Topic 13 - Electromagnetic induction

Question 1

What is electromagnetic induction?

Answer 1

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

Question 2

Induces is a fancy word for….

Answer 2

Creates

Question 3

How would you produce an electric current by the relative movement of a magnet and a conductor? (ON A SMALL SCALE IN THE LAB)

Answer 3

1. You could rotate/move either a magnet in a coil of wire or a conductor in a magnetic field
2. If you move or rotate the magnet in the opposite direction, then the p.d/current will be reversed too
3. If you keep the magnet (or coil) moving backwards and forwards, or keep it rotating in the same direction, you produce an alternating current

Question 4

When a current is induced in a wire, that current always produces its own….

Answer 4

magnetic field

Question 5

The magnetic field created by an induced current always acts against ……

Answer 5

the change that made it

Question 6

What do transformers do?

Answer 6

They change the size of the pd of an alternating current

Question 7

How do transformers work?

Answer 7

1. They all have two coils of wire, the primary and secondary coil, joined with an iron core
2. When an alternating pd is applied across the primary coil, it produces an alternating magnetic field
3. The iron in the core is a magnetic material that is easily (de)magnetised, as the coil is producing an alternating magnetic field, the magnetisation in the core also alternates
4. This changing magnetic field, induces a pd in the secondary coil

Question 8

What do step up transformers do?

Answer 8

• They step the pd up (increase it)

- They have more turns on the secondary coil than the primary coil

Question 9

What do step down transformers do?

Answer 9

They step the pd down (decrease it). They have more turns on the primary coil than the secondary

Question 10

How efficient are transformers?

Answer 10

Nearly 100% efficient

Question 11

What formula shows that the input power is equal to the output power in transformers?

Answer 11

Vp * Ip = Vs * Is

V = pd
I = current
p = primary coil
S= secondary coil

Question 12

How do dynamos generate direct current?

Answer 12

1. Generators apply a force to rotate a coil in a magnetic field, like a motor (or a magnet in a coil)
2. As the coil (or magnet) spins, a current is induced in the coil. This current changes direction every half turn
3. Dynamos are dc generators. They have a split ring commutator
   4, This swaps the connection every half turn to keep the current flowing in the same direction

Question 13

What increases the current induced in a dynamo or alternator?

Answer 13

1. Having more turns of wire in the coil
2. The magnetic flux density is increased
3. If the speed of rotation is increased

Question 14

What is the only main difference between dynamos and alternators?

Answer 14

1. Instead of a split ring commutator, a.c. generators have slip rings and brushes so the contacts dont swap every half turn
2. This means an alternator produces an alternating p.d and therefore an a.c. if the coil is part of a complete circuit

Question 15

How do microphones generate current from sound waves?

Answer 15

1. Mics use electromagnetic induction to generate an electrical signal
2. Sound waves hits a flexible diaphragm, attached to a coil of wire. The coil of wire surrounds one pole of a permanent magnet and is surrounded by the other pole
3. This means as the diaphragm moves, a current is generated in the coil
4. The movement of the coil depends on the properties of the sound wave (louder sounds makes the diaphragm move further
5. Therefore, mics convert pressure variations of a sound wave into variations in current in an electric circuit

Question 16

How do loudspeakers work?

Answer 16

1. In loudspeakers, the diaphragm is replaced with a paper cone
2. The coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil
3. When the current is reversed, the force acts in the opposite direction
4. These movements makes the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that causes a sound wave

Question 17

Describe how a power station uses a turbine to generate electricity

Answer 17

1. Burning fuel is used to heat water, and convert it to steam, which turns a turbine
2. The turbine is connected to a powerful (electro)magnet inside a generator - a huge cylinder wound with coils of copper wire
   3, As turbine spins, the magnet spins with it, inducing a large pd and alternating current in the coils
3. Coils are joined together in parallel, to produce a single output from the generator

Question 18

What is the only type of power generation that doesnt use a turbine and generator?

Answer 18

Solar

Question 19

What is the national grid?

Answer 19

A network of wires that connects UK power stations to consumers. This is where electricity is taken to once it has been generated

Question 20

The national grid has to transfer loads of …….., which means it transfers electricity at a high….. (power = energy transferred * time)

Answer 20

1. Energy each second

2. Power

Question 21

To transmit the huge amounts of power needed, what would you need?

Power = current /pd

Answer 21

Either a high pd or a high current

Question 22

Whats the disadvantage with having a high current?

Answer 22

It makes the wires heat up so lots of energy is wasted to thermal energy stores

Question 23

What formula can you use to find out the power lost due to resistive heating?

Answer 23

Electrical power = current (squared) * resistance

Question 24

How could the national grid reduce these losses

Answer 24

• By using more efficient, high voltage, low resistance cables
• by using transformers, which are almost 100% efficient

Question 25

Why is using high voltage an advantage?

Answer 25

For a given power, as you increase the pd across a coil, you decrease the current through it (Vp * Ip = Vs * Is)

Question 26

What transformer boosts up the pd really high (400,000 V) and keeps the current low in power stations?

Answer 26

Step up transformers

Question 27

What transformer brings the pd back to safe usable levels at the consumers end?

Answer 27

Step down transformers

Question 28

What is the transformer equation?

Answer 28

Vp/ Vs = Np/ Ns * (this equation also works if Vs and Ns are on top) ``` Vp = input pd Vs = output pd Np= number of turns on primary Ns = number of coils on secondary ```