Download Phys241ManualUnit1

Unit 1: Students Absolutely Must Learn…
Weekly Activity 1: Elementary Circuits








The relation between voltage and electrical potential energy.
To make a circuit based on a circuit diagram (practice!).
To think of current as marbles or water flowing in a garden hose.
How multiple parallel paths affect equivalent resistance.
How power is related to both current and voltage.
To use a DMM to measure current, voltage and resistance (all have a different
procedure!).
How to compare the properties of different circuits.
How to design and conduct an experiment to address an open-ended question.
Weekly Activity 2: Magnetism and Electrostatics








How the different materials behave with charge: conductors vs. insulators vs. insulators
with polar molecules.
How to draw magnetic field lines.
How to measure the direction of a magnetic field using a compass.
How like charges repel and unlike charges attract.
How positive charges lower their electrical potential energy by moving to lower
voltages.
How negative charges lower their electrical potential energy by moving to higher
voltages.
How the process of charging by induction works.
How to design and conduct an experiment to address an open-ended question.
1
Unit 1 Grading Guidelines
Staple the lab report, then graphs, and finally worksheets together. Please put
your worksheets in order. Turn in your work to your TA at the beginning of the
next lab meeting following the completion of the unit.
Unit Lab Report [50%, graded out of 25 points]
Write a separate section using the section titles below (be sure to label these sections in your
report). In order to save time, you may add diagrams and equations by hand to your final
printout. However, images, text or equations plagiarized from the internet are not allowed!
Remember to write your report alone as collaborating with a lab partner may make you both
guilty of plagiarism. Pay close attention to your teacher for any changes to these guidelines.

Title [0 points] – A catchy title worth zero points so make it fun.

Concepts & Equations [9 points] – {One small paragraph for each important concept, as
many paragraphs as it takes, 2+ pages.} Go over the lab activities and make a list of all
the different concepts and equations that were covered. Then simply one at a time
write a short paragraph explaining them. You must write using sentences & paragraphs;
bulleted lists are unacceptable.
Some example concepts for this unit report include (but are not limited to):
 Battery voltage
 How multiple batteries add or subtract (or neither).
 Current at a point in a component.
 Behavior of currents.
 Resistance of a component and equivalent resistance of a circuit.
 Power consumed by a resistor vs. total power supplied by the voltage source.
 "In-parallel" components and "in-series" components.
 How to use the digital multimeter (DMM) to measure constant DC voltage,
constant current, and resistance.
 Magnetism, magnetic fields, magnetic field lines, magnetic forces (and how the
magnetic force equation works), right-hand-wrap rule, use of compass.
 Compare and contrast the electrostatics of conductors and insulators (and
insulating material with polar molecules).
 Attractive force caused by charge separation. Discuss examples like the pith ball,
electroscope, and picking up tiny pieces of polar insulating material.
 The process of charging a conductor by induction.
 How the Faraday ice pail works.
 Any equations that were used in the activities will need explained.
 Any other specific TA requests:
 ____________________________________________________________
2
 ____________________________________________________________
 ____________________________________________________________
 ____________________________________________________________
 ____________________________________________________________
 ____________________________________________________________

Selected In-Class Section [6 points]: {3-5 paragraphs, ~1 page}
This week's selection is: Weekly Activity 2, In-Lab Section 3
Write a "mini-report" for this section of the lab manual. Describe what you did
succinctly, and then what you found accurately. Then explain what the result means
and how it relates to some of the concepts in the previous section. You must write using
sentences & paragraphs; bulleted lists are unacceptable.
o Procedure: Do not provide a lot of specific details, but rather you should
summarize the procedure so that a student who took the course a few years ago
would understand what you did.
o Results: Do not bother to rewrite tables of data, but rather refer to the page
number on which it is found. State any measured values, slopes of ilnes-of-bestfit, etc. Do not interpret your results, save any interpretation for the discussion.
o Discussion: Analyze and interpret the results you observed/measured in terms of
some of the concepts and equations of this unit. It is all right to sound repetitive
with other parts of the report.

Open-Ended / Creative Design [6 points] – {3-5 paragraph, ~1 page} Choose one of the
open-ended experiments from the two weekly activities to write about. Describe your
experimental goal and the question you were trying to answer. Explain the ideas you
came up with and what you tried. If your attempts were successful, explain your results.
If your attempts resulted in failure, explain what went wrong and what you would do
differently in the future. You must write using sentences & paragraphs; bulleted lists are
unacceptable.

Graphs [4 points] - {attach to typed report} Graphs must be neatly hand-drawn during
lab and placed directly after your typed discussion (before your quizzes and selected
worksheets). Your graphs must fill the entire page (requires planning ahead) and must
include: a descriptive title, labeled axes, numeric tic marks on the axes, unit labels on the
axes, and if the graph is linear, the line of best fit written directly onto the graph.
3
Thoroughly Completed Activity Worksheets [30%, graded out of 15 points]

Week 1 In-Class [7 points]: Pages assigned to turn in:
_TA signature page, Post-lab pages, ____________________________________
___________________________________________________________________

Week 2 In-Class [8 points]: Pages assigned to turn in:
_TA signature page, Post-lab pages, ____________________________________
___________________________________________________________________
The above lab report and worksheets account for 80% of your unit grade. The
other 20% comes from your weekly quizzes, each worth 10%. These will be
entered into D2L separately.
4
Weekly Activity 1: Elementary Circuits
Pre-Lab
!
You must complete this pre-lab section before you attend your lab to prepare
for a short quiz. Be sure to complete all pages of the pre-lab.
Continue until you see the stop pre-lab picture:
Subsection 0-A
Circuit diagrams are simple ways to represent actual physical circuits. An example is provided
showing three unique ways to represent a certain battery-light bulb circuit. (Technically, the
last diagram shown is called the 'circuit diagram'.) Note that the positive terminal of the
battery is represented by a wider line than the negative terminal, and the resistive circuit
component (the bulb) is represented by zig-zag lines.
Note that most physicists are sloppy and write V for potential difference when they really mean
V, the drop in voltage from one side of a circuit component to the other.
¿
0-A-1
Sketch a circuit diagram below for the following physical system.
5
Subsection 0-B
The SI unit of charge is the coulomb [C]. This is a large charge unit because a charge of -1.0 [C]
represents the total charge of 1.6x1019 individual electrons added together. The other common
SI quantities you need to know are: second [s], meter [m], kilogram [kg], and newton [N].
Circuit concepts are often described using an analogy to plumbing. This is because in (good)
plumbing, the water must stay in the pipes. In circuits, the electrons have to stay in the wires.
It is not a perfect analogy so be sure to discuss any misconceptions you may develop with other
students and your TA. Another analogy used in teaching is to think of the wires of a circuit as
garden hose and the electrons flowing in the wires as marbles flowing through the garden hose.
The following picture is meant to illustrate the water-in-pipes analogy:
In the above picture, you can see that the battery's purpose is to impart energy to the
electrons. The wires provide a path for the electrons to flow through. The light bulb absorbs
the energy of the electrons as they pass through it. Another wire returns the electrons to the
battery. Meanwhile, you can see that the pump gives energy to the water causing it to flow
through the pipes. The paddle wheel will absorb the water's energy. The water will then return
to the pump through more pipes.
Current is the rate of flow of charge at a point in a circuit. The SI unit for current is the ampere
[amp] or [A] which is equivalent to coulombs per second. Current in a circuit is often
compared to how fast water may be flowing through the pipes of a plumbing system. However,
you should keep in mind that a few charges moving quickly may have the same current as many
charges moving slowly. Note that it is technically incorrect to discuss "the current of a circuit".
Instead we should discuss "the current at a specific point of the circuit".
6
If a current carrying wire in a circuit branches into two wires, then the current of the first wire
will be divided between the two paths. For any point of a circuit, the amount of current flowing
into that point must equal the amount flowing out of the point. There can never be any
imbalance of incoming and outgoing current for if there were, then charges would pile up at
that point of the circuit and since like charges repel each other (strongly) that point of the
circuit would become unstable and explode! Alternatively, if more positive charge was leaving
a point than was entering, there would be an excess of negative charge and this too would be
unstable. This is analogous to a river branching into two separate rivers. The water from the
incoming river must follow one of the two branches. Water cannot pile up at the point where
the river divides into two rivers. The amount of water "in" equals the amount of water "out"!
Since the amount of charge flowing through a point in a circuit must be in balance so that
neither positive nor negative charge can accumulate indicates that for current to flow, there
must always exist a closed circuit loop. If you connected the positive terminal of a battery to
the ground, you might think that positive charge would be pushed into the ground by the
battery. But if this happened, the battery itself would be losing positive charge and therefore
accumulate a negative charge. This would be an unstable configuration, or alternatively, the
positive charge can’t flow from the positive terminal because it is attracted to the negative
charge that would remain in the battery. However, if you let positive charge flow back into a
battery by attaching the negative terminal to the ground, then current could flow since there
would not be the issue of charge accumulation. The following picture illustrates this point:
Current is also a vector meaning that it has a magnitude and direction. The direction of a
current describes how positive charges would flow: from high voltages to low voltages.
Unfortunately, Benjamin Franklin's experiments with electricity caused electrons to be
described as negatively charged. Yet these are the particles that move in a circuit to carry
current. Therefore, when we describe current as flowing in a certain direction of a circuit,
electrons are actually flowing in the opposite direction!
7
¿
0-B-1
A circuit is made from a constant voltage supply (source) and two equal
resistors. At the positive terminal of a battery (point 'a'), 4 [A] of current flow
upwards away from the battery. Find the current (magnitude and direction) at
the other points, b-h. (Use below figure.)
A voltage difference (also called electric potential) and describes the difference in electrical
potential energy per unit charge between two points of a circuit. The SI unit of voltage is [volts]
or [V], but can also be written as joules per coulomb. Unlike current which describes a
physical quantity at a point of a circuit, voltage describes the change in electrical potential
energy of a unit charge if the unit charge were to move from one point of the circuit to another.
For example, if you measure the voltage difference between the positive terminal and negative
terminal of a AA-cell battery, you will find it to be approximately 1.5 volts. A constant voltage
gives rise to a constant current flow also called 'direct current' or 'DC'.
!
Take care as many people and books write V instead of V to save time.
A D-cell battery and a AAA-cell battery both provide 1.5 volts of electric potential. That means
that for a given circuit (like in a flashlight), each gives the same energy to each electron as it
leaves the battery so that both produce exactly the same circuit behavior (flashlight brightness).
¿
0-B-2
Explain what the advantage of the D-cell battery would be versus the AAA-cell
battery if they both generate the same circuit behavior.
8
If a positive charge flows from a high electric potential to low, electrical potential energy is
released as work. This could mean lighting a bulb, turning a motor or even heating a wire. An
example of a circuit component that absorbs electrical potential energy from the flowing
charged particles is the resistor. Resistance is usually defined as a property of a particular
circuit object, and has SI units of [ohms] or []. For instance, one usually talks about the
resistance of a light bulb, a wire, the internal resistance of a battery, or even the total
resistance of the circuit.
The resistance indicates how difficult it will be for current to flow from high electric potential to
low electric potential. For example, if 3 volts are applied to a circuit with little resistance, then
more current will flow than if 3 volts were applied to a circuit with large resistance.
When resistors are placed in series in a circuit, their combined resistance (also called
'equivalent resistance') is the sum of the individual resistances. Thus, if two identical resistors
are placed in series, less current will flow from the battery than if a single resistor were used.
Specifically, since using two identical resistors in series doubles the resistance, only half as
much current will flow.
In the figure below, if the applied voltage was 12 [volts] and each resistor's resistance was 2
[ohm], then the circuit on the left would have 6 [amps] of current (at all points of the circuit),
and the circuit on the right would only have 3 [amps] of current (at any point of the circuit).
¿
0-B-3
Since adding an additional equal resistor in series doubles the total resistance of
the circuit and therefore halves the current, what happens when you add a third
equal resistor in series?
9
When resistors are placed in parallel in a circuit, their equivalent resistance is smaller than
either of the individual resistances. This is due to providing an extra path for current to flow (a
formal proof will be discussed later).
In the above figure, if the applied voltage was 12 [volts] and each resistor's resistance was 6
[ohm], then the circuit on the left would have 2 [amps] of current (at all points of the circuit),
and the circuit on the right would only have 4 [amps] of current flowing out of the positive
battery terminal, but only 2 [amps] of current flowing through either resistor.
¿
0-B-4
Since adding an additional equal resistor in parallel halves the total resistance of
the circuit and therefore doubles the total current, what happens when you add
a third equal resistor in parallel?
Copper wire offers very little resistance to current while nichrome wire and graphite have
larger resistances than copper. Air can be treated as having infinite resistance so that a battery
cannot discharge itself when not being used in a circuit. Technically, if the voltage difference is
high enough, air molecules will ionize (separate into charged particles/molecules called 'ions')
and that ionized air can be very conductive (think of lightning)).
10
¿
0-B-5
Why doesn't a battery discharge through your hand if you hold both terminals
simultaneously with your hand?
The power supplied by the battery is equal to the current emanating from the battery times
the voltage of the battery, Psupplied  I battery Vbattery . The power dissipated by a resistive circuit
component is found by multiplying the voltage drop across the component and the current
flowing through the component, Pdissipated  I component Vcomponent . The SI unit of power is [watts] or
[W]. If you have ever imbibed the soft drink Mr. Pibb then you can remember the power
formula by thinking of Mr. PIV.
¿
0-B-6
In the following circuit with I=0.5 [A], find the power supplied by the battery
and the power dissipated by the resistor. Be sure that your answer makes sense
when considering the principle of energy conservation.
11
Subsection 0-C
A digital multimeter (DMM) is used to measure constant voltage differences between two
points of a circuit. A lead is a wire that plugs into a DMM with a rigid end for touching circuitry.
Often any wire with banana plug ends will work in lieu of expensive leads. Sometimes alligator
clips can be placed onto the banana plugs to better grip a circuit component.
A DMM may measure the voltage difference of a constant power source. For example, simply
place the two DMM leads on the terminals of a AA-cell battery to measure 1.5 [volts]. A DMM
may also measure the drop in electric potential (voltage) across a circuit component by
inserting the two leads on either side of the component while current is flowing. The figure
below demonstrates how to measure the voltage drop across a light bulb as well as the
equivalent circuit diagram. Don't worry that current will be diverted through the DMM; the
DMM has a huge resistance when measuring voltage.
Note that most DMMs have three input terminals (holes) even though you only use two at any
given time. Often, one positive terminal is for measuring large currents (depicted by the
leftmost hole in this picture). You must read the fine print around the terminals to determine
which one is only for measuring large currents.
12
¿
0-C-1
Why is making a voltage measurement with a DMM considered a parallel type
of measurement? {Hint: examine the circuit diagram in the above picture.}
A DMM may also measure constant current through a circuit component (see the following
figure). However, to measure the flow of charge, the river of electrons must actually be
diverted through the DMM. This is done by inserting a single lead into the DMM. Then you
must break the circuit and wire the DMM into the circuit (see the following figure). Don't worry
that current will be diminished by the DMM; the DMM measuring current has a very small
resistance.
Note that if you expected to measure a large current, you would want to use the high-current
terminal of the DMM to protect the circuitry of the DMM. Current tolerances are usually
written directly on the DMM. If you don't know what to expect, start with the large current
terminal and then switch to the other positive terminal if using the large-current terminal
indicates that the current is small enough.
13
¿
0-C-2
Why is making a current measurement with a DMM considered a series type of
measurement? {Hint: examine the circuit diagram in the above picture.}
A DMM can also measure the resistance of a component once it has been disconnected from
the circuit.
!
You can't actually measure the resistance of a light bulb this way. A light bulb
gets hot when in use and so its resistance increases.
14
In-Lab Section 1: practice with voltage concepts
The circuit diagram of a battery powering a light bulb is shown above. Introductory students
sometimes confuse the new concepts of voltage, current, resistance, etc. Where voltage is
concerned, a good way to think about what is happening is to think of voltage as a kind of
height.
The battery "lifts" the voltage to a "height" of 1.5 [V] at the positive terminal of the battery.
Then as the current flows through the light bulb, the voltage "falls" back to a "height" of 0 [V].
One could equivalently say that the electrical potential energy of the charge carriers is
increased by the battery and then the charge carriers lose that potential energy as they flow
through the light bulb.
Note also that the wires are drawn nearly parallel to the x-axis. This is because wires are highly
conductive (very low resistance) and so there is no appreciable drop in electrical potential along
a wire.
15
The following pictures attempt to make this concept very clear by providing different ways to
visualize the previous situation at various points of the circuit:
The circuit with labeled points:
The circuit with labeled points:
A way to visualize the voltages at the points of the circuit (above).
16
Another way to visualize the voltages at the points of the circuit (below).
Examine the labeled points on the pictures above. Both pictures represent that same circuit
labeled at the same points. Moving from point a to point b, the voltage increases by 1.5 [V],
Va to b  1.5 [V] . Points b and c are at the same voltage "height". Therefore,
Vb to c  0 [V] . As the current travels through the light bulb, the voltage decreases from 1.5
[V] to 0 [V] so that Vc to d  1.5 [V] . Points d and e are at the same voltage height so there
is no voltage change between them. If you put the positive lead of the DMM at point b and the
negative lead at point d, the DMM display would show a voltage of
Vd to b  1.5 [V] .
¿
1-1
For the previous circuit, if you put the positive lead of the DMM at point e and
the negative lead at point c, what value would the DMM display? {Hint: what is
Vc to e ?} Introductory students are often confused by negative DMM readings.
The negative lead is the starting point and the positive is the final, the DMM
tells the voltage going from negative lead to positive.
17
If you were to simply hook a wire to both sides of a battery without a light bulb, then you would
see that the entire voltage drop must be through this bare wire. Due to a wire’s low resistance,
a very large current would flow from the battery and through the wire and things would really
heat up!
Rwire~0 [] and this causes an giant current to flow, Igiant. The giant current through the wire
still “falls” by -1.5 [V] so that the power going into the wire is also giant (using P=IV):
Pwire=1.5 Igiant.
!
Always use some resistance in your circuit when measuring amps or you will
blow the fuse in your DMM.
18
Now imagine adding another identical light bulb in series with the first as shown in the
following picture. Charge flowing through this circuit will lose half of its electrical potential
energy traversing the first light bulb and the other half of its electrical potential energy
traversing the second light bulb.
The only three unique voltage points of the circuit were chosen. As the current travels from the
voltage source at a through the first bulb to b, the voltage drops 0.75 [V]. When the current
passes the second bulb from b to c, the voltage drops again by 0.75 [V].
19
¿
1-2
Explain what the voltage drop across each of the light bulbs in series must be
and why.
¿
1-3
Predict how the total current of the circuit (the current flowing through the
battery) would be affected by adding this extra bulb in series?
20
¿
1-4
If 0.10 [amps] were flowing from the battery, determine the power supplied by
the battery and the power dissipated by each light bulb. Be sure your answer
makes sense using the principle of energy conservation.
Now imagine adding the extra identical light bulb in parallel with the first (shown in the
following figure). Charge flowing through this circuit will lose all of its electrical potential
energy if it follows the path to the left or all of its electrical potential energy if it follows the
path to the right. The moving charge cannot flow through both bulbs simultaneously so half
the current must go one way and half the other.
21
¿
1-5
Explain what the voltage drop across each of the light bulbs in parallel would
be.
¿
1-6
Predict how the total resistance of this circuit would compare to that of the
previous single light bulb circuit. {Hint: because the charge carriers have two
paths to choose from, there will be less resistance than if a single path were
available.}
22
¿
1-7
Predict how the total current of the circuit (the current flowing through the
battery) would be affected by adding this extra bulb in parallel? What would
the current be as a multiple of the single light bulb circuit? (Using the previous
figure.)
¿
1-8
Use an energy-circuit diagram to investigate the power dissipated for a circuit
with two 1.5 [volt] batteries placed in series and discharged through a single
bulb where the current observed is 0.10 [amps].
¿
1-9
Explain which would last longer and which would be brighter: powering a single
light bulb with two batteries in series or powering a single light bulb with two
batteries in parallel.
23
In-Lab Section 2: circuit basics
Remember that you measure voltage by using two DMM leads and placing the DMM in parallel
with the component you are measuring (see following figure).
!
Be sure your DMM is set to measure voltage so that the internal circuit of the
DMM provides an enormous resistance. Otherwise you may blow a DMM fuse
(or worse).
!
Too much electrical energy in the charge carriers can melt the tungsten
filament of the bulb:
¿
2-1
Measure the voltages of the following batteries (if available): D-cell, AA-cell,
AAA-cell, C-cell, 9-volt, 6-volt. Measure the voltage of two 1.5 [volt] batteries in
series. Measure the voltage of two 1.5 [volt] batteries in parallel. Make a short
table of your results:
24
¿
2-2
If you measure the voltage of a battery to be +1.5 [V], then switching the DMM
leads will cause you to measure -1.5 [V]. Explain how this fact could help you
determine the direction of the current through a circuit component.
Remember that you measure current by using one DMM lead and placing the DMM in series
with the component you are measuring (see following figure).
!
When you are not sure how large the current may be, always use the DMM on the large
current setting, then switch to the small current setting if appropriate.
¿
2-3
Measure the current through a single small incandescent light bulb powered by
a 1.5 [V] battery. Check your result with another group's to be sure that your
DMM measures current correctly. Write your measured current in SI units.
Note that many times in lab you may only need t describe the magnitude of a
current while on a lecture exam you usually need to describe its direction as
well.
25
Remember that you measure a component's resistance when it is disconnected (see following
figure).
¿
2-4
Measure the resistance of a single, cold unpowered light bulb. This is not a
useful observation since a light bulb is non-ohmic: it's resistance changes when
used in a circuit (its resistance grows with increasing temperature). Write your
measured resistance in SI units.
¿
2-5
Measure the resistance of your finger (from tip to base) and write the result in
SI units. Compare your measurements to those of other students. Explain what
might account for such a wide range of varying finger resistances?
26
One way of thinking about a circuit with constant current is to use a sort of voltage "height"
diagram. The y-axis represents voltage (potential energy per unit positive charge).
¿
2-6
Experimentally verify that the voltage supplied by the battery is equal and
opposite to the voltage drop across the light bulb. What fundamental physical
law assures that this happens? (Using the previous figure.)
Now examine two light bulbs in series:
¿
2-7
Experimentally verify that the V supplied by the battery is equal to the sum of
the V's across each light bulb. Then experimentally verify that the current
leaving the battery is the same as the current in the first bulb is the same as the
current in the second bulb. Finally, measure the resistance of two separate
single cold bulbs, and then measure their total resistance when placed in series.
Record your results and check with neighbors. (Using the previous figure.)
27
In-Lab Section 3: circuit behavior
In this section, you will make predictions about different circuits, then build and test them.
Always use SI units.
Subsection 3-A
¿
3-A-1
For the previous figure showing various arrangements of batteries, predict the
voltages that would be measured. Explain your predictions. Be sure to apply
the positive and negative leads of your DMM correctly (as indicated) to obtain
the correct sign of the voltage (compare the sign of the voltage in circuits a and
b). (Write both your predictions and explanations.)
¿
3-A-2
Now set up each of the circuits and test your predictions. What are the actual
voltages of the battery arrangements? If some of your predictions are wrong,
ask around and figure out why, then explain why you were mistaken.
28
Subsection 3-B
¿
3-B-1
For the previous figure, predict the order of brightness of the bulbs from least to
most bright and then explain why you think this will be the case. (Write both
your predictions and explanations.)
!
Always make the circuit first before you approach it with your DMM.
¿
3-B-2
Now set up each of the circuits and test your predictions. What is the actual
order of the bulbs from least to most bright? If some of your predictions are
wrong, ask around and figure out why, then explain why you were mistaken.
29
Subsection 3-C
¿
3-C-1
For the previous figure, predict the order of the magnitude of current through
the light bulb from least to greatest and then explain why you think this will be
the case. (Write both your predictions and explanations.)
¿
3-C-2
Now set up each of the circuits and test your predictions. What is the actual
order of the magnitude of bulb current from least to most? If some of your
predictions are wrong, ask around and figure out why, then explain why you
were mistaken.
30
Subsection 3-D
The following circuit uses two 1.5 [V] batteries in series to power three identical light bulbs.
Marbles are shown to represent the unit charge carriers that produce the current in the circuit.
Thus each marble represents 1 [coul].
For every second of time that passes, 9 marbles flow from the top of the battery so we say that
ITOTAL= 9 [amp]. The marbles are pushed through the circuit by the battery and must push each
other out of the way to proceed through the. The marbles must return to the bottom of the
batteries to replenish the batteries' reservoir.
¿
3-D-1
Calculate the current of marbles for each of the five delineated parts of the wire
(IA, IB, IC, ID, IE).
¿
3-D-2
Explain what is meant when an electrical engineer says that they will lower the
total resistance of a circuit by adding a component in parallel.
¿
3-D-3
Challenge your reasoning by constructing the circuit and testing it. Record your
findings. If you were wrong, ask around to figure out why and then explain why
you were wrong.
31
Subsection 3-E
In the following simple circuit, four locations along the circuit’s wires are labeled.
¿
3-E-1
If the grounding lead of your DMM was placed at location d, predict what the
voltage readings would be on the DMM screen if the positive lead was placed at
each other location in turn. Be sure to include the sign of the electric potential
(voltage difference). Your predictions in SI units:
¿
3-E-2
If the ground of your DMM was placed at location a, predict what the voltage
readings would be on the DMM screen if the positive lead was placed at each
other locations in turn. Be sure to include the sign of the electric potential
(voltage difference). Your predictions in SI units:
32
¿
3-E-3
Now set this circuit up and test your predictions. Record each of your results
and if some of your predictions were wrong, explain the mental misconceptions
you held. Your observations in SI units and any explanations of misconceptions:
¿
3-E-4
What would the magnitude of the current be at each of these locations.
33
Subsection 3-F
Now imagine the circuit from the previous subsection with a section of wire removed.
¿
3-F-1
If the grounding lead of your DMM was placed at location d, predict what the
voltage readings would be on the DMM screen if the positive lead was placed at
each other location in turn. Be sure to include the sign of the electric potential
(voltage difference). Your predictions in SI units:
¿
3-F-2
Now set this circuit up and test your predictions. Record each of your results
and if some of your predictions were wrong, explain the mental misconceptions
you held. Your observations in SI units and any explanations of misconceptions:
34
In-Lab Section 4: comparing circuit behaviors
Subsection 4-A
Below are three light bulb configurations made with identical bulbs. Imagine that each light
bulb carries 1 [ of resistance regardless of its temperature (unrealistic). Answer all the
following questions without making observations. Note: drawing energy-circuit diagrams like
those of the previous section is sometimes helpful.
¿
4-A-1
Calculate the total resistance (equivalent resistance) for each circuit. Your
answers in SI units:
¿
4-A-2
Calculate the voltage across each light bulb. Your answers in SI units:
¿
4-A-3
If circuit A is known to produce 3 [A] of current through the battery, find the
currents through the batteries in circuits B and C. Your answers in SI units:
35
¿
4-A-4
Use your previous answer to find the current through each single light bulb in
the three circuits. Your answers in SI units:
¿
4-A-5
Use your previous answers to find the power dissipated as heat and light by
each light bulb in the three circuits. Your answers in SI units:
¿
4-A-6
Use your previous answer to compare the brightness of each light bulb in the
three circuits. Your answers:
¿
4-A-7
Finally, use your previous answers to calculate the total power output by the
batteries in each circuit. Your answers in SI units:
36
Subsection 4-B
Below are three light bulb configurations where all light bulbs are identical. Imagine that each
light bulb carries 1 [ of resistance regardless of its temperature (unrealistic). Answer all the
following questions while making observations.
¿
4-B-1
Calculate the voltage across each light bulb then set up the circuit and measure
the result. Your answers and observations in SI units:
¿
4-B-2
Calculate the current through each single light bulb in the three circuits then set
up the circuit and measure the result. Your answers and observations in SI
units:
¿
4-B-3
Use your previous answer to compare the brightness of each light bulb in the
three circuits. Your comparisons:
¿
4-B-4
What is the current flowing through the middle wire of circuit C? You answer in
SI units:
37
Subsection 4-C
¿
4-C-1
An electric field always points from high voltages toward low voltages. In the
following circuit diagram, use dashed lines with arrowheads to correctly draw
the direction of the electric field in each wire segment in the circuit.
¿
4-C-2
Electrons are negatively charged so flow in the opposite direction of the defined
current. In the previous circuit diagram, use solid lines with arrowheads to
show the direction electrons would travel in this circuit.
38
In-Lab Section 5: authentic assessment
A popular video shown to education majors has an interviewer approaching students during
graduation at Harvard and MIT. The interviewer provides a light bulb, a wire and a battery.
Very many of the graduates could not make the bulb light! (They were most likely not
engineering majors.) This video is supposed to teach teachers that simple concepts can be
misunderstood despite expensive training.
Not on our watch! You never know where these video makers might come next so we must be
prepared. Use a single wire, a 1.5 [V] battery and a small bulb, and make the bulb light up.
¿
5-1
Show a student in a different group that you can successfully light a bulb with a
wire and a battery. Once you are successful and have them sign below. Note: if
someone is stuck, please give them advice!
"Yes, I have seen this student light a bulb. They are well-prepared for surprise
interviews!"
Student Signature:___________________________________________________
39
In-Lab Section 6: open-ended / creative design
Listed below are several formulae for finding a total resistance for two resistors combined in
parallel. Most of these formulae are wrong. You need to find the correct formula (or formulae)
for the total resistance of two resistors combined in parallel.
A. Rtotal  R1  R2
B. Rtotal  R1  R2


C. Rtotal 
R1  R2
R1  R2
D. Rtotal  R12  R22



E. Rtotal 
F. Rtotal 
1
R1  R2 
2
1
1
1

R1 R2
G. Rtotal  R1  R2  R1  R2

H. Rtotal  e R1 R 2

You are allowed to "cheat" by talking to other groups for ideas, but are not allowed to "cheat"

by just stating an answer you may already know, looking it up online or asking your TA.
Below you are given three prompts:
hypothesizing/planning, observations/data,
calculations/conclusion. Your job is to figure out the answer using these prompts as your
problem-solving model. In the event that you should run out of time, you may not discover the
correct answer, but you should make an attempt at each prompt. Grades are based on honest
effort.
Your open-ended solution should probably include some of the following items: sketches of
circuit diagrams, tables of data, calculations, recorded observations, random ideas, etc.
Write at the prompts on the next page.
40
¿
6-1
hypothesizing/planning:
¿
6-2
observations/data:
¿
6-3
calculations/conclusion
I, the physics 241 laboratory TA, have examined this student's Weekly Activity pages and found
them to be thoroughly completed.
!
TA signature: _______________________________________________________________
41
Post-Lab: elementary circuits
!
You must complete this post-lab section after you attend your lab. You may
work on this post-lab during lab if you have time and have finished all the other
lab sections.
¿
X-1
Use the following figure to answer the questions below:
a. How does the magnitude of the voltage across the battery (Vab) compare to
the magnitude of the voltage across the lightbulb (Vcd)?
b. In lab, we saw that the amount of current coming out of a lightbulb (point d)
is the same as the amount of current coming in (point c). If the bulb isn't "using
up" the current (i.e. electric charges), what causes the bulb to light up?
42
¿
X-2
Use the following figure to answer the questions below. In circuits A and B, the
two light bulbs have identical physical properties.
a. Which circuit provides the brighter bulb?
b. For each circuit in the previous figure, draw the path taken by a single
electron as it makes one complete loop. Draw arrows directly onto the figure.
If the electron must choose between two or more paths, only draw one of the
options.
c. Explain how your answer to part b justifies your answer to part a.
43
¿
X-3
Use the following figure to answer the questions below. In circuits A and B,
each of the light bulbs a, b, c, and d have identical physical properties.
a. Rank the light bulbs in order of increasing brightness indicating any ties.
b. For each circuit in the previous figure, draw the path taken by a single
electron as it makes one complete loop. Draw arrows directly onto the figure.
If the electron must choose between two or more paths, only draw one of the
options.
¿
X-4
Use the following figure to answer the questions below. In circuits A and B,
each of the light bulbs a, b, c, and d have identical physical properties.
Rank the light bulbs in order of increasing brightness indicating any ties.
44
¿
X-5
Use the following figure to answer the questions below. Each of the light bulbs
have identical physical properties.
If point a of the circuit has a current magnitude of 30 [mA], what is the current
magnitude at the other labeled points?
45
(This page intentionally left blank.)
46
Weekly Activity 2: Magnetism and Electrostatics
Pre-Lab
!
You must complete this pre-lab section before you attend your lab to prepare
for a short quiz. Be sure to complete all pages of the pre-lab.
Continue until you see the stop pre-lab picture:
Subsection 0-A
A particle is an object that exists at a particular location in space. Fields are very different, but
no less important in describing the natural world. A field is an object that exists over a whole
region of space and at each point in that region has a numerical value to describe it. For
instance, the scalar field describing the temperature of the room you are in is simply the
(infinite) list of numbers giving the temperature for each position in the room. For each point
of space, a vector field provides a magnitude and a direction. You would need a vector field to
describe the wind because at each point of space in the atmosphere, you would need to give
the speed of the wind and which direction is it blowing. A field used to describe forces is
necessarily a vector field because at each point in space you must tell hard strong the force is
and in which direction it is pushing.
Electric and magnetic fields are vector fields that describe the different kinds of
electromagnetic forces. Electric fields describe forces between stationary charges while
magnetic fields describe forces between moving charges (currents). Technically, electric fields
and magnetic fields can be interchanged due to Einstein's theory of relativity, but you would
need to take a course on modern physics to learn more about that (the sequel to this course at
most universities).
Charged particles create electric fields that can push on other charged particles with an electric
force. The equation Felectric  qE means that a particle with charge q is being pushed by an

electric field E . Note that the electric force on the charged particle q is in the direction of the

applied electric field E . The SI units for electric field are volts per meter [V/m] or equivalently
newtons percoulomb [N/C]. You choose which way to write the units based on how you plan
to use the electric field (i.e. to get voltages use volts per meter or to get forces use newtons per
coulomb).
47
Since the electric field exist in all space, it is easiest to visualize with a sketch of the electric field
lines. Electric field lines that show the direction of the electric force at each point in space. In
the following figure, an electric field is created by the presence of two charged particles. Field
lines are sketched to show the direction of the electric force that would be exerted on a third,
positive test charge if it was introduced to the system. A test charge is a very small charge
whose presence will not significantly alter an electric field.
¿
0-1
(Using the previous figure.) If a positive test charge was placed directly
between the two charges that created the shown electric field, in which
direction would the electric field exert a force on the test charge?
48
Subsection 0-B
Magnetic fields are more complex than electric fields because they are created by moving
charges (currents). That may seem counterintuitive since a macroscopic magnet looks
stationary, but a magnet is often modeled as a collection of magnetic atoms each having a
valence electron circling around creating a tiny magnetic field. Though this description is
incomplete, the basic idea is correct: only moving charges create magnetic fields.
An electric field pushes on a charged particle in a direction parallel to the electric field, but a
magnetic field pushes a moving charged particle in a direction perpendicular to the direction of
the magnetic field. This magnetic force is described mathematically by Fmagnetic  qv  B . The
appearance of the velocity of the charge in the force equation indicates that the force is
proportional to the speed of the charged particle while the use of the vector cross product
indicates that the force is perpendicular to both the direction of the magnetic field and the

direction of the particles motion. The SI unit for magnetic field is the [tesla] or [T].
¿
0-2
If an electron (qe = -1.6x10-19 [C]) is at rest in a magnetic field pointing in the
positive z-direction with magnitude Bz = 1.5 [T], what is the magnitude of the
magnetic force on the particle by the magnetic field.
These are microscopic descriptions of nature, but we will now examine what happens with the
macroscopic magnetism of a bar magnet. From experience we know that a magnet has two
different sides because magnets can attract or repel. We call these kinds of sides North and
South poles. These sides can be determined microscopically by examining the direction of the
current. (The following picture illustrates how to use your right hand to find the poles of the
magnetic field produced by a circulating current.)
49
The magnetic field lines created by moving charges begin at the north pole of a magnet and end
on a south pole whether or not they belong to the same magnet:
Example 1
Example 2
50
Whether drawing magnetic field lines or electric field lines, it is important to note that field
lines may never cross as this would indicate a point in space where the direction of the force on
a particle is undetermined.
Note in the following figure that the same poles of a magnet will experience a repulsive force.
This corresponds to the magnetic field lines “repelling” each other (for north poles the field line
arrows would be reversed). Also, this figure does not show the north poles so we can only
imagine that the magnetic field lines reach around outside the field of view to connect with
some north poles.
¿
0-3
A confused student begins to draw electric field lines in a slightly complicated
arrangement of charges shown in the following figure. Explain why the
student's crossing of electric field lines cause "force direction confusion" by
examining the behavior on a positive test charge placed at the point of
intersection.
51
Pure iron is very magnetic because it contains the most "magnetic atoms". If
iron is initially unmagnetized, it will become magnetized in the presence of a
magnetic field.
Permanent magnets are usually made of steel, which is iron with a small amount of carbon (or
other substance) added to increase the material's hardness. The increased hardness of steel
allows steel to maintain its magnetization even in the presence of strong magnetic fields.
Sometimes classroom magnets and compasses are not hard enough and become magnetized
differently than they are marked. You should always check your magnets with a compass and
check your compass with the Earth's magnetic field to make sure all markings are correct.
Compasses are easily remagnetized with a stack of good magnets and a quick hand. Soft
magnets must be remagnetized in the presence of a strong magnetic field while being tapped
with a hammer to "jiggle" the atoms into place.
52
¿
0-4
Sketch a picture or set of pictures (cartoon) that describes what would happen if
the end of a needle made of soft steel was tapped vigorously against the north
pole of a permanent magnet. You may include words in your sketch for clarity.
53
Subsection 0-C
The right-hand-wrap rule is useful for finding the poles of the magnetic field when you know
the direction of the current. But you cannot see the microscopic currents in a bar magnet so
you must find the poles of the magnet experimentally by using a pole-finding device: a
compass. A compass typically has a marked tip pointing to the geographic north pole of the
Earth. However, the geographic north pole of the Earth is really a south magnetic pole. That
means that the marked tip of the compass is a north magnetic pole because it is attracted to
the Earth’s south magnetic pole (which is the geographic north pole!):
54
¿
0-5
In the following picture, draw what the compass needle would look like when
placed next to the permanent magnet. Remember that the arrow-tip end of the
compass needle is a north magnetic pole.
Sources of magnetism have only been found experimentally to come in north/south pairs. This
means that the magnetic lines of force (field lines) always begin at a north pole and end at the
south pole. We rarely study the strength of the attraction or repulsion between two magnets
(at this course level). Interacting magnetic fields are very complicated to calculate. Usually we
are most interested in the effect a magnetic field produces on nearby moving charges, which is
much easier to calculate.
Subsection 0-D
Electrostatics is the study of stationary charges. That means you try to understand physical
systems where excess charge has been placed on an object, or systems where the net charge is
zero (neutral) but there is some degree of charge separation.
The first kind of system to describe is the conductor, which is a system where charges can move
around freely (usually a metal). If you deposit excess charge on a conductor, the excess charges
will repel each other and spread out uniformly over the surface of the conductor. They will
never collect underneath the surface. This is the reason that you are mostly safe in your car if it
is struck by lightning; the excess charges will flow around the metallic surface of the vehicle.
Nevertheless, you will probably still need to put your clothes into the laundry.
55
If a neutral conductor comes into the presence of an electric field (say from another charged
object), the charges of the conductor will redistribute so that there is macroscopic charge
separation across the entire conductor. Again, the charges that redistribute will lie on the
surface while the inside of the conductor will be neutral.
If you want to remove excess charge (positive or negative) from a conductor, merely touch it to
a ground (metallic plumbing, metallic laboratory gas lines, etc.). The excess charge will be
removed nearly instantaneously.
56
The other kind of material we will study is that of the insulator. Charges cannot move around
on the surface of an insulator. If any excess charge is placed on an insulator, it is stuck at the
location where it was placed. If you rub a balloon on hair or fur you will deposit electrons deep
into the polymer structure of the balloon's molecules and the excess electrons will be trapped
there and difficult to remove.
Of course if your hair deposited electrons into the balloon, then your hair will be left positively
charged, and it will thus be attracted to the negatively charged balloon.
If you wish to discharge an insulator with excess charge, you will either have to rub it on a
ground for a long time, or wet it with an evaporative alcohol then blow it dry with a hair dryer.
57
Some insulators are made of polar molecules that can rotate at their position in the material
when placed in the presence of an electric field. This leads to microscopic charge separation:
An insulating material made of polar molecules can be attracted to a charged object even
though it is neutral (uncharged). That is because its polar molecules can rotate so that the end
with opposite charge (compared to the charged object) is closest to the charged object. The
force attracting the oppositely charged end will be slightly stronger than the force repelling the
same charged end and thus a net force of attraction. In the previous figure, if the yellow
material is light enough, it will actually float upward toward the positively charged object.
¿
0-6
Imagine that a positively charged object comes near to an uncharged conductor.
Explain why the conductor will be attracted to the positively charged object.
(Hint: if the object was negatively charged rather than positively charged, the
uncharged conductor would still be attracted to it.)
58
Many students find the process of charging by induction to be quite confusing. This is where a
charged object is brought near a conductor, the conductor interacts with a charge reservoir,
and the reservoir is removed to leave a charged conductor. The easiest way to explain this
process is with cartoons.
The next three figures demonstrate how to use the Earth as a charge reservoir in order to
charge by induction.
59
The next three figures demonstrate how to use the human body as a charge reservoir in order
to charge by induction.
60
61
¿
0-7
Draw a 3-frame cartoon that describes how to induce a positive charge onto a
conductor via charge by induction. You should include some text in your
cartoon in order to be precise in your explanation. (Frames provided below.)
¿
0-8
After negative charge has been induced on a conductor using a positively
charged object, the positively charged object may be removed. How will the
negative charges on the conductor arrange themselves in the absence of the
positively charged object? Draw you answer on the right half of the figure
below.
62
In-Lab Section 1: magnetism
!
Check that your compass is aligned correctly with the Earth's magnetic field.
If it is not, fix it or get your TA to help.
!
Use your compass to check the labeling of any magnets provided to you.
Students in previous labs often remagnetize soft iron magnets. If your
magnet(s) is magnetized incorrectly, get your TA to help you fix it (by tapping it
while in a strong magnetic field or while a strong DC current moves through it;
there may be a specialized magnetizing device in the lab).
¿
1-1
Sketch the magnetic field produced by a bar magnet by placing it underneath
this worksheet and sprinkling some iron filings onto the top of your page. The
flakes will show you the field lines, but you will need to sketch the direction of
the field lines by identifying the magnetic poles using your compass. Don’t let
the magnet under the paper touch the filings or things will get messy.
63
¿
1-2
For the following double bar magnet arrangements in the following figure,
predict the magnetic field lines by sketching what you think they will look like in
the entire area surrounding the bar magnets. (Some of the field lines will
disappear out of the drawing area only to reenter in another location of the
drawing area.) Discuss your predictions with other lab groups.
¿
1-3
Use your compass to test your prediction for each of the arrangements in the
previous figure.
Discuss how this checking is done and explain any
inconsistencies between your measurements and predictions.
64
In-Lab Section 2: electrostatics basics
Subsection A
If you rub a glass or plastic rod with some fabric or synthetic fur, electrons will be transferred
between the rod and the fabric leaving a charged rod with which to experiment. If the charged
rod is brought into the presence of insulating material containing polar molecules, the charged
rod may attract the dielectric material even though the insulating material is neutral by causing
the rotation of the polar molecules. If the charged rod transfers some of its charge to the
insulator, then both the rod and the insulator would have the same charge and thus would
repel each other.
¿
2-A-1
Predict what would happen if the presence of the rod caused the polar
molecules of the insulator to initially rotate, but once the insulator touched the
rod charge transfer took place.
¿
2-A-2
Use a charged rod to pick up packing peanuts or pieces of paper. Explain
whether there is polar molecule rotation in the insulator, charge transfer
between the rod and insulator, or both.
65
¿
2-A-3
Draw a series of pictures (cartoon) with explanatory text explaining why the
insulating material sticks to the rod. Use the concept of microscopic charge
separation (dipoles). Show plus/minus signs to signify where excess charges
have accumulated or dipoles to signify how charges have microscopically
separated. If you do not know whether the excess charge on the rod is positive
or negative, then assume it is negative.
Subsection B
A silver coated pith ball (i.e. a conductor) has virtually no mass so we can easily see how it
reacts to charge. Note that it is basically a piece of round cork covered in silver to become a
conductor.
When a charged object approached the pith ball (conductor), the pith ball is at first attracted to
the charged object.
66
However, when the pith ball touches the charged object, it immediately becomes repelled by the
charged object.
¿
2-B-1
Take the charged glass rod and slowly bring it near the pith ball. Make
observations of the behavior of the pith ball. If you do not observe the
repulsive feature of the pith ball activity, then your pith ball may not have
enough silver paint on it, and is therefore not a good conductor. In this case,
find another lab group that has a nicely conducting silver-painted pith ball.
Write your observations:
67
¿
2-B-2
Draw a series of pictures (cartoon) with explanatory text describing why the
pith ball is first attracted to the charged object, and then repelled. Use the
concept of macroscopic charge separation on a conductor. Show plus/minus
signs to signify where excess charges have accumulated signify how charges
have macroscopically separated. If you do not know whether the excess charge
on the rod is positive or negative, then assume it is positive.
68
Subsection C
A gold leaf electroscope is a device used to detect charged objects. It is made by fastening a
thin strip of pure gold to a metallic bar encased in a conducting housing. Note that the solid
metallic bar and the gold strip act as a single conductor since they are connected. The strip of
gold has been processed to be extremely thin (a few hundred atoms) so that it is actually only
worth a few dollars (but a real pain to install so please don't touch it as it will disintegrate upon
contact with your finger).
An electroscope detects excess charge by the rising of its gold leaf. The electroscope may
detect the presence of a charged object that is brought near to (but not touching) the top of the
scope through a process involving macroscopic charge separation. In this case, there is no
excess charge on the metallic bar/gold leaf conductor. The electroscope may also be charged
by touching it with a charged object. The gold leaf will still rise, but this time through a process
related to the repulsion of excess charge.
!
If you touch the scope with a highly charged object, the leaf will be ripped
from the scope due to intense electrostatic pressure. It won't be really fun to
watch, either.
69
¿
2-C-1
Be sure your electroscope your electroscope is discharged by grounding it.
Usually your body can remove any excess charge on the electroscope (so simply
touch it). Now bring a charged rod near to but not touching the electroscope
and examine the rising of the gold leaf. Use the picture-template provided
below to show and explain how the charges on the metallic bar/gold leaf are
arranged that cause the gold leaf to rise. Show plus/minus signs to signify how
charges have separated, but remember that the metallic bar/gold leaf
conductor is still neutral. If you do not know whether the excess charge on the
rod is positive or negative, then assume it is positive. Include explanatory text
in your sketch. Also, check to see if there is any excess charge on the
electroscope by withdrawing the charged rod.
70
¿
2-C-2
With an initially neutral electroscope, touch the end of the electroscope with a
charged rod and transfer excess charge to the metallic bar/gold leaf conductor.
Use the picture-template provided below to show and explain how the charges
on the metallic bar/gold leaf are arranged that cause the gold leaf to rise. Show
plus/minus signs to signify where excess charges have accumulated, and
remember that the metallic bar/gold leaf conductor has a net charge. If you do
not know whether the excess charge on the rod is positive or negative, then
assume it is positive. Include explanatory text in your sketch. Also, check to see
if there is any excess charge on the electroscope by withdrawing the charged
rod.
71
In-Lab Section 3: charging by induction
With a little ingenuity you can charge a conductor with either positive or negative charge by
using the process of induction. If you bring a neutral conductor near a positively charged object
(but without touching the conductor to the charged object), and then touch the conductor with
your finger, then negative charge will rush from your body onto the conductor in order to be
near the positively charged object. Then remove your finger so that the negative charge
remains on the conductor. If you then pull the conductor away from the positively charged
object, your conductor will be negatively charged. To induce positive charge on a conductor,
simply place the conductor near a negatively charged object and touch it with your finger then
remove. Of course you can always charge an insulator by rubbing it with wool or imitation fur,
etc.
¿
3-1
Charge a flat sheet of plastic insulator by rubbing vigorously with fake fur. Set a
flat conductor on top of it using an insulated handle (i.e. don't touch the
conductor, yet). Sketch a labeled diagram of how the charge is vertically
separated in the conductor while neutral overall (net charge equal to zero).
¿
3-2
Pull the conductor from the charged insulator (still without touching the
conductor) and see that the conductor is still neutral. Test this using the
electroscope (or Faraday cage). You may detect some small amount of charge
transfer to the conductor itself or the plastic handle, the conductor or even your
hand because there are such enormous electrostatic fields at work. Write your
observations:
72
¿
3-3
Recharge your flat sheet of plastic insulator and again place the flat conductor
on the plate without touching it. This time momentarily place your finger on
the metal. Remove your finger and then lift the conductor disc from the plate.
You may hear electrical crackling during this if your insulator was initially highly
charged. DO NOT LET THIS TOUCH THE ELECTROSCOPE. See that your
conductor now has net charge and determine the sign of the excess charge with
the electroscope or Faraday cage. Make a cartoon that shows how this process
of induction works and what the net charge of the conductor is (positive or
negative).
73
In-Lab Section 4: faraday ice pail
Because electrons are negatively charged, an electron always moves toward regions of higher
electric potential. If you 'see' an electron move from point A to point B, then you can be sure
that VBA  VB  VA  0 (or equivalently, VAB  VA VB  0 ). So an electron moves toward
regions of higher voltage.
A Faraday ice pail (sometimes called a "Faraday cage") detects the presence of excess charge
that is placed inside the pail (without touching the inner wall). If positive excess charge is
placed inside the pail, electrons are forced to the inner wall of the Faraday ice pail (through the
electrometer). This means that the outer wall must have a lower voltage than the inner wall.
Since the grounding lead of the electroscope is attached to the outer wall and the positive lead
is attached to the inner wall, the electroscope thus registers a positive voltage. If a negative
excess charge is placed inside the pail, then the electrons will rush to the outer wall. Thus the
region with the higher voltage is mismatched with the grounding lead and a negative voltage is
registered on the electrometer.
¿
4-1
Use the Faraday cage and electrometer to check the signs of the excess charge
on several classroom objects. Make a table of your observations. Be sure to see
what happens when tape is placed on a plastic surface. Do electrons 'stick' to
the tape? What happens if you put tape on a plastic surface and then another
layer of tape on the first layer of tape?
74
¿
4-2
Find two different non-conducting insulators (blue and white paddles if
equipped) and place the uncharged insulators into the pail. Rub them against
each other. Since they are made of different materials, it is likely that the
difference in electronegativites will cause electrons to be transferred from one
material to another. Pull one insulator out of the pail at a time to determine
the sign of the net charge on the paddle remaining in the pail. Even if charge is
transferred between them, together they should be net neutral. If you don't
observe this, then you should 1) use alcohol and hairdryers to remove any initial
excess charge on the paddles (don't forget the handles) and 2) make any highly
charged lab partners stand some distance away. If your lab is in a region with
high humidity, it is difficult to keep significant excess charge on objects. If the
air is dry, excess charge can end up everywhere. Record your results.
¿
4-3
Now prove to yourself that a charged conductor will transfer charge to an
uncharged conductor when they touch. Induce excess charge into a conductor
(as done earlier in the lab) and record the sign of the excess charge. Take
another conductor with a handle (often a paddle with a metallic face) that is
initially neutral and use the Faraday ice pail to prove that it is neutral. Transfer
some of the excess charge from the first conductor to the second conductor by
touching them together. Prove that charge was successfully transferred from
the one conductor to the other. Record your results.
75
(This page intentionally left blank.)
76
In-Lab Section 5: authentic assessment
Magnets surround you in your everyday life. How upsetting it is to think that most people are
incapable of determining a simple north or south pole on an unlabeled magnet.
¿
5-1
Find an unlabeled magnet in the lab and use a compass to determine the north
pole of the unlabeled magnet. First be sure your compass agrees with the
Earth's magnetic field. Explain your work to a student in a different lab group as
you show them your solution.
"Yes, I have seen this student determine the north pole of an unlabeled magnet
and their verbal explanation of the process is correct. They are well-prepared
for owning a refrigerator!"
Student Signature:___________________________________________________
77
In-Lab Section 6: open-ended / creative design
If one conducting sphere is charged to a constant positive electric potential (voltage), and
another neutral conducting sphere is brought near to it, then the charge on the neutral sphere
will separate. On the second sphere, a certain amount of negative charge will be attracted to
the sphere held at the constant positive potential, and an equal amount will be repelled. It is
extremely difficult to calculate the arrangement of charge on the neutral sphere. Sometimes a
mathematical theory is of little use in a complex system and you just have to experiment to get
answers.
It would be useful to know how the amount of charge separation on the neutral sphere is a
function of the separation distance between the spheres. For example, the strength of charge
separation could be inversely proportional to the separation distance or it could fall off as d 1.5 .
You are allowed to "cheat" by talking to other groups for ideas, but are not allowed to "cheat"
by just stating an answer you may already know, looking it up online or asking your TA.
Below you are given three prompts:
hypothesizing/planning, observations/data,
calculations/conclusion. Your job is to figure out the answer using these prompts as your
problem-solving model. In the event that you should run out of time, you may not discover the
correct answer, but you should make an attempt at each prompt. Grades are based on honest
effort.
Your open-ended solution should probably include some of the following items: sketches of
circuit diagrams, tables of data, calculations, recorded observations, random ideas, etc.
Write at the prompts on the next page.
78
¿
6-1
hypothesizing/planning:
¿
6-2
observations/data:
¿
6-3
calculations/conclusion
I, the physics 241 laboratory TA, have examined this student's Weekly Activity pages and found
them to be thoroughly completed.
!
TA signature: _______________________________________________________________
79
Post-Lab: magnetism and electrostatics
!
You must complete this post-lab section after you attend your lab. You may
work on this post-lab during lab if you have time and have finished all the other
lab sections.
¿
X-1
Use the following figure of a neutral electroscope to answer the questions that
follow.
a. Explain what happens when a negatively charged object is brought near, but
not touching, the top of a neutral electroscope.
b. Explain what happens when a you touch the top of a neutral electroscope
while a negatively charged object is near the top of the electroscope.
c. Explain what happens when a negatively charged object is rubbed against the
top of a neutral electroscope and then removed.
80
¿
X-2
Use the following figure of a positively charged electroscope to answer the
questions that follow.
a. Explain what happens when a positively charged object is brought near to,
but not touching the top of the positively charged electroscope.
b. Explain what happens when a negatively charged object is brought near to,
but not touching the top of the positively charged electroscope.
c. Explain what happens when a negatively charged object touches the top of
the positively charged electroscope.
81
¿
X-3
Use the following figure of a dipole in the presence of a single charge to answer
the following questions about polarization attraction.
a. Find the net force in newtons on the upper charge from the dipole beneath
(two rigidly connected opposite charges). Note that e = 1.6x10-19 [C] and
kE = 9x109 [N m2 / C2].
b. Use this result to explain why a polarizable piece Styrofoam is attracted to a
charged plate.
82