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Unit
Unit 11.03:
11.03:
Electromagnets
Prepared in Dec 1998
Second editing in Feb 2000
Learning objectives
At the end of this unit you should be able to :
1. describe the design and use of electromagnets.
2. give examples of the use of magnetic materials
e.g. audio / video tapes.
3. describe the designs and functions of electric bell
and magnetic relays.
• An electromagnet is
a magnet that
operates on
electricity.
• It consists of a
solenoid of many
turns of insulated
copper wire wound
on a core of soft
iron.
(continue on next slide)
• When d.c. is passed through the solenoid, the
core is magnetized and therefore produced a
strong magnetic field.
• When the current is switched off, the core
loses its magnetism.
– Increase the number of turns of the coils.
– Increase the strength of current (or potential
difference)
– Use soft iron as the core.
Soft-iron core
It is used on cranes in steelworks and
scrap yards to lift heavy iron object.
• When the bell-push
completes the circuit, a
current flows through the
electromagnet and the
electromagnet produces a
strong magnetic field.
• The soft iron armature is
then attracted towards the
electromagnet and the
hammer will hit the gong.
(continue on next slide)
• This movement breaks the
circuit at point X, so that
the current stops flowing
and hence switches off the
electromagnet.
• The spring pulls the
armature back so that
contact is made again and
the sequence repeats.
• The make-and-break
contact screw at X is
adjustable to give the best
sound.
!
• When an electric current (a.c.) from the mains flows
through the coil, the soft iron hammer is magnetized
with south pole and north pole alternately at the
frequency equal to that of the mains. Thus the hammer
is set into vibration at the same frequency.
• Use T = 1/f,
hence the time
is obtained.
Electromagnet
•A relay is a switching device
which uses an electromagnet.
• It has at least two circuits. One
is used to control another
circuit without any direct
electrical contact between
them.
• The input circuit supplies
current to the electromagnet.
(continue in next slide)
A simple magnetic relay
Electromagnet
• Only a small current is needed.
• The electromagnet attracts one
end of iron armature which is
pivoted so that the other end
acts as lever.
• The lever opens or closes
(depending on design) the
contacts in the second output
circuit.
(continue in next slide)
A simple magnetic relay
Electromagnet
• In short, the input circuit which
works on a low current (that is
safe) can control or activate
another circuit which works on
high circuit (that is dangerous).
• This is the main function of a
relay.
A simple magnetic relay
GCE O-Level
Past Examination Paper
Science (Physics)
All rights go to University of Cambridge Examinations Syndicate and other sources
Nov 1991
14. The diagram shows resistance wire wrapped
on to a rod to make an electrical heater.
A
Which of the following materials would be
most suitable for the rod ?
A polished copper
C coloured plastics
B soft iron
D hard steel
O’ level Physics
Nov 1995
14. An experiment was carried out using four rods
made of different materials which were placed, in
turn, in a coil of wire.
A large direct current
was passed through the
coil for a few seconds
and was then switched off.
As a result one of the rods was permanently
magnetised by this experiment. Which one?
D
rod
A
C
material
glass
plastic
rod
B
D
material
iron
steel
GCE O
Nov 1997
13. The diagram shows a coil in which there is a
current.
What will happen to the magnetic field when an
iron bar is inserted into the bar ?
A It becomes much strong.
B It becomes slightly
weaker.
C It changes direction.
D It stays the same.
A
Nov 1998
12. The diagram shows a 12 V d.c. Power supply,
connected across a coil of wire with a metal core.
The core becomes a stronger magnet when the
current is switched on. It remains a magnet after
the current is switched off.
Which metal is the core made of ?
A
B
C
D
aluminium
copper
soft iron
steel
D
(Cont. …) Q. 11
Nov 1995
11.(b) Describe the structure of a simple electromagnet.
State one situation in industry or medicine in
which an electromagnet could be used, where a
permanent magnet would not be suitable. [4]
The electromagnet
usually make by
soft iron. The soft
iron is wound by
many turns of wire.
When electric
current is supplied,
the soft iron will then induce magnetism.
In industry, electric crane must use electromagnet.
(continue on next slide)
November 1992
6. The diagram below shows two metal rods
placed inside a solenoid.
When a battery
is connected to
A and B, the
two rods rolled
away from each
other.
(a) Explain this
observation.
Because same end of the ends of two rods
have the same type of magnetism, therefore
repel one another,
(continue on next slide)
Q. 4
Nov 1990
4.(c) Explain how, using suitable equipment.
(i) a direct current may be used to magnetise a
[2]
magnet.
Wind insulated copper
wire round and
round a large iron
nail. Then connect
the ends of wire to a
battery (d.c.). After few seconds, the nail
will magnetize and able to pick up iron
nails.
(continue in next slide)
(Cont. …) Q. 4
Nov 1990
4.(c)(ii) an alternating current may be used to
[2]
demagnetise a magnet.
Wind insulated
copper wire round
and round
magnetized steel
nail . Pass
alternative current
(a.c.) through the
coil. Gradually pull the nail out of the coil.
The magnetism will then destroyed.