Unit 11.03: Electromagnetssuccesstutor.weebly.com/uploads/9/4/1/9/941932/electromagnets.pdf · •An electromagnet is a magnet that operates on electricity. • It consists of a solenoid

Unit 11.03:

ElectromagnetsUnit 11.03:

ElectromagnetsElectromagnets

Prepared in Dec 1998Second editing in Feb 2000

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.

Learning objectivesLearning objectives

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• 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)

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• 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.

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– Increase the number of turns of the coils.– Increase the strength of current (or potential

difference)– Use soft iron as the core.

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�� Soft-iron core

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�� It is used on cranes in steelworks and scrap yards to lift heavy iron object.

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�� • 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.


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�� • 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.

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• 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.

A simple magnetic relay(continue in next slide)

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. A simple magnetic relay(continue in next slide)

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 OGCE O--LevelLevelPast Examination PaperPast Examination Paper

Science (Physics)

All rights go to University of Cambridge Examinations Syndicate and other sources

Nov 1991

14. The diagram shows resistance wire wrappedon to a rod to make an electrical heater.

Which of the following materials would bemost suitable for the rod ?

A polished copper B soft ironC coloured plastics D hard steel

A

Nov 1995O’ level Physics

14. An experiment was carried out using four rods made of different materials which were placed, in turn, in a coil of wire.

rod material rod materialA glass B ironC plastic D steel

D

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 permanentlymagnetised by this experiment. Which one?

GCE O Nov 1997

13. The diagram shows a coil in which there is acurrent.What will happen to the magnetic field when aniron 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 199812. 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 aluminiumB copper C soft ironD steel

D

11.(b) Describe the structure of a simple electromagnet.State one situation in industry or medicine inwhich an electromagnet could be used, where a permanent magnet would not be suitable. [4]

(Cont. …) Q. 11 Nov 1995

The electromagnetusually make bysoft iron. The softiron is wound bymany turns of wire.When electric current is supplied,the soft iron will then induce magnetism.In industry, electric crane must use electromagnet.


November 1992

6. The diagram below shows two metal rods placed inside a solenoid.When a battery is connected to A and B, thetwo rods rolled away from eachother.(a) Explain this

observation.


Because same end of the ends of two rods have the same type of magnetism, thereforerepel one another,

Q. 4 Nov 1990

4.(c) Explain how, using suitable equipment.(i) a direct current may be used to magnetise a

magnet. [2]

Wind insulated copperwire round and round a large iron nail. Then connect the ends of wire to a battery (d.c.). After few seconds, the nailwill magnetize and able to pick up iron nails.

(continue in next slide)

4.(c)(ii) an alternating current may be used todemagnetise a magnet. [2]

(Cont. …) Q. 4 Nov 1990

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.

Documents

Unit 11.03: Electromagnetssuccesstutor.weebly.com/uploads/9/4/1/9/941932/electromagnets.pdf · •An electromagnet is a magnet that operates on electricity. • It consists of a solenoid