Tom is trying to bring two magnets close to each other in such a way, so that the North Pole of the magnet faces the South pole of the other magnet. In this activity which type of force he will experience?
Answers
Answer:
When two magnets are brought together, the opposite poles will attract one another, but the like poles will repel one another. This is similar to electric charges. Like charges repel, and unlike charges attract.
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Experiment 1
If a bar magnet is taped to
a piece of cork and allowed
to float in a dish of water,
it turns to align itself in an
approximate north-south
direction. The end of a mag-
net that points north is the
north pole. The other end is
the south pole.
A magnet that is free to pivot
like this is called a compass.
A compass will pivot to line up
with a nearby magnet.
Experiment 3
Exploring magnetism
If the north pole of one magnet is brought near the north pole of
another magnet, they repel each other. Two south poles also repel
each other, but the north pole of one magnet exerts an attractive
force on the south pole of another magnet.
Cutting a bar magnet in half produces two weaker but still complete
magnets, each with a north pole and a south pole.
Experiment 4
Magnets can pick up some objects, such
as paper clips, but not all. If an object is
attracted to one pole of a magnet, it is also
attracted to the other pole. Most materials,
including copper, aluminum, glass, and
plastic, experience no force from a magnet.
Experiment 5
When a magnet is brought near an elec-
troscope, the leaves of the electroscope
remain undeflected. If a charged rod is
brought near a magnet, there is a small
polarization force like the ones we studied
in Chapter 21, as there would be on any
metal bar, but there is no other effect.
Experiment 2
S
N
N
S
W E
North
South
The needle of a
compass is a small
magnet.
N
S
S
N
N
S Compass
Bar magnet
Experiment 5 reveals that magnetism is not the same as electricity. Magnetic poles
and electric charges share some similar behavior, but they are not the same.
■ Experiment 2 shows that magnetism is a long-range force. Magnets need not
touch each other to exert a force on each other.
■ Experiments 1 and 3 show that magnets have two types of poles, called north and
south poles, and thus are magnetic dipoles. Cutting a magnet in half yields two
weaker but still complete magnets, each with a north pole and a south pole. The
basic unit of magnetism is thus a magnetic dipole.
■ Experiments 1 and 2 show how the poles of a bar magnet can be identified by
using it as a compass. Other magnets can be identified by testing them against a
bar magnet. A pole that repels a known south pole and attracts a known north pole
must be a south magnetic pole.
■ Experiment 4 reveals that only certain materials, called magnetic materials, are
attracted to a magnet. The most common magnetic material is iron. Magnetic
materials are attracted to both poles of a magnet.
When we studied the electric force between two charges in ◀◀ SECTION 20.4, we devel-
oped a new way to think about forces between charges—the field model. In this
viewpoint, the space around a charge is not empty: The charge alters the space around
it by creating an electric field. A second charge brought into this electric field then
feels a force due to the field.
The concept of a field can also be used to describe the force that turns a compass
to line up with a magnet: Every magnet sets up a magnetic field in the space
around it. If another magnet—such as a compass needle—is then brought into this
field, the second magnet will feel the effects of the field of the first magnet. In this
section, we’ll see how to define the magnetic field, and then we’ll study what the
magnetic field looks like for some common shapes and arrangements of magnets.
Measuring the Magnetic Field
What does the direction a compass needle points tell us about the magnetic field at
the position of the compass? Recall how an electric dipole behaves when placed in
an electric field, as shown in FIGURE 24.1a. In Chapter 20 we learned that an electric
dipole experiences a torque when placed in an electric field, a torque that tends to
align the axis of the dipole with the field. This means that the direction of the electric
field is the same as the direction of the dipole’s axis. The torque on the dipole is
greater when the electric field is stronger; hence, the magnitude of the field, which
we also call the strength of the field, is proportional to the torque on the dipole.
The magnetic dipole of a compass needle behaves very similarly when it is in a
magnetic field. The magnetic field exerts a torque on the compass needle, causing the
needle to point in the field direction