Magnetism

What is a magnet?

An object which always produces a magnetic field

How can we use a compass to find out the poles?

Place a compass at one end and if it points towards the magnetic, that is the south pole.

If it points away, that is the north pole.

What is a permanent magnet?

An object that always produces a magnetic field.

What is an induced magnet? Examples?

A piece of iron, cobalt, nickel, or steel which can be influenced by a magnetic field and have the potential to be a magnet.

What happens when nickel is placed near a bar magnet?

Nickel will be attracted to the magnet and become a temporary magnet.

What materials are magnetically soft?

Iron, nickel.

They lose their magnetism quickly

What material is magnetically hard?

Steel

It loses it’s magnetism slowly.

Describe the right hand rule to find out magnetic field of an object with current.

Fingers wrapped in the direction of magnetic field

Thumb pointing in direction of current

Describe the magnetic field in a coil.

• Same rules apply to a coil.

• Though the magnetic field changes direction from beginning to end of the coil (wire).

• Because current flows down.

What is a solenoid?

A wire with many coils.

Describe the magnetic field on a solenoid.

It acts as would a normal bar magnet would.

Though there are no north or south poles.

Magnetic field would be attracted towards the end of current flow.

How could we increase the strength of a solenoid?

Increase current flow

Increase number of coils, though keep the length the same

Decrease the length, though keep the number of turns the same

Place an iron core inside, to become an induced magnet.

What is the motor effect?

When a current carrying wire is in the presence of a magnetic field, a force on the wire will be experienced.

How can magnetic field strength and current be maximised to give the highest force possible?

Place the wire perpendicular to the magnetic field

Describe flemmings left hand rule

Forefinger - Magnetic field direction

Middle finger - Current direction

Thumb - Force direction (wire pushed out)

How does an electric motor work without a split ring commutator?

There is a DC, so the coil would only make a half turn, not a full turn.

This is because current is always acting in one direction, so the direction of force on each side of the coil would be different.

How does an electric motor work with a split ring commutator?

The split ring commutator causes the positive and negative connections to switch every half turn.

This causes direction of current to switch every half turn, resulting in an AC.

This causes the direction of force on each side of the coil to be the same.

This causes the coil to rotate in one direction.

How can we increase the speed of the rotation in an electric motor?

Increase the number of coils

Increase the magnetic flux density/stronger magnet

Increase the strength of current.

What is electromagnetic induction?

When a coil moves up and down through a magnetic field, inducing an alternating potential difference every time it moves up and down.

When the coil stops moving, there is no more potential difference as there is no change in magnetic field.

What happens when a circuit is placed into a magnetic field?

An alternating current will be induced as the circuit moves up and down.

How can we increase P.D in electromagnetic induction?

Use a stronger magnet, to provide a stronger magnetic field.

Move the circuit/coil up and down more quickly, to increase the speed of change in magnetic field

Increase the number of coils, to increase the strength of magnetism.

What happens when a magnet moves through a solenoid?

As it moves backwards and forwards, an AC would be induced.

How do dynamos work?

A coil is introduced to a magnet, a current will be induced in the coil due to the magnets magnetic field from north to south.

The coil will make a half turn, and the split ring commutator causes the negative and positive connections to switch every half turn.

This means current is always flowing in one direction, along with potential difference.

How do alternators work?

A coil is introduced to a magnet, a current will be induced in the coil due to the magnets magnetic field.

The coil will make a half turn, the slip ring and brushes causes negative and positive contacts to never change, unlike a dynamo, so current alternates every half turn.

How does a loudspeaker work?

Converts electrical signals into sound waves.

An alternating current flows through the coil which induces a magnetic field.

The magnetic field of the coil interacts with the magnetic field of the permanent magnet.

The magnetic field interactions exert a force onto the coil to move back and forth along the permanent magnet, along with the cone as it is attached to the coil.

The current is alternating, causing the magnetic fields and force to alternate, meaning the coil and cone will rapidly move back and forth.

The cone vibrations are so fast that they’ll create pressure variations in the air, which are sound waves.

How do microphones work?

Converts sound waves into electrical current.

Sound waves hit the diaphragm, which is attached to a coil of wire around a permanent magnet.

The sound waves cause the coil of wire to move, causing the diaphragm to move.

An alternating current will be induced in the wire as it moves up and down the magnetic field of the permanent magnet.

The amplitude and frequency of the current is determined based on the how much the diaphragm moves, which is determined based on the amplitude and frequency of sound waves.

How does the national grid work?

Power stations supply electricity.

Step up transformers will increase voltage to 400,000V to minimise energy loss.

This passes through wires and pylons.

Step down transformers will decrease the voltage to 230V to safely be used in homes.

How does a step up transformer work?

An alternating potential difference is supplied to the primary coil.

This induces an alternating current across the coil, which forms an alternating magnetic field around the coil.

This induces an alternating magnetic field in the iron core between both coils.

This induces a potential difference in the secondary coil, which induces an alternating current.

Why doesn’t electricity pass through the iron core in a step up transformer?

The coil’s wires are insulated in plastic