P15 Electromagnetism

Question 1

What objects can be magnetised? (4)

Answer 1

Iron, steel, cobalt and nickel

Question 2

What are permanent magnets usually made of and why?

Answer 2

Steel, because magnetised steel does not lose its magnetism easily

Question 3

What is a magnetic field?

Answer 3

The region around a magnet in which a magnetic material experiences a force

Question 4

What is induced magnetism?

Answer 4

When an unmagnetised magnetic material is placed in a magnetic field and becomes magnetised

Question 5

What is induces a magnetic field around a wire?

Answer 5

Electric current passing along it

Question 6

What does the corkscrew rule do?

Answer 6

Give the direction of the magnetic field for each direction of the current

Question 7

What is a solenoid?

Answer 7

A long coil of insulated wire

Question 8

How are the magnetic field lines inside a solenoid placed?

Answer 8

Parallel to the axis of the solenoid and all in the same direction

Question 9

What does reversing the direction of current do to a single wire and solenoid?

Answer 9

Reverse the direction of the magnetic field

Question 10

What does increasing the current do to a single wire and solenoid?

Answer 10

Increases the strength of the magnetic field

Question 11

What is an electromagnet?

Answer 11

A solenoid in which an insulated wire is wrapped around an iron bar

Question 12

How could you cause the iron bar in an electromagnet to lose its magnetism?

Answer 12

Switching the current through the solenoid off

Question 13

Why are electromagnets useful?

Answer 13

They allow a magnetic field to be switched on and off by switching the current on and off

Question 14

What devices are electromagnets used in? (4)

Answer 14

Scrapyard cranes, circuit breakers, electric bells and relays

Question 15

How do electromagnets work in a scrapyard crane?

Answer 15

A current is switched on to attract the steel cars to the electromagnet in the crane. The current can then be switched off to release it

Question 16

How do electromagnets work in a circuit breaker?

Answer 16

Too great a current makes the electromagnet open a switch, which cuts the current off

Question 17

How do electromagnets work in an electric bell?

Answer 17

When the bell is pushed on, an electromagnet attracts an iron armature, which hits a bell. Because the iron armature is pulled away from the make-and-break switch, the electromagnet loses is magnetism and the armature springs back. This process repeats itself rapidly

Question 18

How do electromagnets work in a relay?

Answer 18

The small current through the coil of an electromagnet is used to attract an iron armature, which switches on a much larger current in another circuit

Question 19

What is the motor effect?

Answer 19

When a wire carrying an electric current inside a magnetic field experiences a force

Question 20

During a motor effect, when is the force at its maximum?

Answer 20

When the wire is at an angle of 90 to the magnetic field

Question 21

During a motor effect, when is the force at zero

Answer 21

When the wire is parallel to the magnetic field

Question 22

What is magnetic flux density and what is its unit?

Answer 22

A measure of how strong a magnetic field is, Tesla

Question 23

When would the direction of force on a wire be reversed?

Answer 23

If either the direction of the current of the magnetic field is reversed

Question 24

When would the force on a wire be increased?

Answer 24

If either the size of the current or the strength of the magnetic field is increased

Question 25

What makes the coil spin in an electric motor?

Answer 25

When a current passes through the coil a force acts on each side of the coil, in opposite directions due to the motor effect

Question 26

What does a split-ring commutator do to that direction of the current around a coil?

Answer 26

It reverses the direction every half turn so that the coil is always pushed in the same direction