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Phys 203A
Spring 2009
Lab 5– Magnetism – Some Investigations
Name__________________
Objective: To investigate magnetic interactions and magnetic fields.
Magnetic Interactions and Magnetic Materials
1. Investigate different objects (magnets, metals, cork, plastic, a battery, a piece of wood
etc.). Separate the objects into three classes based on their interactions with each
other. List the objects in each of your classes.
Class I
Class II
Class III
2. Fill in the table below with a one-word description of the interaction between members
of the same or different classes.
Class 1
Class 2
Class 3
Class 1
Class 2
Class 3
1. Are all metals in the same class?
2. To which class do magnets belong? Are all objects magnets in this class?
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Magnetic and Electric Interactions – are they the same?
3. How can you use a suspended uncharged pith ball to test whether something is
charged or not?
4. Prediction: What do you think will happen if you bring one of the poles of your
magnet near a suspended uncharged pith ball? What about the other pole of the
magnet?
Test your prediction! Record your observation.
4. Attach a paper clip to a string and suspend it from a pencil. Lay the pencil across
a foil lined cup.
a) Predict what will happen to the paper clip when a charged rod is brought near the
cup (keep the rod outside the cup). Explain in terms of the electric field inside the
foil-lined cup.
Test your prediction and record your observations below!
b) Bring a magnet near the cup and record your observations.
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c) Based on your observations, would you say that a magnetic interaction is the
same as or different from an electrical interaction? Explain.
Magnetic Fields
We introduced the idea of an electric field as a way to understand the long-range
electric force. A charge alters the space around it by creating an electric field. A second
charge then experiences a force due to the presence of the electric field. The electric field
is the means by which charges interact with each other. Although this idea seemed rather
abstract, it turned out to be useful. We need a similar idea to understand the long range
force exerted by magnets on each other. We introduce the magnetic field to understand
magnetic interactions.
5. a) Observe a compass needle as you bring it slowly near one pole of the long bar
magnet. Then move it slowly around the magnet and towards the other pole.
Describe the behavior of the needle.
b) To which class of objects (Class I, II and III) does the compass belong?
c) Move the compass far away from all the magnets. Describe the behavior of the
compass needle. Does the compass behave as if it is in a magnetic field?
Explain.
The compass is responding as if the earth is a large magnet. We label the
pole that points “north” as the north pole of a magnet.
d) Find the north pole of one of the unlabeled ceramic magnets.
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6. Suspend a “cow” magnet with a string. Does it behave like a compass? Can you
determine the north pole of this magnet? Record your observation.
7. Place a bar magnet on your sheet in the space below. Draw the outline of the magnet.
Now slowly move the compass near the magnet from one pole to another. Draw an
arrow to show the direction in which the north pole of the compass points at various
points around the magnet.
What happens as you move the compass further and further away from the magnet?
8. Place the bar magnet on the table and place a Plexiglas sheet on top of it. Now
gently sprinkle iron filings on the sheet and tap the sheet (don’t touch the filings
directly) until the filings arrange themselves. Do you recognize this pattern?
Conserve Iron filings! Please put them back in the container after you finish
using them!
10a) Use the iron filings to plot the field of a cow magnet.
b) Separate the cow magnet into 2 parts. How many poles result from this division?
How do you know?
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a) Place the right half of the cow magnet on the Plexiglas and plot the field lines on
a separate sheet after you determine the lines using the iron filings. You may
want to use a colored pen or pencil.
b) On the same figure plot the field lines of the left part of the cow magnet using a
pen of a different color.
c) Do magnetic fields obey the principle of linear superposition? Explain.
d) What can you conclude about the field lines inside a bar magnet? Draw these
lines for the cow magnet below including the field lines inside the magnet.
h) How does the strength of the magnetic field inside the magnet compare with that
outside?
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