Induced potential, transformers and the National Grid (7.3)

Question 1

long description

What is the generator effect?

Answer 1

If an electrical conductor moves relative to a magnetic field (magnetic field fixed and conductor moving around it) or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.

If the conductor is part of a complete circuit, a current is induced in the (whole) conductor

Question 2

An induced current generates what?

Answer 2

A magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field

Question 3

What happens if a permanent magnet is move into a solenoid (or vice-versa)?

Answer 3

the induced magnetic field tends to repel the magnet back out of the coil

Question 4

3 factors

What are the factors that affect the size of the induced potential difference/induced current?

Answer 4

• the speed of movement is increased
• the magnetic field strength is increased
• the number of turns on the coil is increased

Question 5

What is the factor that affects the direction of the induced potential difference/induced current?

Answer 5

The orientation of the poles of the magnet

Question 6

2 ways

When will current reverse when using a magnet and solenoid?

Answer 6

• if the magnet is moved out of the coil
• if the other pole of the magnet is moved into the coil

Question 7

Why is it harder to pull a magnet out of a solenoid?

Answer 7

As you take the magnet out of the solenoid, the pole of the solenoid becomes opposite to that of the magnet, so it is harder to pull out and therefore work is done

Question 8

The generator effect is used in an alternator to generate…

Answer 8

ac

Question 9

The generator effect is used in an dynamo to generate…

Answer 9

dc

Question 10

Explain how a dynamo works

Answer 10

A direct current and pd are induced causing the coil to rotate due the force created.

The side of the coil moving up is connected to part A of the split-ring commutator and the side of the coil moving down is connected to part B.

When current is perpendicular to the magnetic field, pd at peak/maximum (positive).

However, when it completes a half rotation (pd is zero at parallel), the side now moving up is still connected to part A, and vice versa with part B.

Therefore, the current does not alternate as the direction of the current and potential difference do not reverse when the coil rotates.

Question 11

A current, and therefore pd, will not be induced, if the wire is (…) to the magnetic field

Answer 11

parallel

Question 12

What do split ring commutators do in a dynamo

Answer 12

A split ring commutator changes the coil connections every half turn.

As the induced potential difference is about to change direction, the connections are reversed.

This means that the current to the external circuit always flows in the same direction.

Question 13

How does an alternator work?

Answer 13

• the coil moves through the magnetic field
• a potential difference is induced (across the coil)
• there is a complete circuit, so a current is induced (in the coil)
• However, the direction of pd reverses (up to down or down to up) every half turn, so (every half turn) the current changes direction; this produces an alternating current

at 0^o zero pd, 90^o max positive pd, 180^o zero pd, 270^o max negative pd

Question 14

What does an oscilloscope look like for a dynamo?

Answer 14

Question 15

Other than increasing number of turns or strength of mag field, how else can a greater pd be produced in alternators and dynamos?

Answer 15

by increasing the rate of rotation

Question 16

Alternators have (…) commutators, dynamos have (…) commutators?

Answer 16

slit-ring

split-ring

Question 17

What does an oscilloscope look like for an alternator?

Answer 17

Question 18

Microphones use the generator effect to do what?

Answer 18

to convert the pressure variations in sound waves into variations in current in electrical circuits

Question 19

4 marks

Explain how a moving-coil microphone works

Answer 19

1. sound (waves) cause the diaphragm to vibrate
2. the diaphragm causes the coil / wire to vibrate
3. the coil / wire moves through the magnetic field
4. a potential difference is induced (across the ends of the coil / wire)

Question 20

A basic transformer consists of what?

Answer 20

a primary coil and a secondary coil wound on an iron core

Question 21

Why is iron used in a transformer?

Answer 21

it is easily magnetised

Question 22

The greater the number of coils, the (…) the potential difference induced

Answer 22

greater

Question 23

Describe a step-up transformer

Answer 23

There is a greater number of coils and therefore greater voltage induced on the secondary coil compared to the primary coil

Question 24

Describe a step-down transformer

Answer 24

There is a lesser number of coils and therefore less voltage induced on the secondary coil compared to the primary coil

Question 25

What would happen if transformers were 100% efficient with regards to electrical power input/output?

Answer 25

the electrical power output would equal the electrical power input.

Question 26

Describe how a transformer works?

Answer 26

1. An alternating potential difference is supplied to the primary coil
2. Which causes an alternating current to flow in the priamry coil
3. So a magnetic field (which alternates/changes) is produced around the primary coil
4. This induces an alternating magentic field in the iron core
5. Which induces an alternating potential difference in the secondary coil
6. If it is part of a complete cirucit, this causes currnet to flow

Question 27

Why can’t current just flow from one coil top another directly in a coil through the iron core?`

Answer 27

Coils are insulated in plastic, which prevents current flowing through the iron core

Question 28

The ratio of the potential differences across the primary and secondary coils of a transformer depends on what?

Answer 28

the ratio of the number of turns on each coil

this is more of a concept, it is ratio, not abundance you need to think about in the exam

Question 29

Why is a larger current produced when the magnet leaves the solenoid, than when it enters it?

Answer 29

The magnet is moving faster when it leaves the solenoid

Question 30

What is voltage caused by in magnetic field?

Answer 30

the change in the magnetic field

the stronger the change in the magnetic field, the greater the voltage produced

Question 31

What is the difference between direct and alternating potential difference?

Answer 31

Alternating p.d. constantly changes direction. Direct p.d. always goes in same direction (one direction only).

Question 32

Does a current induce voltage, or voltage induce current?

Answer 32

An induced voltage induces current (which in turn induces a magnetic field around the conductor/wire)

Question 33

Explain

Answer 33

when moved vertically, the wire cuts through field lines

Which induces pd in wire

Why moved in other directions, less field lines are cut

When moved horizontally, no field lines are cut

Question 34

Why are insulated wires used in transformers?

Answer 34

tp prevent current flowing through/entering the core

Question 35

What is the use of a transformer being adjusted to have a different number of turn son the secondary coil?

Answer 35

to vary the (output) potential difference

so that you don’t need a different generator for each type of device

Question 36

What is the purpose of slip rings (alternator)?

Answer 36

provides a continuous / moveable contact

Question 37

Why is the handle of an alternator much easier to turn after it is disconnected from an electrical circuit?

Answer 37

there is no induced current

so no magnetic field produced around coil

to oppose the movement of the coil