5 s/25 s) *100% = 2%
By measuring the time for a longer time period, you have managed to reduce the uncertainty in your time
measurement by a factor of 5!
Of course you should not forget about the obvious way of reducing relative uncertainties by minimizing
absolute uncertainty with better design, decreasing effect of assumptions or increasing the accuracy of
instrument if it is possible.
Uncertainty in calculated value
It is important to estimate data uncertainties because uncertainties propagate through the calculations to
produce uncertainty in results.
Consider now the following example. Suppose you know the average mass of one apple m with the
uncertainty Δm. If you want now to calculate the mass of the basket of the 100 apples you will get the value M
± ΔM = 100m ± 100Δm. It means that relative uncertainty of calculated value M remains the same as the
relative uncertainty of the single measured parameter m
ΔM / M = Δm / m.
If you have more than one measured parameter, estimating uncertainty of the result may be more complicated.
However, if one of your sources of uncertainty is much larger than the others (comparing relative
uncertainties!) then you can neglect other sources and use the weakest link rule.
Weakest link rule
The percent uncertainty in the calculated value of some quantity is at least as great as the greatest percentage
uncertainty of the values used to make calculation.
Thus to estimate uncertainty in you calculated value you have to:
1. Estimate the absolute uncertainty in each measured quantity used to find calculated quantity.
2. Calculate the relative percentage uncertainty in each measured quantity.
3. Pick out the largest percentage uncertainty. This will be the percent uncertainty in your calculated
quantity.
Comparable uncertainties
In case you have measured values with comparable uncertainties the rules are more complicated: they depend
on the type of the mathematical relationship you use for calculation. In the most frequent case of multiplying
two values their relative uncertainties add. One of the consequences of this rule is that the raising to the
second power doubles the relative uncertainty and the raising to the third power triples it. Thus, in the
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example above, the relative uncertainty in the calculated volume of the baseball will be three times larger then
the relative uncertainty in the measured radius.
Why do you need to know uncertainty?
Is the measured value in agreement with the prediction? Do the data fit the physical model? Are two measured
values the same? You cannot answer these questions without considering the uncertainties of your
measurements. Indeed, how can you compare two values if the difference between them is smaller than the
uncertainty in their measurement?
Which bunch of grass is higher? You cannot tell
this because their heights are determined with the
uncertainty larger than the height difference. If you
cannot tell which of two values are larger you can
claim them the same.
Thus to make judgment about two values X and Y you have to find the ranges where these values lie. If the
ranges X ± ΔX and Y ± ΔY overlap you can claim the values X and Y the same within your experimental
uncertainty.
Summary
When you are doing a lab and measuring some quantities to determine an unknown quantity:
• Decide which factors affect your result most.
• Wherever possible try to reduce effects of these factors.
• Wherever possible, try to reduce uncertainties by measuring longer distances or times etc.
• Decide what the absolute uncertainties of each measurement are.
• Then find the relative uncertainties of each measurement.
• If one uncertainty is much larger than the others you can ignore all other sources and use this
uncertainty to write the value of the uncertainty of the quantity that you are calculating.
• Find the range where your result lies. Make a judgment about your results taking into the account the
uncertainty.
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