13.4
Practice
switch: a device that is used to make or
break an electric circuit
Understanding Concepts
3. An electromagnet is able to exert a force of 1.5 × 102 N when lifting
an object. The electromagnet has 1000 turns, a current of 1.5 A, and a
material in the core with a relative magnetic permeability of 2.0 × 103.
What force will the electromagnet exert if the following changes are
made, each considered separately?
(a) The current is increased to 6.0 A.
(b) The number of turns in the coil is increased to 1400 without
increasing the length of the coil.
(c) All of the above changes are considered simultaneously.
Answers
(a) 6.0 × 102 N
(b) 2.1 × 102 N
(c) 8.4 × 102 N
Applications of Electromagnetism
Many appliances, tools, vehicles, and machines use a current-carrying coil to
create a magnetic field. In most cases, the magnetic field is used to cause another
component to move by magnetic attraction. A few examples will illustrate how
electromagnets are used.
Lifting Electromagnet
Large steel plates, girders, and pieces of scrap iron can be lifted and transported
by a lifting electromagnet (Figure 8). A soft ferromagnetic core of high relative
magnetic permeability is wound with a copper conductor. The ends of the coil
are connected to a source of electric potential through a switch. Closing the
switch causes an electric current in the coil, and the soft iron core becomes a very
strong induced magnet.
When the switch is opened and the electric current stops, the soft iron core
becomes demagnetized and releases its load. A U-shaped core is often used, with
a coil wrapped around each leg of the device. If the coils are wound in opposite
directions, the legs become oppositely magnetized and the lifting ability of the
magnet is doubled.
soft
iron
core
battery
switch
N
S
S
N
Figure 8
A lifting electromagnet
soft iron core
switch
spring
Electromagnetic Relay
A relay is a device in which a switch is closed by the action of an electromagnet
(Figure 9). A relatively small current in the coil of the electromagnet can be used
to switch on a large current without the circuits being electrically linked.
A pivoted bar of soft iron, called an armature, is held clear of the contact
point by a light spring. There is no current in the left-hand circuit, and the lamp
is off. When the switch is closed, there is a current in the right-hand circuit, and
the soft iron U-shaped core becomes magnetized. The magnetized core attracts
the armature and pulls it to the right until it touches the contact point, completing the circuit. Now there is a current in the left-hand circuit, and the lamp
goes on.
When the switch is opened, the electric current drops to zero, the core
becomes demagnetized, and the armature is released. When the spring pulls the
armature away from the contact point, current drops to zero in the left-hand circuit and the lamp goes off.
If the contact points were on the opposite side of the armature from the electromagnet, the relay would operate in reverse. Closing the switch would then
turn the left-hand circuit off, and vice versa.
battery
soft iron
armature
contact point
soft iron core
Figure 9
An electromagnetic relay
relay: a device in which a switch is closed
by the action of an electromagnet
armature: a pivoted bar of soft iron
Electromagnetism 487
switch
Electric Bell
spring
battery
soft iron
armature
N
contact
adjusting
screw
soft
iron
core
S
contact
bell
Figure 10
An electric bell
In an electric bell, a small hammer is attached to the armature. The armature is
vibrated back and forth several times a second, striking a metal bell. Figure 10
shows the circuit that causes the armature to move.
When a button is pushed, the switch is closed. An electric current flows
through the contacts and the spring to the coils, and the soft iron cores become
magnetized. The cores attract the iron armature, which moves toward the electromagnet, causing the hammer to strike the bell. As the hammer strikes the bell,
the movement of the armature opens the contacts. The electric current stops
flowing to the coils and the soft iron cores become demagnetized, releasing the
armature. A spring pulls the armature back to re-establish contact, thereby completing the circuit, and the entire cycle begins again. Small sparks, evidence of
charge jumping across the gap, can be observed at the contact points as the circuit is alternately completed and broken.
Try This
Activity
Testing an Electromagnet
Perform an activity in which you can use an electromagnet to lift iron
objects, such as washers. The coil of the electromagnet can be connected to a variable DC power supply. Remember not to leave the current flowing for more than a few seconds at a time. Test the
electromagnet both with and without the core, and with various currents
and numbers of loops. Summarize your findings in tabular form.
Practice
Understanding Concepts
Answers
4. Figure 11 shows coils of wire wound on cardboard cylinders. Copy
the diagrams into your notebook, and on each diagram mark the
direction of electric current, the direction of the field lines at each end
of the coil, and the N-pole and S-pole of the coil.
5. (a) 60.0 N
(b) 45.0 N
(c) 90.0 N
(a)
5. A coil with an iron core is used as an electromagnet. With 500 loops
and a current of 1.5 A, it can exert a lifting force of 30.0 N. What force
will it be able to lift if the following changes are made?
(a) The current is increased to 3.0 A.
(b) The number of loops is increased to 750 without increasing the
length of the coil.
(c) Both the above changes are made together.
(b)
–
+
(c)
6. In both electromagnetic relays and doorbells, soft iron cores are used
in the electromagnets. If a hard iron core is used, explain what would
happen in
(a) a relay that is used to turn a light on and off
(b) an electric bell
(d)
+
–
–
+
Figure 11
For question 4
488 Chapter 13
Making Connections
7. Describe some commercial or industrial applications in which electromagnets are better than permanent magnets. Describe some applications in which permanent magnets are better than electromagnets.