Magnetism and electromagnetism

Question 1

Identify some magnetic metals

Answer 1

1. Iron
2. Steel
3. Nickel
4. Cobalt

Question 2

What is a magnet?

Answer 2

It attracts magnetic materials at a distance, such as iron

It has a north and south pole.

Question 3

Describe what happens when the poles of two different magnets are brought together.

Answer 3

1. A north pole will attract a south pole.

2. Two poles which are the same will repel each other.

Question 4

What is a magnetic field?

Answer 4

The area around a magnet in which a magnetic force acts on magnetic objects or other magnets

Question 5

How do you plot a magnetic field?

Answer 5

1. Use a bar magnet and a plotting compass
2. Place the compass close to the north pole and put a mark at the end of the compass to show the direction of the field. move the compass to the mark and make another.
3. Sketch in the shape of the magnetic field remembering that field lines never cross.

Question 6

Explain how you can use a compass to follow a course or find your direction.

Answer 6

A compass contains a small bar magnet which can rotate. When the compass is held flat the needle always settles in the north - south direction.
This is because the earth has a magnetic field and north and south magnetic poles.

Question 7

Explain the properties of a permanent magnet

Answer 7

They always have a north and a south pole
They produce their own magnetic field.
Check whether magnets are permanent or induced seeing if they can repel as well as attract.

Question 8

Explain the properties of an induced magnet

Answer 8

A material which becomes magnetic when placed in a magnetic field.
It is a temporary magnet.
When it is taken away from the magnetic field it will lose most of it’s magnetism.

Question 9

What is the right hand grip rule?

Answer 9

It shows the direction of a magnetic field on a wire carrying an electric current
Make a fist with your thumb pointing upwards using your right hand.
Point your thumb in the direction of the current. Your fingers will show the direction of the magnetic field.

Question 10

What is magnetic flux density?

What unit is it measured in?

Answer 10

The number of lines of magnetic flux in a given area.

Measured in Tesla (T)

Question 11

How do you calculate magnetic flux density? (B)

Answer 11

F = BIL

Force (N) = magnetic flux density(T) x current (A) x length (m)

Question 12

What is the motor effect?

Answer 12

The force produced between a conductor carrying a current within a megnetic field and the magnet producing the field.

Question 13

How can you demonstrate the motor effect in a visual practical way?
(motor effect experiment)

Answer 13

Place a strip of aluminium foil between two magnets.
Pass an electric current through the foil.
The magnetic field produced by the current pushes against the magnetic field acting between the magnets, causing movement of the foil strip up or down depending on the direction of the current.
Flemings left hand rule can be used to predict the direction of the movement.

Question 14

What does each finger and thumb represent in Flemings left hand rule

Answer 14

1. Thumb = the direction of movement ThuMb = M
2. First finger = field F = F
3. Index finger = current I = Index

Question 15

What are the factors which determine the size of the force acting on the foil in the motor effect experiment.

Answer 15

1. The magnetic flux density between the poles
2. The size of the current in amps
3. The length of the foil used between the poles.

Question 16

The direction of a current in a wire is shown by either an x or a dot in a circle. What does each mean.

Answer 16

A dot shows the current travelling towards you thumbs up in the RH grip rule

A cross shows the current travelling away, thumbs down in the RH grip rule.

Question 17

An electric motor contains a coil of wire , surrounded by a magnet which rotates on an axle. When an electric current passes through the coil there is an upward force on one side and a downward force on the other causing the coil to rotate through 180 degrees. How does the design of an electric motor allow the coil to rotate through 360 degrees?

Answer 17

1. The coil is connected to a split ring communicator.
2. The current is passed into carbon brushes which form contact points with each half of the split ring.
3. As the split ring rotates through the vertical position it makes contact with each carbon brush in turn changing the contact.
4. This causes the direction of the current in the coil to reverse with every half turn therefore allowing the coil to continue to spin clockwise.