SP12 - Magnetism And The Motor Effect

Question 1

What is a permanent magnet?

Answer 1

An object which always produces a magnetic field (e.g. a bar magnet)

Question 2

What metals are magnetic?

Answer 2

Iron, steel, nickel and cobalt

Question 3

What is a magnetic field?

Answer 3

The space around a magnet where it can attract or repel other magnets/magnetic materials

Question 4

What is an induced magnet?

Answer 4

When a piece of magnetic material is placed in a magnetic field it becomes a magnet itself (induced magnet). It stops being magnetic when it is taken out of the field again. (e.g. chain of paper clips underneath a bar magnet being held up)

Question 5

How can the shape of magnetic field be drawn?

Answer 5

Plotting compasses can be used to show the direction of the field lines, which can then be drawn. Iron filings also show the layout of the field lines, but they do not show direction.

Question 6

Describe the shape of the magnetic field around a bar magnet.

Answer 6

Field lines move in ellipticals from north to south, with the lines being closer together at the poles (indicating a stronger field)

Question 7

Describe the shape of the magnetic field of a solenoid (coil of wire).

Answer 7

Inside the solenoid, the magnetic field is uniform and very strong (shown by close together parallel lines). Outside the solenoid, the field lines are similar to a bar magnet (elliptical shape, north to south, strongest at poles).

Question 8

Describe the shape of the Earth’s magnetic field.

Answer 8

Similar to that of a bar magnet (field lines move in an elliptical shape from North to South, lines closest together at poles where the field is strongest).

Question 9

How do compasses work?

Answer 9

The needle of a compass is a small magnet which is weighted to keep it level. The needle is attracted to the magnetic pole, so it points North. Compasses do not work when placed near strong magnets, as the compass responds to the magnetic field of the magnet rather than of the Earth.

Question 10

Why does the Earth have a magnetic field?

Answer 10

It is thought to be caused by electric currents in the molten outer core which is made of a mixture of iron and nickel.

Question 11

What is the shape of the magnetic field around a current carrying wire?

Answer 11

Circular, with field lines becoming further apart the further you get from the wire.

Question 12

What is Fleming’s right hand rule and how can it be used?

Answer 12

Fleming’s right hand rule is a trick to work out the direction of the magnetic field in a current carrying wire. You have to make a fist with your right hand and stick your thumb out. The direction of your thumb is the direction of the current, while the direction of your fingers is the direction of the magnetic field?

Question 13

What symbols are used to show the direction of the current in a right hand rule question?

Answer 13

Circle with a filled in circle inside - current flowing out of the paper (towards you)
Circle with a circle with a cross inside - current flowing into the paper (away from you)

Question 14

What is an electromagnet?

Answer 14

A coil of wire with a current flowing through it.

Question 15

How can the strength of an electromagnet be increased?

Answer 15

Put a piece of iron (an iron core) inside the coil. The iron becomes a temporary magnet - it is only magnetic while the field from the electromagnet is affecting it.

Question 16

What is the motor effect?

Answer 16

When a wire carrying a current experiences a force when it is placed between two magnets.

Question 17

Why does the motor effect occur?

Answer 17

The current in the wire creates a magnetic field around the wire which interacts with the magnetic field between the magnets, causing a force to be experienced by the wire.

Question 18

When is the force greatest during the motor effect and when is it zero?

Answer 18

The force is greatest when the wire is at right angles to the magnetic field produced by magnets, and is zero when the wire is in the same direction as this magnetic field.

Question 19

What is Fleming’s left hand rule and how can it be used?

Answer 19

Fleming’s left hand rule is a trick to work out the direction of the force experienced by a wire during the motor effect. You have to place you thumb and first two fingers at right angles to each other. The thumb is the direction of the force, the first finger is the direction of the magnetic field, the second finger is the direction of the current (remember FBI).

Question 20

What is the equation for force in the motor effect?

Answer 20

Force on conductor carrying current at right angles to magnetic field (N) = magnetic field strength/magnetic flux density (N/Am or T) x current (A) x length of wire (m)

F = B x I x L

Question 21

How do electric motors work?

Answer 21

A coil of wire has a current supplied to it, causing it to have a magnetic field. It is placed in a magnetic field between two permanent magnets, so the magnetic fields interact, resulting in the wire experiencing a force. The current is moving in a different direction on each side of the coil, so one side experiences an upward force, while the other experiences a downward one. The split-ring commentator changes the direction of the current every half turn so that the wire rotates all the way round (otherwise it would flip back and forth due to each side of the coil always experiencing an upwards or downwards force). Carbon brushes are used as electrical contacts between the circuit and the motor.