SC13 Electromagnetic Induction

Question 1

Define electromagnetic induction

Answer 1

A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it

Question 2

What is the difference between the motor effect and the generator effect?

Answer 2

• In the motor effect, there is already a current in the conductor that experiences a force
• whereas in the generator effect, there is no initial current in the conductor, but one is induced when it goes through the magnetic field

Question 3

How can a current be induced in a wire?

Answer 3

If a wire is moved into a magnetic field, such that the field lines are cut, a potential difference is induced between the ends of the wire and a current can flow

Question 4

How can we induce a voltage in a wire with coils?

Answer 4

We can move the magnet around it

Question 5

What is the size of the potential difference that is induced determined by?

Answer 5

• the speed at which the coils or the magnet is moved
• the number of turns on the coils of the wire
• the size of the coils
• the strength of the magnetic field

Question 6

What are the rules for induction?

Answer 6

• reversing the direction of the poles reverses the induced p.d and current
• magnetic field produced in the coil always opposes the magnetic field that induces the potential difference
• moving the coils or the magnet faster induces a greater p.d and current

Question 7

What is the direction of the potential difference determined by?

Answer 7

The orientation of the poles

Question 8

Why will the number of turns of the coil of a wire increase the induced potential difference?

Answer 8

• each coil will cut through the magnetic field lines
• total potential difference induced = result of all the coils cutting the magnetic field lines

Question 9

Why will increasing the area of the coils increase the potential difference?

Answer 9

More wire to cut through the magnetic field lines

Question 10

If a magnet is pushed north first into a coil of wire, the end of the coil closest to the magnet will also become the north pole. Explain why.

Answer 10

• due to the generator effect, a potential difference will induced in the coil
• induced potential difference always opposes the charge that produces
• coil will apply a force to oppose the magnet being pushed into i
• therefore, closest end will become a north pole
• which repels the magnet

Question 11

What can the generator effect be used to generate?

Answer 11

• a.c. in an alternator
• d.c. in a dynamo

Question 12

What is a simple alternator?

Answer 12

A type of generator that converts mechanical energy to electrical energy in the form of an alternating curent

Question 13

What does a meter pointer show in an alternator?

Answer 13

It swings from side to side to show the alternating potential difference

Question 14

Describe what happens in an alternator?

Answer 14

Every half turn, the current will switch direction, as the wire will be in the opposite direction compared to its starting position

Question 15

What is a dynamo?

Answer 15

• same as an alternator, but at the end, there is a commutator

Question 16

What is the role of a commutator in a dynamo?

Answer 16

The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction
This happens each time the coil is perpendicular to the magnetic field lines

Question 17

How does a microphone use the principles of electromagnetic induction?

Answer 17

They convert the pressure variation in the sound waves into variation in current in electrical circuits

Question 18

Describe how the microphone works

Answer 18

1.pressure variations in sound waves cause the diaphragm to vibrate
2. the vibrations of the diaphragm cause vibrations in the coil
3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
5. the changing size and direction of the induced current matches the vibrations of the coil
6. the electrical signals generated match the pressure variations in the sound waves

Question 19

Describe how a loudspeaker works

Answer 19

1. Uses a coil which can slide forwards and backwards over the central pole of a circular permanent magnet
2. Wire from the amplifier moves bakwards and forwards at the same frequency as the changing current
3. Paper cone then moves the air backwards and forwards which creates the sound

Question 20

What is a transformer?

Answer 20

A transformer is an electrical device that can be used to increase or decrease the potential difference of an alternating current

Question 21

How does a transformer work?

Answer 21

1. AC in first coil creates a changing magnetic field
2. this changing magnetic field cuts through the second secondary coil, inducing an alternating current within it

Question 22

What does a basic transformer consist of?

Answer 22

• primary coil
• secondary coil
• iron core

Question 23

Why is an iron core used in a transformer?

Answer 23

It is easily magnetised

Question 24

What are the uses of a transformer?

Answer 24

• increase the p.d. of electricity as it is transmitted through the national grid
• decrease the high voltage electricity as it travels through power lines to the low voltages used in houses
• in adapters to lower mains voltage used by many electrical devices

Question 25

What is a step up transformer?

Answer 25

Increases the voltage and decreases current

Question 26

Describe the coils in a step up transformer

Answer 26

coils on primary < coils on secondary

Question 27

What is a step down transformer?

Answer 27

Decreases voltage and increases current

Question 28

Describe the coils in a step down transformer

Answer 28

coils on primary > coils on secondary

Question 29

What does the output potential difference of a transformer depend on?

Answer 29

• input potential difference
• number of turns on primary and secondary coils

Question 30

What is the equation used to calculate the output potential difference of a transformer?

Answer 30

number of coils on primary/ number of coils on secondary = p.d of primary/ p.d of secondary

Question 31

What is the national grid?

Answer 31

Used to transfer electricity across the country

Question 32

Why are high voltages initially used in the national grid?

Answer 32

• A high current produces a high heating effect
• P = IV, and since power is constant, to decrease th current, we need to increase the pd.
• therefore, by using high voltages, we improve the efficiency of the transfer

Question 33

Why are the high voltages lowered as they get closer to domestic areas?

Answer 33

• high voltages are dangerous
• so they lower the voltage as it travels to domestic areas, making it safer to use

Question 34

Describe the journey of electricity from the power stations to factories

Answer 34

1. Power stations
2. Step-up transformers increase the voltage to 400,00 voltages
3. electricity sent through transmission lines
4. step-down transformers reduce voltage to 33,000 volts
5. Electricity is then delivered to factories

Question 35

Describe the journey of electricity from the power stations to your house

Answer 35

1. Power stations
2. Step-up transformers increase the voltage to 400,00 voltages
3. electricity sent through transmission lines
4. step-down transformers reduce voltage to 33,000 volts
5. transformers at local substations reduce voltage to 230V from homes, shops and offices
6. Electricity delivered to your homes

Question 36

State the ideal transfomer equation

Answer 36

Vp X Ip = Vs X Is
where p = primary coil
and s= secondary coil

Question 37

What is the voltage produced in power stations?

Answer 37

25kV

Question 38

What is the voltage used in transmission lines?

Answer 38

400 kV

Question 39

What is the voltage used by small factories?

Answer 39

11kV

Question 40

What is the voltage used by large factories?

Answer 40

33kV

Question 41

What is the voltage used in homes, shops and schools?

Answer 41

230 V