Download Magnets and Magnetism

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Investigation 14
Magnets and Magnetic Fields
I.
Magnetic materials
A.
Two small magnets are included in the plastic bag. Choose one of the magnets to be your reference
magnet and draw a small mark on it. You are now going to investigate how various objects that you
have around your home, including the other magnet, interact with your reference magnet. Class 1 will
be objects that are attracted to and repelled from your reference magnet. Class 2 will be objects that are
only attracted to the reference magnet. Class 3 will be objects that are only repelled from the reference
magnet. Class 4 will be objects that do not interact at all with the reference magnet. Use as many
objects made from different materials as you can (e.g., the other magnet, metals: aluminum, copper,
and steel, cork, plastic, wood, rubber, paper, etc.).
1.
When performing this exercise, first: write the type of object, second: make a guess as to what will
happen, and third: write what happened.
object
your guess
what happened?
2.
List the objects in each of your classes.
Class 1
attracted and repelled
2.
Class 2
attracted
Class 3
repelled
Class 4
no interaction
Are all metals in the same class? Do they all interact with magnets?
B.
Suppose you have one permanent bar magnet and another bar-shaped 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 different types of poles do you have evidence for so far?
What are their names?
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.
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A.
Compass
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.
When you hold a compass in the palm of your hand, away from all other objects, and shake the
compass, the compass needle wiggles back and forth and then finally comes to rest pointing in
one particular direction. Is this the behavior is should have if it is in a magnetic field? What is
the needle lining up with?
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.
B.
Magnetic field around a bar magnet.
Draw the orientation of the compass when placed at points A, B, C, D, and E. The compass can be
represented as an arrow with north being the point of the arrow.
C
A
B
D
S
N
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E
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 as shown. A bar
magnet placed in the outline drawn at the center of the earth produces a magnetic field that looks
very similar to the field surrounding the earth. Draw the lines of force surrounding the bar magnet
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 and the magnetic north and south poles of the bar magnet in
the earth. Also draw arrows to show the directions of the lines of force of the magnetic field.
Geographic North Pole
Geographic South Pole
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