P4

Question 1

What is the magnetic flux density?

Answer 1

The magnetic field strength
Number of field lines passing through an area

Question 2

What is an induced magnet?

Answer 2

Attracted to a magnet even though it’s not a permanent magnet

Question 3

What does the magnetic flux density (magnetic field strength) around a wire depend on?

Answer 3

• magnitude of the current (bigger = stronger field)
• distance to the wire (nearer = stronger field)

Question 4

What is magnetic field strength measured in?

Answer 4

Tesla (T)

Question 5

What is a solenoid?

Answer 5

Many parallel loops of wire

Question 6

How do you model the direction of current in wires?

Answer 6

A dot = coming towards you (RH rule)
A cross = going away
Use darts imagery

Question 7

What is the catapult effect?

Answer 7

When the current goes inwards and a uniform field goes to the left combine, a catapult field is formed

Question 8

What is Flemings left hand rule?

Answer 8

Thumb = force
First finger = field
Second finger = current (positive to negative

Question 9

How do you find the force on a conductor carrying a current?

Answer 9

Force on conductor (at right angles to magnetic field) carrying a current = magnetic flux density x current x length
F = BIL

Question 10

What is the motor effect?

Answer 10

A wire with current flowing through it is placed in a magnetic field and experiences a force from two interacting magnetic fields

Question 11

What is a D.C electric motor?

Answer 11

Direct current motor (split ring commutator)
Uses a coil of wire which rotates in a uniform magnetic field
The split ring commutator swaps the contacts of the coil so it rotates in the reverse direction every half turn

Question 12

How to increase the speed of the coil rotating?

Answer 12

• increasing the current
• use a stronger magnet

Question 13

How to increase the force supplied by the motor?

Answer 13

• increase the current
• increase the strength of the magnetic field
• increase the number of turns

Question 14

What is electromagnetic induction?

Answer 14

If a conductor is in a changing magnetic field, a voltage or p.d can be induced from it.
This is an induce voltage which can be used to generate electricity
The direction of the voltage acts opposite to the changing magnetic field

Question 15

What is the generator effect?

Answer 15

When the conductor or coil cuts through the magnetic field lines of the magnetic field

Question 16

How is the generator effect different from the motor effect?

Answer 16

There is already a current in the conductor of a motor
No initial current in the conductor as one is induced

Question 17

How do you increase the induced p.d.?

Answer 17

• move the wire faster
• use a stronger magnetic field
• use more coils/loops

Question 18

What is an alternator?

Answer 18

Type of generator
Converts mechanical energy to electrical energy in the form of a.c (alternating current)
Connected to commutator rings (slip rings)

Question 19

What would the graph for an a.c generator look like?

Answer 19

Would go between both and positive axis
Smooth curve

Question 20

What is a dynamo?

Answer 20

D.c generator
P.d doesn’t change direction
Split ring commutator to ensure direction doesn’t change

Question 21

How can you increase the output of both alternators and dynamos?

Answer 21

• use a stronger magnetic field
• use more turns on the coil
• spin the coil faster

Question 22

What is a transformer?

Answer 22

A device that induces a voltage in a second coil from a changing magnetic field

Question 23

What does a basic transformer consist of?

Answer 23

Primary coil
Secondary coil
Iron core (easily magnetised)

Question 24

How does a transformer work?

Answer 24

1) an a.c is supplied to the primary coil
2) the current produces a changing magnetic field and as the iron core is easily magnetised the second coil also has a changing magnetic field
3) there is now an induce p.d

Question 25

What is a step up transformer?

Answer 25

Increases the p.d of a power source
- more turns on the secondary coil than on the primary

Question 26

What is the equation for output of p.d?

Answer 26

Potential difference across primary coil/p.d across the secondary coil = number of turns on primary coil/number of turns over secondary coil

Question 27

How does a microphone work?

Answer 27

1) when sound waves reach the microphone the pressure variation causes the diaphragm to vibrate
2) this causes the coil to move back and forth through the magnetic field
3) as it does the coil cuts through the field lines and induces an alternating p.d and current
4) this allows sound to be transmitted, amplified and recorded

Question 28

How do loudspeakers and headphones work?

Answer 28

Use the motor effect
Contains a coil of wire wrapped in a permanent magnet
1) an a.c is passed through the coil of the loudspeaker
2) a changing magnetic field is created
3) the magnetic field interacts with the field from the permanent magnet and exerts a force in the coil
4) the coil oscillates as the force is constantly changing direction
5) sound waves are created