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ELECTROMAGNETISM LEARNING OUTCOMES

Draw the field pattern of the magnetic field due to currents in straight wires and in solenoids

State qualitative variation of the strength of magnetic field over salient parts of the pattern

State effect on the magnetic field of changing the magnitude and/or direction of the current

Describe application of the magnetic effect of a current in an electric bell and circuit breaker

Describe experiments to show the forces on a current-carrying conductor, a beam of charged

particles and in a magnetic field including the effect of reversing

o The current

o Direction of the field

State the relative directions of force, field and current when any two of these quantities are at

right angles to each other using Fleming’s Left Hand Rule

Explain how a current carrying coil in a magnetic field experiences a turning effect and that the

effect is increased by increasing

o The number of turns on the coils

o The current

Discuss how this turning effect is used in the action of an electric motor

Describe the action of a split-ring commutator in a two-pole, single coil motor and the effect of

winding the coil onto a soft-iron cylinder

FUNDAMENTALS OF MAGNETISM

Why is repulsion the only sure test for magnetism?

How can a compass be used as a navigational tool? What is the principle behind its

operation?

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The variation of a magnet/the Earth’s magnetic

field strength can be described by its

_____________________________________

When placed in a magnetic field, the north

seeking end of a compass needle will

___________ itself to the magnetic ________

at that point

Recall the significance of

The spacing between the field lines

Direction of field lines

Does the magnetic field strength of the Earth vary in a similar fashion to that of a bar

magnet?

There is no magnetic attraction at the middle of a bar magnet but the Earth’s equator is

still able to attract/deflect a compass needle. Why?

What is the source of the Earth’s magnetism? Why do some substances exhibit

magnetic properties?

Magnetism is a fundamental property of matter caused by ____________________________

__________________________ (hence its association with electricity)

________________________________________________are magnetic as a result of their

atomic configuration

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SHORT QUIZ

1. Sketch the magnetic field pattern between the ends of 2 magnets with like and unlike

poles facing one another [4]

2. From the magnetic field patterns, account for the fact that

a) When two equally strong magnets are placed with opposite poles facing each other,

magnetic field between poles become stronger [2]

b) If the poles are of the same polarity, magnetic effects are cancelled out and a

neutral point is a place where resultant magnetic field strength is zero [2]

Write down a few aspects of magnetism

that you already know prior to the

lesson.

Write down 2 other questions /aspects

about the nature or the application of

magnetism for discussion.

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3. By sketching the magnetic field through it, explain why the soft iron bar is attracted to

the permanent magnet.

Magnetic field due to permanent magnet is ____________________ through the iron rod,

causing it to become an _______________

The end of the induced magnet nearer to the permanent magnet has ___________________

____________________ to that of the permanent magnet

________________ occurs between unlike poles of permanent and induced magnet

4. Two steel needles are hanging from the ends of a magnet as shown above. Why are the

needles are leaning towards each other?

5. Explain what is meant by the term ‘Magnetic Field Strength’?

6. By sketching the magnetic field through it, explain why the soft iron shields point X

from the magnetic effects of the permanent magnet. In what situations are magnetic

shielding required?

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METHODS OF MAGNETISATION (REVISION/SELF-STUDY)

1. Heating and hammering a steel bar to make a weak magnet due to the effect of the earth’s

magnetic field:

a. place steel bar in a north-south direction

b. heat it to a high temperature

c. hammer while it cools

2. Stroke the steel bar several times in the same direction along its length

a. The end of the steel bar where the strokes end always has the opposite polarity to that

of the stroking magnet’s end in contact

b. What are the poles at the 2 ends of the induced magnet?

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3. *Using the magnetic effect of an electric current (direct current)

a. pass a direct electric current through a solenoid

b. current flowing through the wire magnetizes the steel bar

METHODS OF DEMAGNETISATION (REVISION/SELF-STUDY)

1. Hammering

a. Place the magnet in the east-west direction

b. Hammer the magnet vigorously until it demagnetises

2. Heating

a. Heat the magnet until it becomes red-hot

b. Place the magnet in the east-west direction to cool until it demagnetises

3. *Using an electric (alternating current)

a. Strong magnetic field produced when the alternating current passes through the

solenoid magnetises the steel bar in opposite directions

b. Withdraw the magnet slowly in an east-west direction further and further away until it

becomes demagnetised

MAGNETIC EFFECT OF AN ELECTRIC CURRENT

How would you increase the magnetic field strength of an electromagnet?

Increase the _________________________ through the solenoid – ___________________

in the solenoid produces a larger magnetic effect

Insert a __________________ into the solenoid – ________________ the magnetic field

Increase the _________________________ in the solenoid – ________________ produces

its own magnetic effect

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What are the similarities/differences between the magnetic field patterns of a

permanent bar magnet and an electromagnet (e.g. current-carrying coil)

Direction of the magnetic field lines can be determined by a plotting compass or right hand

grip rule.

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Circular magnetic field pattern of a current carrying wire can be verified using a plotting

compass

A compass is used to investigate the magnetic field around a current-carrying wire. Where

would the compass needle point when placed in the following positions?

What are the uses of electromagnets?

Magnetic Relay

Magnetic relay used in starter motor of cars

What is the advantage of using a magnetic relay?

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Electric Bell

Why does the electric bell ring continuously until the switch is open?

Circuit Breaker

What adjustments should be made to the circuit breaker to reduce the maximum

permitted current through the household circuit?

How can the circuit breaker be reset?

What materials are used to make permanent magnets and temporary magnets? What

makes these materials suitable?

Iron, easier to _________________ but loses magnetism ___________, called __________

magnetic material

Steel, harder to magnetise but does not lose magnetism easily, called __________ magnetic

material

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In which applications are permanent magnets preferred over electromagnets and what

are the situations in which electromagnets are more suitable?

FORCE ON A CURRENT-CARRYING CONDUCTOR

Interaction between magnetic field of 2 current-carrying wires

Conductors with current flowing in the same direction attract

Conductors with currents flowing in the opposite direction repel

Which attractive force, F1 or F2 is larger?

Explain in terms of their magnetic field interactions, why the two wires experience an

attractive force.

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Interaction between magnetic field of a permanent magnet and an electromagnet

(current-carrying wire)

Explain in terms of magnetic field interactions, why the current-carrying rod placed on the

rails experience a force to the left/right.

Sketch the magnetic field pattern due to current-carrying rod, the permanent magnets and

their combined magnetic field

The two fields acting in the same direction combine to give a stronger field, but the two

fields opposing each other combine to give a weaker field.

The __________________ fields on both sides exert produce a __________ that exerts

on the wire.

Flemming’s Left Hand Rule

Can be used to predict that (but not explain why) the force on the current-carrying wire

acts downwards

Note that the directions of Force, Field and Current are mutually perpendicular

Flemming’s Left hand Rule can be used to determine the direction of force acting on

conductor when the current and magnetic field are perpendicular to each other but NOT

when they are parallel

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Force on a beam of charged particles

In what direction should an electric and magnetic field be directed to deflect a beam of

electrons upwards?

How is the path taken by the electrons when deflected upwards by a magnetic and electric

field different?

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The beam is deflected to move in a _______________ path

This is because the moving charges experience a force ________________ to its velocity

What is the effect of a magnetic field on a stationary charge?

DC MOTOR

Basic components in electric fans, hair dryers and many other electrical appliances

Makes use of the ______________________ of a current-carrying coil in magnetic field to

convert electrical energy to mechanical (kinetic) energy

Works on direct current

Rectangular loop of wire is mounted on an axle (represented by the AB) such that it can rotate

freely about the axis AB

A DC supply is connected to it

It is placed between the poles of a powerful magnet

Explain in terms of the magnetic field interactions between the current-carrying wires in the

coil and the permanent magnets, why the coil turns anti-clockwise about the axis AB

A catapult field is produced when the field produced by the coil interacts with the field of

the permanent magnets

The two forces produce a ____________________, which turns the loop anti-clockwise

about the axis PQ

Left side of the coil experiences a ___________ force and the right-hand side experiences

an equal _________ force, causing the coil to rotate _________ about the axis AB

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Generally, for continuous clockwise rotation of the coil,

the current must _______________ direction each time the coil passes thru the vertical

The right hand side wire must always carry current __________ the loop and always

connected to the __________ terminal of the D.C. supply

The left hand side wire must carry current __________ the loop and always connected to

the _________ terminal of the D.C. supply

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The ends of the coil of wire are connected to a _____________________________.

Two ______________________ are made to press lightly against the commutator.

Once the loop passes the vertical position, the current direction is reversed as the commutator

________________________________________________________________.

This ensures that the loop turns in one direction

Summary of DC motor principles (for self-study)

- Coil spins on axle between poles of permanent magnet

- Ends of coil connected to a split-ring, commutator

- Commutator turns with coil, rubs against carbon brushes

- When circuit is closed, current flows from battery through coil and back to battery

- Left side of coil experiences downward force

- Right side experiences an equal upward force

- Pair of forces makes coil rotate anticlockwise until it’s vertical

- Current cut off because neither commutator in contact

- But momentum of coil will carry it slightly beyond this vertical position

- Half-ring Y will touch P

- X comes into contact with Q

- Same as start except commutators exchange places

- Turning forces will act

- Coil rotates in same direction

- Direction of current in coil reverses each time coil passes vertical position

- If a soft iron cylinder is placed between the curved poles of the magnet, radial field created

- Keeps pair of forces acting on coil almost constant as it turns

- Increases magnetic field strength and turning effect

When the coil is in the vertical position, it is not in contact with the terminals of the DC source

via the split ring commutator and carbon brushes. No current passes through it and hence no

force acts on it. Why does it continue to turn pass this vertical position despite the absence of

forces?

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What adjustments should be made to the motor to increase its turning effect?

increasing the __________________________

increasing the __________________________

inserting a ________________________ within the coil to ________________ the

magnetic lines of force

SUMMARY OF KEY CONCEPTS

1. Like poles attract each other. Unlike poles repel each other

2. Iron, cobalt, nickel and their alloys are magnetic materials

3. Only repulsion can confirm that a specimen is a magnet

4. A magnetic material becomes an induced magnet when placed near a permanent magnet

5. The end of the induced magnet nearer the permanent magnet has an opposite polarity to that

of the permanent magnet

6. Strong magnets are usually made by putting a piece of magnetic material inside a solenoid

carrying a direct current

7. A magnet can be demagnetised by putting it in a solenoid carrying an alternating current and

then withdrawing it

8. Iron, a soft magnetic material, is easier to magnetise but loses it magnetism easily

9. Steel, a hard magnetic material, is harder to magnetise but does not lose its magnetism easily

10. The hard magnetic materials such as steel are used in the making of permanent magnets

11. The soft magnetic materials such as soft iron are used as temporary magnets

12. A straight current carrying wire produces a circular magnetic field. The further away from the

wire, the weaker is the magnetic field

13. Reversing the current will reverse the direction of the magnetic field

14. A solenoid has a magnetic field similar to that of a bar magnet

15. The strength of a magnetic field produced by a solenoid can be increased by

- Increasing the magnitude of the current

- Increasing the number of turns of the solenoid

- Inserting a soft iron core into the solenoid

16. Magnetic relays, electric bells and circuit breakers make use of the magnetic effect of current

17. A current-carrying conductor experiences a force when placed in a magnetic field

18. The direction of the force can be deduced by using Fleming’s left-hand rule

19. When either the direction of the magnetic field or the direction of the current is reversed, the

force is also reversed

20. Conductors with currents flowing in the same direction attract. Conductors with currents

flowing in the opposite direction repels

21. A current-carrying coil in a magnetic field experiences a turning effect which can be increased

by - Increasing the number of turns on the coil

- Increasing the magnitude of the current

- Inserting a soft iron core within the coil to concentrate the magnetic lines of force

22. A d.c. motor needs a split ring commutator to reverse the current in the coil