Download Physics 241 Lab Magnetism and Electrostatics

Physics 241 Lab
Magnetism and Electrostatics
Overview:
This week’s lab is like several little experiments with lots of things to learn and think about.
Introductory Discussion:
1. Magnetism: magnetic poles (north/south), magnetic fields, Lorentz force law and perpendicular
force, the Earth, using a compass (carefully).
2. Electrostatics: conductors, insulators and dielectrics, and learn which is able to accept charge,
conduct charge or become polarized. Polarizability of dielectrics, charge embedding in
insulators, and charge separation in conductors.
3. Demonstrate: the pith ball, using the electroscope, charge by induction, using the Faraday cage,
grounding properly and the conducting spheres.
Worksheet:
Experiment 1: Magnetic Fields
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. Note: one thing that is rarely studied is the strength of attraction/repulsion between two magnets.
Usually we are interested in the effect of the magnetic field produced by the magnet on nearby moving
charges.
1. Use your compass to check the labeling of the magnetic poles of your weakly magnetized soft
iron bar magnet. If your magnet is labeled incorrectly, let your instructor know and maybe they
can use a solenoid and hammer to remagnetize it correctly, otherwise use a pencil to lightly
label it correctly.
Your results:
2. Sketch the magnetic field produced by your bar magnet by placing it underneath your
worksheet and sprinkling some iron flakes 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 using your
compass.
Your sketch:
3. For the following double bar magnet arrangements, 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.)
4. Use your compass to test your prediction for each of the above arrangements. Explain any
inconsistencies between your measurements and predictions.
Your results and explanations:
Experiment 2: Electrostatics
Like charges repel and opposite charges attract.
5. Rub a glass rod with some spare paper. Electrons will be transferred from the rod to the paper
leaving a positively charged rod with which to experiment. Now tear up some paper into tiny
pieces and use your charged rod to pick up the (neutral) pieces. The pieces are uncharged yet
are still attracted to the rod. If the rod could transfer some of its positive charge to the paper,
then they would both be positive and would repel. But this doesn’t happen. Draw a labeled
sketch with explanatory text explaining why the pieces of paper stick to the rod. Be sure to use
the concept of charge separation in a conductor in your answer, and that your sketches show
plus/minus signs to signify where excess charges have accumulated or dipoles to signify how
charges have microscopically separated.
Your sketch and explanation:
6. A silver coated pith ball (i.e. a conductor) has virtually no mass so we can easily see how it
reacts to charge. Take the charged glass rod and slowly bring it near the pith ball. You should
see some perplexing behavior as the pith ball is at first attracted by but then repelled by the
glass rod. (I wouldn’t tell you the punch line, but the coating on these pith balls have worn off
over the years so you may have to rub the rod in the pith ball quite a bit to see the correct
behavior. Use you imagination somewhat.) Draw two labeled sketches with explanatory text
explaining why in the first case the pith ball is attracted to the rod while in the second case it is
repelled. Be sure to use the concept of charge separation in a conductor in your answer, and
that your sketches show plus/minus signs to signify where excess charges have accumulated or
dipoles to signify how charges have microscopically separated.
Your sketches and explanation:
7. A gold leaf electroscope shows when it has become charged by repelling its gold leaf.
WARNING: If you touch the scope with a highly charged object, the leaf will be ripped from
the scope due to the electrostatic pressure. You may still use your scope to detect charge by
bringing highly charged objects near to but not touching the scope.
Place charge your plastic square by rubbing it with paper. Rub the charged plastic plate on the
top of the gold leaf electroscope to transfer excess charge to it. Use the picture provided to
show and explain how excess charge is distributed on the scope to explain its behavior.
8. Discharge your electroscope (by touching it to a ground) and now bring the negatively charged
plastic plate near to but not touching the electroscope. Now show and explain the scopes
behavior using the concept of charge separation in a conductor.
9. Determine if charge was transferred to the scope when you brought the charged plate near to
the scope (but not touching it) by removing the plate and observing the gold leaf.
Your result:
10. (Data and graphs.) Next use your gold-leaf electroscope to make quantitative measurements.
Specifically, charge your plastic plate and measure the displacement of the gold leaf from the
vertical, D1, due to the distance of the charged plate, D2 (see the figure below). It will not be
easy to measure D1 from outside the scope but do your best to
obtain reasonably precise measurements. Make a data table of
D1 and D2 (about five data pairs) and then use these results to
make a hand drawn graph of D1 versus D2 (on a full sheet of
graph paper during class). I would like you to test the
k
relationship D1 = 2 . To do this, make another graph of D1
D2
1
versus 2 . If this graph is linear, then calculate the slope,
D2
which
! is equal to k. Even if this graph is not perfectly linear,
make your best estimate of k. You should be able to determine
k
! from dimensional analysis of D1 = 2 what the correct units
D2
are for k.
Your data:
!
Your result for k with correct units: ____________________________
Experiment 3: Charging by Induction
With a little ingenuity you can charge a conductor with either charge you want.
11. Supercharge your plastic plate and then set your conductor disc on top of it (without touching
the metal part). Sketch a labeled diagram of how the charge is vertically separated in the
conductor while neutral overall (net charge equal to zero).
Your sketch:
12. Pull the disc off the plate (still without touching the metal part) and see that the disc is still
neutral and has no net charge (test this using the Faraday cage and electrometer if this was
demonstrated to you at the beginning of the lab).
13. Re-supercharge your plate and again place the conductor disc on the plate. 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 plate was extra
supercharged. See that your conductor now has net charge and can readily pick up tiny pieces
of paper. Make a cartoon that shows how this process works and what the net charge of the
conductor is (positive or negative). This may require thinking and brainstorming with several
other students. This is the process of charging by induction.
Your explanatory cartoon:
Experiment 4: Faraday Cage
A Faraday cage detects charge placed inside the inner cage. If negative charge is placed in the
cage, electrons on the cage feel a force pushing them outward to the outer cage, which is grounded
and so at zero electric potential (voltage). The inner cage is therefore at a negative electric
potential since electrons feel an outward force, which must be in the opposite direction of the
electric field. An attached electrometer attached to both cages can measure this difference in
electric potential.
If you place positive charges inside the cage, the direction of the electric field reverses and so
will the reading on the electroscope.
14. Use the Faraday cage to check the signs of the excess charge on the glass rod and plastic plate
(an object does not need to fit entirely in the cage to make a reading).
Your results:
15. Place the uncharged white and blue paddle into the cage. Rub them and pull one of them out of
the cage at a time to determine the sign of the net charge on the paddle remaining in the cage.
Your results:
16. Induce excess charge into your conducting disc as done earlier in the lab. Transfer some of
this excess charge to the conducting paddle and place it in the Faraday cage to test if charge was
successfully transferred from the one conductor to the other. (You need to make sure this works to
be able to complete the open ended experiment later.)
17. Polarization attraction. Find the net force in Newtons on the upper charge from the two
polarized charges beneath. Note that e = 1.6x10-19 {C} and kE = 9x109 {N m2 / C2}. Now use this
result to explain why a polarizeable peanut is attracted to a charged plate. [Show your work next
to the picture.]
18. (Authentic Assessment: 2 points) You are given a magnet and a compass and you must
determine which end of the magnet is the magnetic north pole.
YOUR SCORE: ___________________
OPEN ENDED - Experiment 5: Conducting Spheres
19. If one conducting sphere is held at a constant positive electric potential (voltage), and the other
conducting sphere is brought near to it, then the charge on the second 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. You need to find out how the
strength of that charge separation on the second sphere is related to its distance from the first
sphere held at a constant potential. For example, the strength of charge separation could be
inversely proportional to the separation distance.
In the area below and on additional paper if necessary, design an experiment to find the correct
relationship between the strength of the charge separation and the distance between the spheres.
Then implement your experiment and make your observations. You may “cheat” by talking to
other groups for ideas, but not “cheat” by already knowing the answer or looking it up. For this
experiment you MUST make at least two graphs.
Report Guidelines: (Ellipsis means you should not need help here on what to write about). Note that
you are free to leave blank areas in your typed report and then hand-draw pictures in those areas after
you print out your report. This may make it much easier for you to explain something (often with less
typing). Quite honestly I prefer a hand-drawn explanatory diagram instead of a long paragraph when it
is possible to do so.
1. Title - …
2. Goals – Write a sentence for each experiment that describes the “big idea” for that particular
experiment.
3. Concepts & Equations – be sure to address the concepts for each experiment:
• Magnetism and magnetic fields.
• Electrostatics of conductors, insulators and dielectrics. Be sure to describe how each
material responds differently to charge either via charge separation in a neutral object,
the accumulation of excess charge on either a conductor or insulator, or the polarization
of dielectric materials.
• The process of charging by induction.
• How the Faraday cage works.
• Your open-ended experiment (#18).
4. Procedure – Just describe your experimental design for your open-ended experiment (#18)
5. Results – Write a separate short paragraph for each experiment and your open-ended
experiment simply stating the results of each without explaining the results (that comes in the
next section).
6. Conclusion – Interpret your results based upon things you discussed in your theory section.
Write a separate paragraph for each experiment and your open-ended experiment. Describe
how what you actually did in each experiment demonstrated the main idea for that experiment.
7. Attachments
• This worksheet.
• At least 4 graphs, each neatly made occupying the entire sheet of graph paper with
correctly labeled axes along with the correct units. Put any calculation of slopes
directly onto the graphs.
• Any additional notes.