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Introduction to Magnetism

In form 3, we learned that

1. a magnet can attract certain type of metal.

2. the metals that can be attracted by a magnet are called the “magnetic materials” of

“ferromagnetic materials”. Examples of magnetic materials are iron, steel, nickel and

cobalt.

3. a magnet has 2 poles-the North Pole and the South Pole.

4. there is a magnetic field surrounding the magnet. A magnetic field is a region in the

surrounding of a magnet which a magnetic material experiences a detectable force.

Magnetic Field Line

(The magnetic field is represented by the magnetic field lines)

1. The magnetic filed of a magnet is represented by the magnetic field lines. The

magnetic field lines flow out from the North pole and flow into the South pole.

2. The distance between the field lines represent the strength of the field, the closer the

field line, the stronger the field. In the diagram, the magnetic field A is stronger than

magnetic field B because the line in magnetic field A is closer.

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Compass in a Magnetic Field

(Figure(a): The pointer of a compass point towards the North pole of a magnet)

(Figure(b): The direction of the pointer of a magnet is always in the same direction of the

magnetic field)

1. The pattern and the direction of a magnetic field can be determined by a compass.

2. First of all, we need to know that, in SPM, normally we use a circle with an arrow to

represent compass. The arrow represents the pointer of a compass and it always points

towards the North pole of a magnet.

3. Second, we also need to know that the pointer of a compass is always in the direction

of the magnetic field.

4. In figure (b) above, we can see that when a few compasses are put near to a bar

magnet, the pointer of the compasses are all in the direction of the magnetic field.

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Electromagnetism and Electromagnet

1. When current passes through a conductor, magnetic field will be generated around the

conductor and the conductor become a magnet. This phenomenon is called

electromagnetism.

2. Since the magnet is produced by electric current, hence it is called the electromagnet.

3. An electromagnet is a type of magnet in which the magnetic field is produced by a

flow of electric current. The magnetic field disappears when the current ceases.

4. The magnetism of an electromagnet is switched on or off using electric current.

5. In short, when current flow through a conductor, magnetic field will be generated.

When the current ceases, the magnetic field disappear.

6.

An electromagnet can be made by sending an electric current through a coil of

wire wound around an iron core.

When a current flows through the coil, it produces a magnetic field. The soft iron

core becomes temporarily magnetized when the current is switched on. When the current is switched off, it loses its magnetism.

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Magnetics Effects of a Current Carrying Conductor - Straight Wire

Magnetic Field Pattern

(Figure (a))

1. The magnetic field generated by a straight wire are concentric circles around the wire

as shown in figure (a) above.

2. Take notes that when the direction of the current is reversed, the direction of the

magnetic field line is also reversed.

3. The direction of the magnetic field line can be determined by the Maxwell's Screw

Rule or the Right Hand Grip Rule.

(Figure (b): The plan view of the magnetic field generated by a straight wire)

4. Sometime, the magnetic field pattern may be given in plan view, as shown in figure

(b).

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5. In plan view, a dot in the wire shows the current coming out from the plane whereas a

cross in the wire shows the current moving into the plane.

(Figure (c): A dot indicates the current move out from a plane whereas a cross indicates the

current move into the plane)

Direction of the Magnetic Field

The direction of the magnetic field formed by a current carrying straight wire can be

determined by the

1. Right Hand Grip Rule or the

2. Maxwell Screw Rule.

Right Hand Grip Rule

Grip the wire with the right hand, with the thumb pointing along the direction of the current.

The other fingers give the direction of the magnetic field around the wire. This is illustrated

in the figure below.

(Figure (d))

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The Maxwell's Screw Rules

The Maxwell Screw Rules sometime is also called the Maxwell's Corkscrew Rule. Imagine a

right handed screw being turn so that it bores its way in the direction of the current in the

wire. The direction of rotation gives the direction of the magnetic field.

(Figure (e))

Strength of the Magnetic Field

1. The strength of the magnetic field form by a current carrying conductor depends on

the magnitude of the current.

2. A stronger current will produce a stronger magnetic field around the wire as shown in

Figure (f) below.

(Figure (f))

3. The strength of the field decreases out as you move further out. This is illustrated in

figure (g) below. Thus, you must be very careful when you are asked to draw the

magnetic field in your exam.

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(Figure (g)

4. The distance of the field lines must increase as it is further out form the wire.

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Magnetic Effects of a Current-Carrying Conductor - Flat Coil

Field Pattern

1. Figure (a) below shows the field pattern produced by a current flowing in a circular

coil.

2. In SPM, you need to know the field pattern, the direction of the field and the factors

affect the strength of the field.

3. The direction of the field can be determined by the Right Hand Grip Rule. Grip the

wire at one side of the coil with your right hand, with thumb pointing along the

direction of the current. Your other fingers will be pointing in the direction of the field.

Figure (a)

4. Figure (b) shows the plan view of the field pattern.

Factors affecting the strength

There are 3 ways to increase the strength of the magnetic field:

1. increase the current and

2. increase the number of turns of the coil.

3. use coil with smaller radius

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Magnetic Effects of a Current-Carrying Conductor - Solenoid

Magnetic Field Pattern

1. Figure (a) illustrates the field pattern produced by a solenoid when current pass

through it.

2. The field lines in the solenoid are close to each other, indicates that the magnetic field

is stronger inside the solenoid.

3. We can also see that the field lines are parallel inside the solenoid. This shows that the

strength of the magnetic filed is about uniform inside the solenoid.

4. We can also see that the magnetic field of a solenoid resembles that of the long bar

magnet, and it behaves as if it has a North Pole at one end and a South Pole at the

other.

(Figure (a): Magnetic field pattern of a solenoid)

Determining the Pole of the Magnetic Field

1. The pole of the magnetic field of a solenoid can be determined by the Right Hand

Grip Rule.

2. Imagine your right-hand gripping the coil of the solenoid such that your fingers point

the same way as the current. Your thumb then points in the direction of the field.

3. Since the magnetic field lines always come out from the North Pole, hence the thumb

points towards the North Pole.

[Figure (b)]

Strength of the Magnetic Field

The strength of the magnetic field can be increased by

1. increasing the current,

2. increasing the number of turns per unit length of the solenoid,

3. using a soft-iron core within the solenoid

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Uses of Electromagnet - Electric Bell

1. When the switch is on, the circuit is completed and current flows.

2. The electromagnet becomes magnetised and hence attracts the soft-iron armature and

at the same time pull the hammer to strike the gong. This enables the hammer to strike

the gong.

3. As soon as the hammer moves towards the gong, the circuit is broken. The current

stops flowing and the electromagnet loses its magnetism. This causes the spring to

pull back the armature and reconnect the circuit again.

4. When the circuit is connected, the electromagnet regain its magnetism and pull the

armature and hence the hammer to strike the gong again.

5. This cycle repeats and the bell rings continuously.

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Uses of Electromagnet - Electromagnetic Relay

1. A relay is an electrical switch that opens and closes under the control of another

electrical circuit.

2. The switch is operated by an electromagnet to open or close one or many sets of

contacts.

3. A relay has at least two circuits. One circuit can be used to control another circuit.

The 1st circuit (input circuit) supplies current to the electromagnet.

4. When the switch is close, the electromagnet is magnetised and attracts one end of the

iron armature.

5. The armature is then closes the contacts (2nd switch) and allows current flows in the

second circuit.

6. When the 1st switch is open again, the current to the electromagnet is cut, the

electromagnet loses its magnetism and the 2nd switch is opened. Thus current stop to

flow in the 2nd circuit.

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Uses of Electromagnet - Circuit Breaker

1. Figure above shows the structure of a circuit breaker.

2. A circuit breaker is an automatic switch that cut off current in a circuit when the

current become too large.

3. When the current in a circuit increases, the strength of the electromagnet will increase

in accordance; this will pull the soft iron armature towards the electromagnet.

4. As a result, the spring pulls apart the contact and disconnects the circuit immediately,

and the current stop to flow.

5. We can reconnect the circuit by using the reset button. The reset button can be pushed

to bring the contact back to its original position to reconnect the circuit.

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Uses of Electromagnet - Telephone Earpiece

1. An electromagnet is used in the earpiece of a telephone. The figure shows the simple

structure of a telephone earpiece.

2. When you speak to a friend through the telephone, your sound will be converted into

electric current by the mouthpiece of the telephone.

3. The current produced is a varying current and the frequency of the current will be the

same as the frequency of your sound.

4. The current will be sent to the earpiece of the telephone of your friend.

5. When the current passes through the solenoid, the iron core is magnetised. The

strength of the magnetic field changes according to the varying current.

6. When the current is high, the magnetic field will become stronger and when the

current is low, the magnetic field become weaker.

7. The soft-iron diaphragm is pulled by the electromagnet and vibrates at the frequency

of the varying current. The air around the diaphragm is stretched and compressed and

produces sound wave.

8. The frequency of the sound produced in the telephone earpiece will be the same as

your sound.

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Force on a Current Carrying Conductor in a Magnetic Field

1. We have learned that when current flows in a conductor, a magnetic field will be

generated.

2. When the current-carrying conductor is placed in a magnetic field, the interaction

between the two magnetic fields will produce a resultant field known as the catapult

field as shown in the figure below.

3. The catapult field is a non-uniform field where the field at one side is stronger than

the other side.

4. As a result, a force is produced to move the current carrying conductor from the

stronger field to the weaker field.

5. The force produced by a catapult field is called the catapult force.

6. The direction of the force can be determined by Fleming's left hand rule as shown in

Figure below.

7. The fore finger, middle finger and the thumb are perpendicularly to each other. The

forefinger points along the direction of the magnetic field, middle finger points in the

current direction and the thumb points along the direction of the force.

8. The strength of the force can be increased by:

a. Increase the current

b. Using a stronger magnet

c. using a longer wire

d. arranging the wire perpendicular to the direction of the magnetic field.

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Explain the factors that affect the

magnitude of the force on a current-

carrying conductor in a magnetic field.

The magnitude of the force on a current-

carrying conductor in a magnetic field

depends on:

1. the size of the current in the

conductor

2. the strength of the magnetic field

The current can be increased by: 1. Increasing the e.m.f. of the power

supply

2. Using a thicker wire of the same

length

3. Using a shorter wire

A stronger magnetic field can be

produced by:

1. Using more powerful magnets

2. Using two pairs of magnets with

like poles side by side

3. Placing the magnets closer to

each other to narrow the gap

between the poles of the magnet

4. Longer rod (conductor)