magnetism + electromagnetism

Describe the attraction and repulsion between the poles of a magnet

- Opposite poles attract

- i.e. south pole is attracted to north pole

- Similar poles repel

What are permanent magnets made from?

- Magnetically hard materials

- e.g. steel

- Keep magnetism once they have been magnetised

How is a magnetically hard material magnetised?

- Placed in a magnetic field

What is a magnetically soft material?

- Loses magnetism easily

- Forms temporary magnets

- e.g. iron

What is the difference between steel and iron’s magnetism?

- Steel → hard magnetic material → retains magnetism

- Iron → soft magnetic material → loses magnetism

Define magnetic field line

- Space around a magnet where magnetism can be detected

Where is the magnetic field around a bar magnet strongest?

- Strongest at the poles

- Magnetic field lines closest together

Where is the magnetic field around a bar magnet weakest?

- Weaker as distance from the magnet increases

- Magnetic field lines get further apart

How may an objects’ magnetism be induced (created)?

- Place a magnetic object (e.g. one made from iron) into a magnetic field

- Object becomes a magnet (magnetism has been induced)

- Magnetism is temporary as object loses its magnetism when it’s removed from magnetic field

How can magnetic field lines be visualised using plotting compasses?

- Place plotting compass at end of magnet

- Mark position of end of compass

- Place compass in new position, mark and repeat

- Link marks together to find the field line pattern

How can magnetic field lines be visualised using iron filings?

- Place magnet underneath sheet of paper

- Sprinkle iron fillings over paper

- Tap paper gently

- Iron filings will move to form field lines

What are the properties of a uniform magnetic field?

- Field lines are parallel

- Field lines are evenly spaced

Explain how to produce a uniform magnetic field

- Use two permanent magnets

- Place magnets close together

- Arrange with North facing South

What happens when an electric current is passed through a conductor (e.g. a wire)?

- A magnetic field is produced

How can the magnetic field around a current carrying wire be made stronger?

- Increasing current

- Wrapping the wire into a coil (solenoid)

- Add an iron core

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

- Increasing the number of turns in the solenoid

- Increasing the current

Explain why a force is experienced by a wire when a current flows through it

- A current flows through the wire creating a magnetic field around the wire

- This temporary magnetic field interacts with the permanent magnetic field of the bar magnets

- A force is created

Describe how a motor works

- A current flows through the coil

- Creates a magnetic field around the wire

- Temporary magnetic field interacts with a permanent magnetic field

- Produces a force on the wires

- Forces on opposite sides of the coil are in opposite directions

- A commutator reverses the direction of current every half turn

- Allows motor to keep spinning

How may the motor be made to spin faster?

- Increase the current

- Increase the number of turns of wire

- Increasing the strength of the magnetic field

Describe how a loudspeaker converts an alternating current into a sound wave

- Current passed through coil in loudspeaker

- Produces magnetic field

- Interaction between induced magnetic field and permanent magnetic field

- Causes a force to be exerted on speaker

- Alternating current changes direction

- Force on speaker changes direction

- Causes speaker to vibrate

How can the force on the loudspeaker be increased?

- Increase the current

- Increase the number of turns of wire

- Increasing the strength of the magnetic field

How can the direction of the force on the wire be predicted when a wire carries a current perpendicular to a magnetic field?

- Use Fleming’s left-hand rule

Describe how voltage (or current) may be induced

- If a wire is moved across a magnetic field at right angles

- A voltage is induced

- If the wire is attached to a complete circuit, a current is induced

- This is electromagnetic induction

Explain how a generator produces a voltage

- Rotate coil of wire in magnetic field

- Coil cuts through magnetic field

- Current (and voltage) is induced

How may the size of the induced voltage (and current) be increased?

- Wrapping the wire into a coil (or increasing number of coils)

- Using a stronger magnet

- Moving the wire more quickly

Describe how a dynamo works in a bicycle light

The core gains a magnetic field
Magnetic field changes directions
Field lines cut by wire
Inducing a voltage across the coil
Causing a current in the wire