Magnetism and electromagnetism

Question 1

Which way do magnetic field lines always go?

Answer 1

from the north to south poles

Question 2

What will two of the same poles of a magnet do?

Answer 2

repel eachother

Question 3

What will two different poles of a magnet do?

Answer 3

attract eachother

Question 4

What do compasses show?

Answer 4

the direction of magnetic field lines

because compasses contain tiny magnets that always point north

Question 5

What are permenant magnets?

Answer 5

magnets that produce their own magnetic field

Question 6

What are induced magnets?

Answer 6

magnetic materials that turn into a magnet when they are put into a magnetic field (near a magnet)

Question 7

What happens when a magnetic material is put near a permanent magnet?

Answer 7

it attracts to it

quickly gains its magnetism

Question 8

What does the right hand thumb rule show?

Answer 8

thumb - current

fingers - direction of magnetic field lines

Question 9

When a current flows through a wire, what happens to the wire?

Answer 9

a magnetic field is created around the wire

this field is made up of concentric circles perpendicular to the wire (with the wire in the centre)

Question 10

What is a solenoid?

Answer 10

a coil of wire

that has a magnetic field

Question 11

What are the magnetic field lines inside and outside a solenoid like?

Answer 11

inside - strong and uniform (same strength and direction)

outside - just like a bar magnet

Question 12

What is an electromagnet?

Answer 12

a solenoid with an iron core

a magnet which has a magnetic field that can be turned on or off

Question 13

What can electromagnets be used for?

Answer 13

in cranes in scrap yards (to pick up scrap metal)

as switches within other circuits

Question 14

What is magnetic flux density?

Answer 14

how many field lines there are in a region

these lines represent how strong the magnet is

Question 15

What is the motor effect?

Answer 15

When a current carrying wire is put between two magnetic poles, the magnetic field around the wire reacts with the magnetic field between the poles, which causes the wire and magnet to exert a force on each other. This can cause the wire to move.

Question 16

What does Fleming’s left hand rule show?

Answer 16

the motor effect
thumb - movement of wire (the direction of the force)
first finger - direction of magnetic field
middle finger - current (wire)

Question 17

What is the generator effect?

Answer 17

The induction of potential difference (and current if there is a complete circuit) in a wire which is experiencing a change in magnetic field.

Question 18

How is the generator effect caused?

Answer 18

By changing the magnetic field lines in a wire.
For example by moving a magnet inside a coil of wire or by moving a conductor (wire) in a magnetic field.
(cutting field lines)

Question 19

How can the size of the induced potential difference be changed in a generator?

Answer 19

• increasing the speed of movement (cutting more magnetic field lines in a given time)
• increasing the strength of a magnetic field (so that there are more field lines that can be cut)

Question 20

What is an alternator?

Answer 20

a generator that generates an alternating current

Question 21

What is a dynamo?

Answer 21

a generator that generates a direct current

Question 22

What are the two types of generators and what makes them different?

Answer 22

alternators generate alternating current
dynamos generate direct current
alternators use slip rings and brushes so that the contacts don’t swap every half turn, meaning that every half turn the magnetic field reverses which means the current flows a different way each half turn (so is alternating)
dynamos use a split-ring commutator so that the connections are swapped each half turn to keep the current flowing in the same direction

Question 23

How do loud speakers (and headphones) work?

Answer 23

using the motor effect
An alternating current goes through a coil of wire, which surrounds one pole of a permanent magnet and is surrounded by the other pole. This current causes a force (and therefore movement) on the coil, so that when the current reverses, the force acts in the opposite direction. The variations in the current make the cone vibrate so that the surrounding air vibrates, creating sound waves.

Question 24

What are transformers?

Answer 24

devices that use the generator effect to change the potential difference for an alternating current

Question 25

What are step up transformers?

Answer 25

transformers that increase the potential difference

have more turns on the secondary coil

Question 26

What are step down transformers?

Answer 26

transformers that decrease the potential difference

have more turns on the primary coil

Question 27

How do transformers work?

Answer 27

Transformers have two coils of wire (primary and secondary) joined by an iron core.
When an alternating current is applied to the primary coil, the iron core magnetises and demagnetises quickly so that there is a changing magnetic field, which induces an alternating potential difference in the secondary coil.
If the secondary coil is part of a complete circuit, a current will be induced too.