Magnetism and Electromagnetism

Physics Paper 2

Magnet

A material or object that produces a magnetic field.

Field lines from North to South

The closer the field lines, the stronger the magnet.

Magnetic field strongest at the poles.

Like poles repel.

Unlike poles attract.

Permanent magnet

Produces its own magnetic field.

Will always be attracted to a Induced magnet as it induces an opposite pole.

Induced Magnet

Magnetic material or object that becomes a magnet when placed in a magnetic field.

Induces a magnetic field.

Loses magnetism when removed from magnetic field of permanent magnet.

WIll always be attracted to a permanent magnet.

Magnetic material

An object that can be influenced by a magnetic field and has the potential to become a magnet.

Examples: Nickel, Cobalt, Iron, Steel

Magnetically hard materials lose their magnetism slowly e.g Steel

Magnetically soft materials lose their magnetism quickly e.g Iron and Nickel.

Electromagnetism

Electric currents produce their own magnetic fields.

When current flows through a conducting wire, a magnetic field is produced around the wire.

Solenoid

An electromagnet made by coiling up wire and inserting an iron core in the middle with current flowing through it.

Increasing strength of an electromagnet

Increase the current.

Increase the number of turns of coils of wire.

Insert iron core.

Decrease length of solenoid whilst keeping coils of wire the same.

Right hand grip rule

Hold a pen in your right hand.

Thumb shows the direction of current.

Curl your fingers around the pen and this shows the direction of the magnetic field lines.

If current moves upwards, magnetic field moves anti-clockwise.

If current moves downwards, magnetic field moves clockwise.

Motor Effect

When a current - carrying wire is placed in a magnetic field, it experiences a force.

The magnetic field of the wire and the magnetic field of the magnet interacts and causes the wire to experience a force which pushes it out of the field.

Experiencing full force

WIre must be perpendicular (90 degrees) to the magnetic field.

If parallel, no force is experienced.

Fleming's left hand rule

Used to find the direction of force, magnetic field and current.

Thumb - Direction of force.

Index - Direction of magnetic field.

Middle - Direction of current.

Father, Mother, Child

Fingers must be placed perpendicular to one another.

F=BIL

When a wire is perpendicular to the magnetic field, you can calculate the strength of the force using:

F=BIL

F - Force (N)

B - Magnetic Flux Density (T)

I - Current (A)

L - Length of wire (m)

Electric motor - Issue

When a current carrying wire is placed in the magnetic field of a permanent magnet, the wire experiences a force which causes it to rotate - the motor effect.

When it turns by 180 degrees, the wire rotates in the opposite direction as the direction of the force acting on the wire changes to the opposite direction and the wire therefore can’t rotate by 360 degrees which is not useful.

Electric motor - Solution

A split ring commutator is used to resolve the issue.

The positive terminal of the wire is attached to one half and the negative terminal is attached to the other half of the split ring commutator.

Every half turn, the commutator reverses the direction of the current so the direction of the force acting upon the wire remains the same and the wire now moves continuously in the same direction and completes full, 360 degree rotations.

Electric Motor - How it works

A current carrying wire is placed in the magnetic field of a permanent magnet.

The positive terminal of the wire is attached to one half and the negative terminal is attached to the other half of the split ring commutator.

Every half turn, the commutator reverses the direction of the current so the direction of the force on the wire remains the same.

The wire continues to move in the same direction.

The wire completes full, 360 degree rotations.

Increasing speed of electric motor

Increase the current.

Increase number of coils.

Use a more powerful magnet.

Generator Effect

When you move a wire through a magnetic field, it induces a potential difference because the wire experiences a change in magnetic field.

Direction of the potential difference swaps every time you change the direction of movement of wire.

If you join the wire to form a circuit, you can generate a current as electrons are free to move around the circuit.

Increasing potential difference

Change the strength of the magnetic field.

Increase the speed the wire or magnet move at.

Shape wire into a coil.

Alternators

Generates an alternating current,

A slip ring commutator is used so contacts don’t swap every half turn.

Coil moves through magnetic field.

Potential difference is induced.

A complete circuit is formed so current is induced.

Every half turn, the direction of the potential difference is reversed.

So direction of current changes.

Dynamos

Generates a direct current.

A split ring commutator is used so contacts swap every half turn.

Coil moves through magnetic field.

Potential difference is induced.

A complete circuit is formed so current is induced.

Each half - revolution, the two ends of the coil swap from one brush to the other.

So direction of current does not reverse every half rotation.

Loudspeakers (Not in specification)

Converts electrical signals into sound waves using the motor effect.

Has a cone and coil of wire connected to it which moves.

Microphones

Converts sound waves into electrical signals using the generator effect.

Has a diaphragm which is attached to a coil of wire that both move.

Sound waves hit the diaphragm.

Causes the diaphragm and coil of wire to move.

As the wire moves with magnetic field of the permanent magnet , it generates a current.

The frequency and amplitude of the sound waves determine how much the diaphragm vibrates and the therefore determine the frequency and amplitude of the induced current.

Transformers

An alternating potential difference is applied across the primary coil.

This causes an alternating current to flow and induces an alternating magnetic field in the primary coil.

The induced alternating magnetic field on the primary coil, induces an alternating magnetic field on the iron core.

The induced alternating magnetic field on the iron core, induces an alternating potential difference across the secondary coil.

If a complete circuit (which it will be in a transformer), it causes a current to flow around the secondary coil.

Role of Transformers

Electricity is transferred from power stations to step-up transformers.

Step-up transformers increase the voltage and decrease the current to reduce energy lost to the surroundings and increase efficiency.

Step-down transformers decreases the voltage, increases the current for safe, domestic use in homes.

In step-up transformers, there are more turns of coil in the secondary coil than the primary coil.

In step-down transformers, there are more turns of coil in the primary coil than the secondary coil.

Transformer calculations

V_p / V_s = n_p / n_s

V - Voltage

N - Number of turns in coil

p - primary coil

s - secondary coil

V_p I_p = V_s I_s

V - Voltage

I - Current

p - primary coil

s - secondary coil