Topic 7 Magnetism

Poles of a magnetic

• The poles of a magnet are the places where the magnetic forces are strongest

• When two magnets are brought close together they exert a force on each other

• Two like poles repel each other. Two unlike poles attract each other

• Attraction and repulsion between two magnetic poles are examples of non-contact force

What is a permanent magnet?

A permanent magnet produces its own magnetic field and remains magnetic without needing an external magnetic field

What is an induced magnet?

An induced magnet is a material that becomes magnetic when placed in a magnetic field. It loses most or all of its magnetism when the magnetic field is removed

What happens when like poles of two permanent magnets are brought together?

Like poles repel each other

• North–North → Repel

• South–South → Repel

What happens when unlike poles of two permanent magnets are brought together?

Unlike poles attract each other

• North–South → Attract

How can you compare permanent and induced magnets?

• Permanent magnet: Produces its own magnetic field and stays magnetic

• Induced magnet: Only becomes magnetic in a magnetic field and quickly loses its magnetism when the field is removed

• Induced magnets always attract a magnet

What are the four types of magnetic material?

• Iron

• Steel (alloy of iron)

• Cobalt

• Nickel

These materials can be made into a permanent or induced magnet

What is a magnetic field?

A magnetic field is the region around a magnet where a force acts on another magnet or on magnetic materials (iron, steel, cobalt, nickel)

What happens when a magnetic material is placed in a magnetic field?

It is always attracted to the magnet (the force is always attraction, not repulsion)

How does magnetic field strength change with distance?

The magnetic field is strongest close to the magnet, especially at the poles, and becomes weaker as distance increases

How can you plot a magnetic field pattern using a compass?

• Place the compass near the north pole of a bar magnet

• Mark the position of the north end of the compass needle (e.g. with a dot or cross)

• Move the compass so that the south end of the needle is placed on the previous mark

• Mark the new position of the north end of the compass needle again

• Repeat this process to form a series of points

• Join the points smoothly to form a magnetic field line

• Show the direction of the field line using an arrow (direction always runs from North pole to South pole)

• Repeat from different starting positions to map the full magnetic field pattern

What is the direction of a magnetic field line?

The direction is the force on a north pole placed in the field. Field lines go from the north pole to the south pole of a magnet

What does the spacing of magnetic field lines tell you?

The closer the field lines are together (high concentration of field lines), the stronger the magnetic field

Where is the magnetic field strongest on a bar magnet?

At the poles, where the field lines are closest together

What does a compass tell us about Earth’s magnetic field?

• A compass needle aligns north–south, showing that Earth has its own magnetic field

• It suggests that Earth has a magnetic source in its core, meaning the core must be magnetic or contain moving magnetic material

What happens when current flows through a wire?

• A magnetic field is produced around the wire

• Conventional current flows from + to -

How can you show that a current-carrying wire produces a magnetic field?

Use a compass:

• Current OFF → needle points N–S (Earth’s field)

• Current ON → needle deflects, showing a new magnetic field is present

What affects the strength of the magnetic field around a wire?

• Size of current (I) → bigger current = stronger field

• Distance from wire → closer = stronger field

What happens to the magnetic field if current direction is reversed?

The direction of the magnetic field also reverses (shown by opposite compass deflection)

What rule is used to find the direction of the magnetic field around a wire?

The Right-Hand Grip Rule:

Do a thumbs up

• Thumb = direction of current (points to North Pole)

• Fingers = direction of magnetic field lines (direction on current)

What is a solenoid?

A coil of wire that produces a strong, uniform magnetic field when current flows through it

How does a solenoid’s magnetic field compare to a bar magnet?

It has a similar shape to a bar magnet, with clear north and south poles

What is an electromagnet?

A solenoid with an iron core, which greatly increases magnetic field strength

How do you find the poles of a solenoid?

Use the Right-Hand Grip Rule:

• Fingers curl in direction of current

• Thumb points to the North pole

How can you increase the strength of a solenoid’s magnetic field?

• Increase current (I)

• Increase number of turns / coils (N)

• Add an iron core

What is a solenoid containing an iron core called?

An electromagnet

Why are electromagnets useful?

• Because the strength of the magnetic field can be changed by changing the size of the current

• An electromagnet can be turned on or off

Why does an iron core increase the magnetic field?

It concentrates magnetic field lines, making the field much stronger

What is the motor effect

The motor effect is the force exerted on a current-carrying conductor placed in a magnetic field

• The magnetic field around the wire interacts with the external magnetic field

• This causes the wire and magnet to exert forces on each other

What conditions are needed for the motor effect to occur?

You need:

1. A current-carrying conductor

2. An external magnetic field

The force is greatest when the conductor is at right angles (90°) to the magnetic field

What is the unit for magnetic flux density?

Tesla (T)

What happens if the conductor is parallel to the magnetic field?

No force acts on the conductor, so it does not move

Exam fact: Force is only produced when the conductor has a component at right angles to the magnetic field

What is magnetic flux density?

Magnetic flux density (B) is a measure of the strength of a magnetic field

What three factors affect the size of the force in the motor effec

The force increases when:

• Magnetic flux density (B) increases

• Current (I) increases

• Length of conductor in the magnetic field (l) increases

How do you use Fleming's Left-Hand Rule?

Hold your left hand so the thumb, first finger and second finger are at right angles

• First finger = magnetic Field (N → S)

• Second finger = Current (+ → −)

• Thumb = Force (motion)

What does Fleming's Left-Hand Rule show?

It shows the relative directions of:

• The magnetic field

• The current

• The force on the conductor

What happens if the current direction is reversed?

The direction of the force reverses, so the conductor moves in the opposite direction

What is the basis of an electric motor?

• The motor effect

• When a current-carrying conductor is placed in a magnetic field, it experiences a force. In an electric motor, these forces cause a coil of wire to rotate

Why is the motor effect important?

The motor effect is the principle behind electric motors, where forces on current-carrying coils cause rotation

Why does a coil of wire rotate in a magnetic field?

urrent flows in opposite directions on opposite sides of the coil.

This causes:

• An upward force on one side

• A downward force on the other side

The opposing forces create a turning effect (torque/moment), causing the coil to rotate

How can Fleming's Left-Hand Rule be used in an electric motor?

It is used to determine the direction of the force on each side of the current-carrying coil

• First finger = magnetic Field

• Second finger = Current

• Thumb = Force

Why would a simple motor stop rotating?

• When the coil reaches about 90°, the forces would reverse direction

• This would push the coil back towards its original position, preventing continuous rotation

What is a split-ring commutator?

A split-ring commutator is a split metal ring connected to the coil that reverses the current direction every half turn

Why is the split-ring commutator important?

• It reverses the current just as the coil passes 90°

• This keeps the forces acting in the same rotational direction, allowing the motor to continue spinning

What is the role of the brushes in an electric motor?

Brushes:

• Maintain electrical contact with the rotating split-ring commutator

• Allow current to flow into and out of the coil

What happens when the current reverses in the split-ring commutator?

• The forces on the sides of the coil also reverse

• This means the turning effect continues in the same direction, producing continuous rotation

What is meant by torque (moment)?

• Torque is the turning effect of a force

• In an electric motor, the upward and downward forces on opposite sides of the coil create a torque that rotates the coil

Explain how the force on a conductor in a magnetic field causes rotation in an electric motor. (6-mark style)

• A current flows through a coil placed in a magnetic field

• Due to the motor effect, opposite sides of the coil experience forces in opposite directions

• These forces create a turning effect (torque), causing the coil to rotate

• A split-ring commutator reverses the current every half turn so that the forces continue to act in the same rotational direction

• This allows the coil to rotate continuously