Transmitting Electricity

Learning Objectives• Recall the parts of the national grid.

• Describe what a transformer does.

• Use the turns ration equation to predict the missing voltage or number of turns.

Question• What happens to electricity when

it is generated in power stations??

The National Grid• All across the UK a network of pylons and

cables distribute electricity to our houses.

How are transformers used to supply electricity?

Transformers• The voltage of an alternating current can be

changed using a transformer.

• A step-up transformer increases voltage.

• A step-down transformer decreases voltage.

Why Change Voltage?• Voltage is increased as it is transmitted

through the national grid.

• High voltage is very dangerous but reduces energy lost through heat.

• Voltage is reduced to 230 V before electricity is supplied to homes.

RULE TO REMEMBER...

• High = Low = Less energy wasted as Voltage Current heat

To know how an electric current can be generated in a wire.

To know how an electric current can be generated in a wire.

A transformer where the voltage rises is called a step-up transformer

To know how an electric current can be generated in a wire.

A transformer where the voltage falls is called a step-down transformer

To know how an electric current can be generated in a wire.

Because transformers rely on induction (changing fields), they only work with alternating current (a.c.)

Direct or alternating current?

The Transformer Equation

Primary voltage

Secondary voltage

No. of turns on Primary coil

No. of turns on Secondary coil=or

Vp =Vs

Np

Ns

To know how an electric current can be generated in a wire.

Example 1A transformer has 1000 turns on its primary coil

and 3000 turns on its secondary. 30 V is applied across the primary coil. What is its output?

S

P

S

P

N

N

V

V

3000

100030

SV

33.030

SV

33.0

1

30SV

330

SV

303SV

90SV V

To know how an electric current can be generated in a wire.

Example 2A transformer has 3800 turns on its primary and 100

turns on its secondary coil. Its output voltage is 6 V.a) Is it a step up or step down transformer?b) Calculate the input voltage.c) Explain why this could be suitable for charging

a mobile phone.

To know how an electric current can be generated in a wire.

a) It is a step down transformer.

b) c) Mains voltage is

230V so this transformer is suitable for stepping down the voltage to a safe level for charging a mobile phone.

s

p

s

p

N

N

V

V

100

3800

6pV

386

pV

638PV

228PV V

Vs

Primary turns Np

Secondary turns Ns

Primary voltage Vp

Secondary voltage Vs

Step up or step down

100 20 3

400 10,000 10

50 240 12

5,000 33,000 11,000

Vp

Ns

Np=

Copy and complete the table

Primary turns Np

Secondary turns Ns

Primary voltage Vp

Secondary voltage Vs

Step up or step down

100 20 15 3 down

400 10,000 10 250 up

1,000 50 240 12 down

15,000 5,000 33,000 11,000 down

Vs

Vp

Ns

Np=