Electromagnetism

Question 1

What is a solenoid?

Answer 1

The magnetic field around a current carrying wire.

Question 2

Which direction do lines of a magnetic field go?

Answer 2

From the north end outwards, splitting of. From the south end, they come inwards, converging.

Question 3

What is induced magnetism?

Answer 3

Placing an unmagnetised magnetic material in a magnetic field.

Question 4

How do you know which direction the field goes in?

Answer 4

Place your RIGHT hand with your fist closed and thumb facing the direction of current (towards negative). The direction of your fingers is the field direction.

Question 5

What is each finger in Fleming’s left hand rule?

Answer 5

Thumb - movement.

Forefinger - magnetic field (back is north, tip is south).

Third finger - current (positive is back of finger, tip is negative).

Question 6

What is Fleming’s left hand rule used for?

Answer 6

The motor effect.

Question 7

What is the generator effect?

Answer 7

Inducing a current using a magnet.

Question 8

What does the generator effect require?

Answer 8

A magnet and a wire (not carrying a current).

Question 9

What happens when you reverse the direction of the movement of the wire or coil in the generator effect?

Answer 9

The direction of the induced p.d is also reversed.

Question 10

How is the size of the induced p.d changed?

Answer 10

• The speed of movement.
• The strength of the magnetic field.
• The number of turns on the coil.

Question 11

When is the force at its maximum in the motor effect?

Answer 11

When the wire is 90* to the magnetic field.

Question 12

What is magnetic flux density?

Answer 12

A measure of how strong the magnetic field is.

Question 13

How is the direction of the force on a wire reversed in the motor effect?

Answer 13

By reversing the current of the direction of the magnetic field.

Question 14

What does an electric motor contain. How does it work?

Answer 14

It contains a coil in a magnetic field. When a current passes through the coil, a force acts on each side of the coil in opposite directions. This makes the coil spin.

Question 15

What does the split ring do to make the rotation in one direction?

Answer 15

It reversed the direction of the current every half turn so the coil is always pushed in the same direction.

Question 16

How do you tell if it is the motor or generator effect?

Answer 16

If there’s current flowing through the wire, it is the motor effect. The generator effect involves a magnet inside a coil of wire.

Question 17

How does a loudspeaker work?

Answer 17

The coil of wire has a AC current flowing through it. There is a permanent magnet inside the coil. The current generates an induced magnetic field. The magnetic fields interact, repelling and attracting. The cone then moves back and forth. Changing the direction of the current (AC) causes this alternating movement.

Question 18

How does an AC current generator work?

Answer 18

• The PD induced is greatest when the plane of the coil is parallel to the magnetic field direction.
• The PD is zero when the coil perpendicular to the magnetic field direction as the coil is moving in the same direction as the magnetic field lines.

Question 19

How does a microphone work?

Answer 19

Sound waves cause a coil to vibrate, producing an alternating PD.

Question 20

How does a transformer work?

Answer 20

• Two coils a wire go around an iron core.
• AC current through the primary coil produces a magnetic field.
• The magnetic field lines pass through the secondary coil, inducing an alternating PD. If part of a complete circuit, an AC is produced.

Question 21

Why must AC be used for a transformer?

Answer 21

A changing magnetic field across the secondary coil is needed to induce a PD in that coil.

Question 22

Why is the wire insulated in a transformer?

Answer 22

So that the current doesn’t short across either the iron core or adjacent turns of wire.

Question 23

Why is iron used in a transformer?

Answer 23

So it is easily magnetised and demagnetised.

Question 24

What does a step-up transformer do?

Answer 24

It makes the PD across the secondary coil greater than the primary coil. The secondary coil has more turns than the primary.

Question 25

What does a step-down transformer do?

Answer 25

It makes the PD across the secondary coil less than the primary coil. The primary coil has more turns than the secondary.

Question 26

Why is PD increased in the National Grid?

Answer 26

To reduce the current, reducing the heating effect and power loss in the cables. So, the system is more efficient.

Question 27

What is the unit for magnetic flux density?

Answer 27

Tesla, with B as the symbol.