Physics: Magnetism

What are the two poles of a magnet?

North pole and south pole.

What happens when two like magnetic poles are brought together?

They repel.

What happens when two unlike magnetic poles are brought together?

They attract.

What is the direction of a magnetic field?

From the north pole to the south pole.

What is the magnetic field pattern around a bar magnet?

Field lines leave the north pole and curve round to enter the south pole.

What is a magnetic field?

A region where a magnetic material or moving charge experiences a force.

What is a hard magnetic material?

A material that is difficult to magnetise but retains its magnetism.

Give an example of a hard magnetic material.

Steel.

What is a soft magnetic material?

A material that is easily magnetised and easily demagnetised.

Give an example of a soft magnetic material.

Iron.

What is induced magnetism?

Temporary magnetism produced when a magnetic material is placed in a magnetic field.

What is the magnetic effect of an electric current?

A current produces a magnetic field around the conductor.

What is the magnetic field pattern around a straight current-carrying wire?

Concentric circles centred on the wire.

How can the direction of the magnetic field around a straight wire be determined?

Using the right-hand grip rule.

What is the magnetic field pattern around a solenoid?

A bar magnet-like field with a strong, nearly uniform field inside.

How can the magnetic field around a solenoid be made stronger?

Increase the current, increase the number of turns per unit length, or insert a soft iron core.

What is the difference between a permanent magnet and an electromagnet?

A permanent magnet produces a constant magnetic field, whereas an electromagnet only produces a field when current flows.

What is the motor effect?

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

Which rule determines the direction of the force on a current-carrying conductor?

Fleming's Left-Hand Rule.

What three factors determine the direction in Fleming's Left-Hand Rule?

Field, current and force.

What factors increase the magnitude of the force on a current-carrying wire?

Greater current, stronger magnetic field and longer wire in the field.

State the equation for the force on a wire at right angles to a magnetic field.

F = BIL.

What does B represent in F = BIL?

Magnetic flux density.

What does I represent in F = BIL?

Current.

What does L represent in F = BIL?

Length of wire in the magnetic field.

How does a simple dc motor produce continuous rotation?

Using a split-ring commutator to reverse the current every half turn.

What is the function of the brushes in a dc motor?

To maintain electrical contact with the rotating coil.

Name one application of an electromagnet.

Relay, scrapyard crane or electric bell.

What is electromagnetic induction?

The production of a voltage when a conductor cuts magnetic field lines or the magnetic field changes.

When is an induced voltage produced?

When there is a change in magnetic flux.

What factors increase the magnitude of an induced voltage?

Faster motion, stronger magnetic field or more turns in the coil.

What determines the direction of an induced voltage?

The direction of motion and the direction of the magnetic field.

How does an ac generator produce alternating current?

A rotating coil in a magnetic field produces an alternating induced voltage.

What happens to the output voltage if the generator rotates faster?

It increases.

What is a step-up transformer?

A transformer that increases voltage.

What is a step-down transformer?

A transformer that decreases voltage.

State the transformer equation.

Vp/Vs = Np/Ns.

What is the power relationship for an ideal transformer?

VpIp = VsIs.

Why is electrical power transmitted at high voltage?

To reduce current and minimise power losses in transmission cables.