Download Magnetism and Electrostatics

Physics 241 Lab: Magnetism and Electrostatics
http://bohr.physics.arizona.edu/~leone/ua/ua_spring_2010/phys241lab.html
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Section 1.
1.1. Charged
particles
create electric fields that can push on other charged particles with an electric
r
r
force Felectric = qE . These electric fields are represented by drawing electric field lines that show the
direction of the electric force. Magnetic fields are more complex 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 valence electron circling around each atom creating a tiny magnetic field.
! Though this description is incomplete and magnetism can only be explained using quantum mechanics,
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 rdirection of the magnetic
r
r
field. This magnetic force is described mathematically with 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.
!
These are microscopic descriptions of nature, but we will now examine what happens with the
macroscopic magnetism of a bar magnet. A bar magnet being comprised of many tiny moving charges
(~Avogadro’s number!) creates a sizeable magnetic field near it’s surface. 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 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:
Note 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):
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):
1.2. 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.
Use your compass to check the labeling of the magnetic poles of your magnetized soft iron bar magnet.
If your magnet is labeled incorrectly, let your instructor know and maybe they can use a strong
magnetic field or DC current to remagnetize it correctly, otherwise use a pencil to lightly label it
correctly. Be sure to first check that your compass is magnetized correctly using the Earth’s magnetic
field. Your results:
1.3. Sketch the magnetic field produced by your bar magnet by placing it underneath this 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 identifying the magnetic poles using your
compass. Don’t let the magnet under the paper touch the filings or things will get messy.
Your sketch:
1.4. 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.)
1.5. 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:
Section 2.
2.1. 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). In this system, if you deposit excess charge on the conductor, the
excess charges will repel each other and spread out uniformly over the surface of the conductor:
If a neutral conductor comes into the presence of an electric field (say from another charged object),
the charges already present on its surface will redistribute so that there is macroscopic charge
separation across the entire conductor:
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:
Some insulators are also dielectrics, materials comprised 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:
2.2. 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 microscopic
charge separation within an insulator 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:
2.3. 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.
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 macroscopic
charge separation in a conductor in your answer, and that your sketches show plus/minus signs to
signify where excess charges have accumulated or how charges have macroscopically separated.
Your sketches and explanation:
2.4. A gold leaf electroscope detects excess charge on its surface by repelling its gold leaf.
WARNING: If you touch the scope with a highly enough charged object, the leaf will be ripped from
the scope due to the electrostatic pressure. You may also use your electroscope to detect charge by
bringing highly charged objects near to but not touching the scope though in this case there is no
excess charge only macroscopic charge separation. Charge your plastic square by rubbing it with
paper or fake fur. Rub the charged plastic plate on the top of the gold leaf electroscope to transfer
excess charge to the electroscope. Use the picture provided to show and explain how excess charge is
distributed on the electroscope to explain its behavior. Note that you may always use your Faraday
cage and electrometer to determine the sign of the charge on a charged object. Your picture and
explanation (be sure to check you answer with the groups around you or the TA):
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 macroscopic charge separation in a conductor. Your picture and explanation (be sure to
check you answer with the groups around you or the TA):
2.5. 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:
Section 3.
3.1. 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. Pull
the conductor away and you have a negatively charged conductor. 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 fake fur.
Charge your flat insulator by rubbing and then set your flat conductor on top of it (without touching the
conductor with your finger, yet). Sketch a labeled diagram of how the charge is vertically separated in
the conductor while neutral overall (net charge equal to zero). Your quick sketch:
Pull the conductor from the charged insulator (still without touching the conductor with your finger)
and see that the conductor is still neutral. Test this using the Faraday cage and electrometer. (You
may detect some small amount of charge transfer to the plastic handle or your hand with your
electrometer on its most sensitive setting, therefore use a LESS sensitive setting.) Your observations:
Recharge your flat insulator and again place the flat conductor 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 insulator was highly charged. See that your
conductor now has net charge and determine the sign of the excess charge with the electrometer and
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). Your explanatory cartoon:
See that you can readily pick up tiny pieces of paper (or Styrofoam) by using the charged conductor to
induce microscopic charge separation inside the paper. Your observations:
Section 4.
4.1. A Faraday cage detects charge placed inside the inner cage (without touching the cage). If
negative charge is placed inside the cage, electrons on the cage feel a force pushing them outward to
the outer cage, which causes you to measure a negative voltage with an electrometer attached to both
parts of the cage. If positive charge is placed inside the cage, the attached electrometer will measure a
positive voltage.
Use the Faraday cage and electrometer to check the signs of the excess charge on the glass rod and flat
insulator (an object does not need to fit entirely in the cage to make a reading). Your results:
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:
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.) Your results:
4.2. (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 piece of paper (or Styrofoam) is attracted to a charged plate.
Your calculations and answers (in SI units):
Section 5.
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 neutral sphere will separate
macroscopically. 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. Use your
conductive paddle, Faraday cage and electrometer to test this. Note that nearby lab groups performing
this test can affect others’ experiments over large distances. Your observations and results:
Report Guidelines: Write a separate section using the labels and instructions provided below.
• Title – A catchy title worth zero points so make it fun.
• Goals – Write a 3-4 sentence paragraph stating the experimental goals of the lab. [~1-point]
• Concepts & Equations – [~10-points] Be sure to write a separate paragraph to explain each of
the following concepts.
• 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, 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.
• Discuss force caused by charge separation. Discuss examples like the pith ball,
electroscope, and picking up tiny pieces of dielectric material.
• The process of charging a conductor by induction. Discuss an example.
• How the Faraday cage works.
• Procedure & Results – Write a 2-4 sentence paragraph for each section of the lab describing
what you did and what you found. Save any interpretation of your results for the conclusion.
[~4-points]
• Conclusion – Write at least three paragraphs where you analyze and interpret the results you
observed or measured based upon your previous discussion of concepts and equations. It is all
right to sound repetitive since it is important to get your scientific points across to your reader.
[~5-points]
• Graphs – All graphs must be neatly hand-drawn during class, fill an entire sheet of graph
paper, include a title, labeled axes, units on the axes, and the calculated line of best fit if
applicable. [0-points]:
o No graphs this lab.
• Worksheet – thoroughly completed in class. [~5-points]