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GCSE Physics

Magnetism and Electromagnetism

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Lesson 6 - TransformersAims:

•To recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides.

•To know and be able to use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer :

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What is a transformer?

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What is a transformer?

Where are transformers used?

What do we call a transformer that increases voltage?

What do we call a transformer that decreases voltage?

A device used to increase or decrease voltage.

In the national grid and household appliances.

A step-up transformer.

A step-down transformer.

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Transformers

•A transformer has two electromagnetic coils.

•When the electricity changes in the first coil an electric voltage is created across the second coil.

•Transformers are used to change the voltage and current of the electricity that comes from BELCO.

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How does it work?

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Transformers•A transformer can change the voltage and current of an electrical supply.

•Some devices need low voltage and some need some need high voltage.

•Only a.c. voltage can be transformed from one voltage to another, this is why mains electricity needs to be a.c.

•To understand a transformer we need to learn more about coils and cores.

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Remember this?

An electric voltage is only induced when the magnet is moving. We need a changing magnetism to make electricity.

Bar magnet have a similar magnetic field patter to a solenoid with current.

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What is what ?Input a.c. voltage

Input or primary

coil

Output or secondary

coilA soft iron ‘O’ core

Output a.c.

voltage

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What is what ?

The input or primary

voltage

The output or secondary

voltage

Number of primary

coils

Number of secondary

coils

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Input side

The primary voltage creates a magnetic field when its current passes through the coil.

The a.c. voltage means that the magnetism through the coil is always changing.

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Output sideThe magnetism from the primary coil is passed through the iron to the secondary coil.

Because the magnetism is changing a voltage is induced in the secondary coil.

The size of the output voltage depends on the size of the two coils.

Changing the magnetism is just like moving a magnet, only

easier!

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Soft iron core• Iron is a soft magnetic material, this means that it is very easy to magnetize and very easy to demagnetize.

•In a transformer the direction of electricity is changing many times each second.

•Every fraction of a second the core needs to change its magnetism. We use iron because it can change its magnetism from one direction to another very easily.

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Lamp demonstration

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Transformers•Transformers change the voltage and current of an a.c. electrical supply.

•In the following experiment mains electricity at 220 volts enters the circuit.

•After leaving the transformer the voltage has been reduced to approximately 11 volts.

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Input circuitThe voltage enters the circuit through the thick 220 volt wire and passes through the coil with 3600 turns.

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Output circuitThe voltage is induced in the smaller part of the transformer that only has 300 turns of wire. It then travels through the thinner wires to the 11 volt lamp.

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The lamp is off because the magnetism is not strong enough to go from one coil the other.

21The ‘C’ core increases the magnetism.

22The ‘O’ core is even stronger.

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Step up and Step down

… and an equation to learn!

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Is this a step-up or a step-down transformer?

primarycoil

secondarycoil

This a step-down transformer because there areless turns in the secondary coil than the primary coil.

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Is this a step-up or a step-down transformer?

primarycoil

secondarycoil

This a step-up transformer because there aremore turns in the secondary coil than the primary coil.

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Less turns = less voltage

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Transformer formulaThe formula for calculating voltages and coils in a transformer has four variables!

coilprimaryonTurnscoilondaryonTurns

voltageimaryvoltageSecondary

____sec__

_Pr_

1

2

1

2

NN

VV

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V1 N1

V2 N2

=

Which way up?

1

2

1

2

NN

VV

The formula is about variables – it does not matter which way up you remember the symbols.

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Transformer example 1A step-down transformer is required to transform 240 V a.c. to 12 V a.c. for a model railway. If the primary coil has 1000 turns, how many turns should the secondary have?

V2 / V1 = T2 / T1

T2 = T1 × (V2 / V1)

T2 = 1000 × (12/240)

T2 = 50 turns

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Transformer example 2A transformer is designed to have 2500 primary turns and 5000 secondary turns. What is the output voltage if the input voltage is 120 V ?

V2 / V1 = T2 / T1

V2 = V1 × (T2 / T1)

V2 = 120V × (5000/2500)

V2 = 240 V

31= 230 V

V2 N2

V1 N1=

N2

N1=V2 x V1

50200=V2 x 920

Transformer example 3A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920 V. What is the output voltage?

32= 500 turns

N2 V2

N1 V1=

V2

V1=N2 x N1

17535=N2 x 100

A transformer has 100 turns on its primary coil. It has an input voltage of 35V and an output voltage of 175 V. How many turns are on the secondary coil?

Transformer example 4

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TransformersTransformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.

Voltage across primary (Vp)No. of turns on secondary (Ns)

Voltage across secondary (Vs)No. of turns on primary (Np)

Words – alternating, magnetic field, induce, step up, voltage

We can work out how much a transformer will step up or step down a voltage:

• If a transformer is 100% efficient, the power produced in the secondary coil should equal the power input of the primary coil.

s

p

s

p

NN

VV

S

p

P

s

s

p

VV

II

NN

power station homeappliance

long transmission line

looks like: Rload

Rwire

Rwire

Power Transmission

Power Dissipated in an Electricity Distribution System

We can figure out the current required by a single bulb using P = VI so I = P/V = 120 Watts/12 Volts = 10 Amps (!) Estimate resistance of power lines: 0.001 Ω/m 2105 m = 20 Ohms

120 WattLight bulb

12 Volt Connection Box

Power Plant on Colorado River

10km

Power dissipate/waste in transmission a line is

P = I2R = 102 20 = 2,000 Watts!!

“Efficiency” is

e = 120 Watts/2120 Watts = 5.6%!!!

What could we change in order to do better?

The Tradeoff• The thing that kills us most is the high current

through the (fixed resistance) transmission lines

• Need less current– it’s that square in I2R that has the most dramatic

effect

• But our appliance needs a certain amount of power– P = VI so less current demands higher voltage

• Solution is high voltage transmission

– Repeating the above calculation with 12,000 Volts delivered to the house draws only

– I = 120 Watts/12 kV = 0.01 Amps for one bulb,

– giving – P = I2R = (0.01)220 = 2010-4 Watts, – so – P = 0.002 Watts of power dissipated in transmission

line

– Efficiency in this case is e = 120 Watts/120.004 = 99.996% 39

Example• An average of 120 kW is delivered to

a suburb 10 km away. The transmission lines have a total resistance of 0.40 Ω. Calculate the power loss if the transmission voltage is:

1. 240 V2. 24000V

P = IV

Power loss:

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1

2

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DANGER!• But having high voltage in each household

is a recipe for disaster– sparks every time you plug something in– risk of fire– not cat-friendly

• Need a way to step-up/step-down voltage at will– can’t do this with DC, so go to AC

A way to provide high efficiency, safe low voltage:

High Voltage Transmission LinesLow Voltage to Consumers

step-up to 500,000 Vstep-down,

back to 5,000 V

step-down to 220 V

~5,000 Volts

Power Transmission• Electric power is usually transmitted over

high voltage power lines.

• Copper wire has a resistance and over long runs some energy will be lost to the surroundings as heat.

• A low current (high voltage) minimizes this loss.