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Investigation 12
Magnets and Magnetic Fields
I.
Magnetic materials
A.
Investigate the objects that you have been given (magnets, metals: aluminum and steel, cork, plastic,
wood, rubber). Place the magnets in class 1. Separate the remaining objects into three classes based
on their interactions with the magnets and each other.
1.
List the objects in each of your classes.
Class 1
Magnets
2.
Class 2
Interacts with Magnets
Class 3
No Interaction with Magnets
Fill out the table below with a word or two describing the interaction between members of the
same and different classes, i.e., do they attract, repel, attract and repel, or is there no interaction?
Table of Interactions
Class 1
Class 2
Class 3
Class 1
Class 2
Class 3
3.
B.
Are all metals in the same class? Do they all interact with magnets?
Obtain a permanent magnet and an object that is attracted to the magnet but not repelled. Imagine
that you do not know which object is the magnet. Using only these two objects, find a way to
determine which object is the permanent magnet. (Hint: Are there parts on either object that do not
interact as strongly as other parts?
C.
The parts of a permanent magnet that interact most strongly with other materials are called the poles
of the magnet.
How many magnetic poles does each of your magnets have? Explain how you found them.
How many different types of poles do you have evidence for so far?
II.
Magnetic fields
We have observed that magnets interact even when they are not in direct contact. In electrostatics
we used the idea of an electric field to account for the interaction between charges that were
separated from one another. With magnetic interactions, we similarly define a magnetic field.
A.
A compass is included in your box.
1.
A compass is a small bar magnet that can rotate freely around a vertical axis. Being a bar
magnet, it can interact with other magnets, and being small, it does not adversely affect the
magnet with which it is interacting.
2.
Move the compass far away from all other objects. Shake the compass. Does the needle behave
as if it is in a magnetic field? How do you know?
We can account for the behavior of the compass needle by supposing that it interacts with the
earth and that the earth belongs to one of the categories from section I. To which class of objects
from section I do your observations suggest the earth belongs? How you can tell?
3.
We define the north (seeking) pole of a magnet as the end that points toward the arctic region of
the earth when the magnet is free to rotate and is not interacting with other nearby objects.
On the basis of this definition and assuming that the earth is a magnet, is the geographic north
pole of the earth the north-seeking pole or the south-seeking pole of a magnet? Explain.
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B.
Magnetic field around a bar magnet.
If we were to break our bar magnet in half, we know that each half would become a new magnet
with its own north and south poles. If we were to break the two halves in half, then each of those
pieces would also become a new magnet. We could continue to smaller and smaller pieces to form
many small magnets. This process can be reversed and a bar magnet can be built by combining
smaller magnets to form a larger magnet.
Obtain some small magnets and stack them north-to-south until you have a bar magnet about the
length shown below. Place your bar magnet on the outline shown on this page. Now use the
compass to trace lines of force of the magnetic field surrounding the bar magnet. Include enough
lines until you have a good feeling for the shape of the magnetic field. Be sure to indicate the
direction of the magnetic field.
N
S
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C.
The magnetic field of the earth.
The magnetic field surrounding the earth is produced by convection currents in the outer core of the
earth in combination with the rotation of the earth. The shape of the field, however, is very much
like that of a bar magnet, and so one can imagine a bar magnet in the earth producing the field.
Below is a circle representing the earth with the geographic north and south poles shown. Place a
small bar magnet (about four small magnets) in the outline drawn at the center of the earth. Use the
compass to trace lines of force of the magnetic field surrounding the earth. Include enough lines
until you have a good feeling for the shape of the magnetic field. Indicate the position of the
magnetic north and south poles of the earth, the magnetic north and south poles of the bar magnet in
the earth, and the direction of the magnetic field.
Geographic North Pole
Geographic South Pole
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