Magnetism and the motor effect

What do like magnetic poles do?

Repel

What do unlike magnetic poles do?

Attract

Where is a magnetic field strongest and how can we see this on a bar magnet diagram?

• At the poles

• The field lines are shown to be closer together and more concentrated

What direction do field lines go on a magnet diagram?

From N to S

What are some magnetic materials?

Cobalt, steel, iron, nickel and magnadur

What is a permanent magnet?

A magnet that has poles which are N and S all the time. They are made from steel or magnadur.

What is a temporary/induced magnet?

A material that can be magnetised by bringing a permanent magnet near it. When the permanent magnet is removed, it loses its magnetism.

What are some uses of permanent magnets?

• Fridge magnets

• Compasses

• Motors and generators

• Loudspeakers

• Door closers on fridges

What are some uses of temporary/induced magnets?

• Electromagnetics

• Circuit breakers

• Electric bells

• Magnetic relays

How do we know that Earth has a magnetic field?

The Earth’s magnetic field has the same pattern as a bar magnet. The behaviour of compasses is proof Earth has a magnetic field

How can you plot the shape and direction of magnetic lines on a magnet?

• Place a bar magnet on a piece of paper

• Put a plotting compass by one pole of the magnet

• Place a dot by the needle that is facing away from the compass

• Move the plotting compass so the other point is by the dot you have just drawn

• Continue doing this until you have mapped all the way around the magnet

How can you tell if a magnetic material is a permanent or temporary magnet?

A temporary magnet will always be attracted by a magnet but a permanent magnetic can both be attracted and repelled

What will an electric current create?

A magnetic field. The shape direction and strength of a magnetic field depends on different factors

What are the factors that affect a magnetic field around a long straight conductor? Mention:

• Shape of the magnetic field

• Direction of the magnetic field

• Strength of the magnetic field

• The shape around this conductor is a series of concentric circles

• The strength depends on:

  • The size of the current in the wire- it is directly proportional to the current

  • The distance from the wire- it is inversely proportional to the distance from the wire

• The direction of the magnetic field depends on the direction of the electric current in the conductor

• If you point the thumb of your right hand in the direction of flow of conventional current (+ to -), the direction your fingers curl will be the direction of the magnetic field

What is the magnetic field of a solenoid like?

• The magnetic lines of the individual coils in the centre of a solenoid add up to give a very strong uniform field

• The field lines cancel to give a weaker field outside of the solenoid

What will happen to a current carrying conductor when placed near a magnet?

It will experience a force due to the interaction of their magnetic fields

What is the motor effect?

• An electric current flowing through a wire has a magnetic field

• If you put a wire into the field of another magnet, the two fields affect each other and the wire experiences a force

When does the maximum force on a wire occur and how can we work out the direction of the force?

• When a current is at right angles to the lines of the magnetic field

• We can work out the direction of the force using Fleming’s left-hand rule

What factors affect the force on a wire between two poles?

• There is no force if the current is parallel to the field lines

• If the direction of either the current or the magnetic field is reversed, the direction of the force is reversed

• The size of the force can be increased by increasing the strength of the magnetic field or increasing the size of the current

How do we find out the direction of the force caused by the motor effect?

Fleming’s left-hand rule

• Your forefinger points in the direction of the magnetic field from the north to south pole

• Your second finger points in the direction of CONVENTIONAL CURRENT (this flows from + to -)

• Your thumb points the direction the force moves

How do you calculate the force on a current carrying wire?

Force (N)= Magnetic flux density (T or N/Am) x current (A) x length of wire (m)

or F= B x I x L

What is the magnetic field of the Earth like?

The geographical north pole is the magnetic south pole and vice versa