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Please turn to the section titled Magnetism from Electric Currents.
In this section, you will examine how magnetism is produced by electric currents. You
will learn about solenoids and electromagnets, two devices that use electricity to generate
magnetic fields. You will also be introduced to the concept of domains. Finally, you will take a
look at devices that make use of the magnetic field produced by electric currents. These devices
are appropriately called electromagnetic devices.
In the eighteenth century, people noticed that a bolt of lightning could change the
direction of a compass needle. People also observed that iron pans sometimes became
magnetized during lightning storms. These observations suggested that a relationship exists
between electricity and magnetism, but the relationship was not understood until 1820.
In 1820, a Danish science teacher named Hans Christian Oersted [UHR STED ] first
experimented with the effects of an electric current on a compass needle. Oersted discovered
that magnetism is produced by moving charges.
Figure 6 shows that an electric current produces a magnetic field. The wire in Figure 6 is
carrying an electric current. Notice what happens to the iron filings that are placed near this
wire. As you can see in Figure 6, the magnetic field causes the iron filings to make a distinct
pattern around the wire.
You learned in the previous section that pieces of iron will align with a magnetic field.
Examine the pattern of the iron filings shown in Figure 6. What type of pattern do these iron
filings form? If you said that these filings align themselves in concentric circles, you are correct.
This arrangement of the iron filings suggests that the electric current traveling through the wire
produces a magnetic field. This magnetic field causes the iron filings to form concentric circles
around the wire. If you were to bring a compass close to a current-carrying wire, you would find
that the needle points in a direction tangent to the circles of iron filings. This is what Oersted
did.
Please look at the next page.
A current-carrying wire produces a circular magnetic field. But is the direction of this
magnetic field clockwise or counterclockwise? An easy way to predict the direction is to use the
right-hand rule. The right-hand rule states that if you imagine holding the wire in your right
hand with your thumb pointing in the direction of the positive current, the direction of
your fingers would curl in the direction of the magnetic field.
Figure 7 illustrates the right-hand rule. Take a look at it. Notice that the wire is grasped
with the right hand with the thumb pointing upward in the direction of the current. Notice how
the fingers wrap around the wire. The fingers point in the direction of the magnetic field. In this
case, the direction of the magnetic field is counterclockwise. Now imagine that the current flows
through the wire in Figure 7 in a downward direction, toward the bottom of the page. In this
case, the wire would be grabbed with the right hand so that the thumb points downward in the
direction of the electric current. In this situation, the magnetic field would point clockwise.
Remember—never grasp or touch an uninsulated wire. You could be electrocuted.
The magnetic field of a coil of wire resembles the magnetic field of a bar magnet.
As Oersted demonstrated, the magnetic field of a current-carrying wire exerts a force on
a compass needle. This force causes the compass needle to turn in the direction of the wire’s
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magnetic field. However, this magnetic force is very weak. Increasing the current in the wire is
one way to increase the strength of the magnetic field, but large currents can be fire hazards. A
safer way to create a strong magnetic force is to wrap the wire into a coil. Notice how the wire
in Figure 8 has been wrapped into a coil. This coiled wire forms a device called a solenoid. A
solenoid is a long, wound coil of insulated wire.
In a solenoid, the magnetic field of each loop of wire adds to the strength of the magnetic
field of the loop next to it. Because of the numerous coils present in a solenoid, the result is a
strong magnetic field similar to the magnetic field produced by a bar magnet. Notice in Figure 8
that a solenoid even has a north pole and a south pole, just like a magnet.
Turn to the next page.
The strength of the magnetic field of a solenoid depends on two factors. One factor is
the number of loops in the wire. The second factor is the quantity of current that is passing
through the wire. The greater the number of loops or the greater the current, the stronger the
magnetic field.
Another way to increase the strength of a solenoid’s magnetic field is to insert a rod
made of iron through the center of its coils. Actually, any potentially magnetic metal can be
used in place of iron. The device made of a solenoid and a magnetic metal is known as an
electromagnet. An electromagnet is defined as a strong magnet that is created when a magnetic
metal is inserted into the center of a current-carrying solenoid. The magnetic field of the solenoid
causes the metal rod to become a magnet as well. Therefore, an electromagnet creates a stronger
magnetic field than does a solenoid alone.
The movement of charges causes all magnetism. For instance, the movement of charged
particles is responsible for the magnetic properties of a bar magnet.
But what charges are moving in a bar magnet? You learned in the chapter on Atoms and
the Periodic Table that electrons are negatively-charged particles that travel around the nucleus
of an atom. All electrons have a property called electron spin. This electron spin produces a
tiny magnetic field around each electron.
In some atoms, the magnetic fields produced by the moving electrons cancel one another.
Materials containing these types of atoms are not magnetic. However, in iron, nickel, and cobalt
atoms, not all of the magnetic fields of the spinning electrons cancel. Therefore, materials
containing iron, nickel, or cobalt atoms are magnetic.
Look at the next page.
Recall that a compass needle rotates to align with a magnetic field. In a similar way,
magnetic atoms rotate to align with magnetic fields of nearby atoms. The result is the formation
of small regions within the material called domains. A domain is a microscopic region composed
of a group of atoms whose magnetic fields are aligned in the same direction.
Take a look at Figure 9A. Notice that the magnetic fields of the domains inside an
unmagnetized piece of iron are not aligned. You can tell that they are not aligned by the arrows
in Figure 9A. These arrows point in all different directions.
Notice in Figure 9B what happens when a strong magnet is brought near the iron. The
magnet causes the domains to line up more closely with the magnetic field. Locate the three
domains in Figure 9B where the magnetic fields have aligned more closely to each other. Notice
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that the direction of the domains becomes more uniform, and the piece of iron becomes
magnetized.
Now let’s take a look at some electromagnetic devices.
Many modern appliances make use of electromagnetic devices. Two such appliances are
hair dryers and stereo speakers. Many scientific instruments also make use of electromagnets.
Galvanometers are devices used to measure current in ammeters and voltages in
voltmeters. A galvanometer can detect the movement of charges in a circuit. Figure 10 shows the
basic components inside a galvanometer.
Notice in Figure 10 that a galvanometer consists of a coil of insulated wire wrapped
around an iron core. The iron core can spin between the poles of a permanent magnet. Consider
what happens when a galvanometer is attached to a circuit.
Please turn to the next page.
When a galvanometer is attached to a circuit, a current will flow through the coil of wire.
As a result, the coil and iron core will act as an electromagnet and produce a magnetic field. This
magnetic field will interact with the magnetic field produced by the surrounding permanent
magnet. The resulting forces will cause the iron core to rotate.
Recall that the greater the current passing through an electromagnet, the stronger its
magnetic field. If the core’s magnetic field is strong, then the force on the core will be great. As a
result, the core will rotate through a large angle. A needle extends from the core to a scale. As the
core rotates, the needle moves across the scale. The larger the current, the greater the movement
of the needle across the scale.
An electric motor is a device that converts electrical energy to mechanical energy.
Electric motors are another type of device that use magnetic force to cause motion.
Figure 11 illustrates a simple direct current, or DC, motor.
The arrow in Figure 11 shows that the coil of wire in a DC motor turns when a current is
in the wire. Recall that the coil and core in a galvanometer rotate back and forth, causing a needle
to move across a scale. However, the coil in an electric motor keeps spinning. If the coil is
attached to a shaft, it can do work. The end of the shaft can be connected to a propeller or
wheel. This design is often used in mechanical toys.
Locate the commutator in Figure 11. A commutator is a device that makes the current
change direction every time the flat coil makes a half revolution. Notice in Figure 11 that the
commutator in this case consists of two half rings of metal. Devices called brushes connect the
wires to the commutator.
Notice in Figure 11 that the two halves of the commutator are separated by slits. As a
result, charges must move through the coil of wire to reach the opposite half of the metal ring.
Use your finger to trace the path the current can take through an electric motor, starting at the
positive terminal of the battery. As the coil and commutator spin, the brushes come in contact
with a different side of the metal ring. As a result, the current in the coil changes direction.
Because the direction of the current through the coil changes, the direction of the
magnetic field produced by the coil also changes. In this way, the coil is alternately repelled by
both the north pole and the south pole of the magnet surrounding it. Because the current keeps
reversing direction, the loop rotates or spins in one direction. If the current did not change
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direction, the loop would just bounce back and forth in the magnetic field. Friction would
eventually cause the coil to come to rest.
Look at the next page.
Magnetic forces can cause motion that produces sound waves. This is how most stereo
speakers, like the one shown in Figure 12, work. Notice in Figure 12 that a speaker assembly
contains a permanent magnet and a coil of wire attached to a flexible paper cone. When a current
is in the coil, a magnetic field is produced. This field interacts with the field of the permanent
magnet. As a result, the coil and cone move in one direction. When the current on the coil
reverses direction, the magnetic force on the coil also reverses direction. As a result, the coil and
cone both move in the reversed direction.
These alternating back-and-forth forces on the speaker cone make it vibrate. Varying the
magnitude of the current changes how much the coil and cone vibrate. These vibrations in turn
produce sound waves in the air. In this way, an electrical signal is converted to a sound wave.
Now let’s review the key concepts from this section that are listed under the Summary.
A magnetic field is produced around a current-carrying wire.
A current-carrying solenoid has a magnetic field similar to that of a bar magnet.
An electromagnet consists of a current-carrying solenoid with an iron core.
A domain is a group of atoms whose magnetic fields are aligned.
Galvanometers measure the current in a circuit using the magnetic field produced by a
current in a coil.
Electric motors convert electrical energy to mechanical energy.
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Magnetism p.8