COPY this please!

Topic p14 TEST! P14 Electromagnetic Effects Monday 14th December

Last lesson - Electromagnetic induction If a magnet is moved inside a coil an electric current is induced (produced)

Generator/dynamo A generator works by rotating a coil in a magnetic field (or rotating a magnet in a coil). A slip-ring commutator produces alternating current (a.c.)

Today’s lesson - Transformers

Transformers – Revenge of the Fallen

Transformers A transformer consists of 3 main parts

Changing a.c. current here… (5) …induces a changing (a.c.) voltage here. (2) Produces a changing magnetic field here… (1) Changing a.c. current here… (4) A changing magnetic field here…. (3) The changing magnetic field is carried by the iron core

Transformers A changing current in the primary coil produces a changing magnetic field in the core. This changing magnetic field induces a changing current in the secondary coil.

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

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

Vp/Vs = Np/Ns Changing voltages The ratio of the number of turns on the coils equals the ratio of the voltages Vp/Vs = Np/Ns # of turns = Ns # of turns = Np

To know how an electric current can be generated in a wire. Example 1 A 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? V

To know how an electric current can be generated in a wire. Example 2 A 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.

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. V

No energy for nothing However, no new energy is made

No energy for nothing Power in = power out VpIp = VsIs

Can you copy this please? a.c. only Because transformers rely on induction (changing fields), they only work with alternating current (a.c.) Can you copy this please?

Why do we need transformers?

Why do we need transformers? Current flowing through wires causes them to get hot and energy is lost.

Why do we need transformers? Keeping the current low means electricity can be transported long distances without losing too much energy.

Why do we need transformers? However, in order to send a large quantity of energy, high voltages are needed.

Transmission of Electricity Step-up Step-down

The effect of lower current When energy is transferred at a low current, it uses a higher voltage, which results in less energy being lost as heat in the cables. The National Grid has to transmit electricity over large distances. If the National Grid was to transmit at 25,000V from the power station, about 40% of the electrical energy would be lost as heat – the electricity would be expensive as a result. Step-up-transformer But at 400kV, only 1% of the energy will be lost as heat. This would also mean that the tall pylons could carry thinner, lighter wires, so they would not have to be as strong.

Some questions!

Vs Ns = Vp Np 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

Vs Ns = Vp Np 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

Vs Ns = Vp Np Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vn Step up or step down 100 20 15 3 down 400 10,000 10 250 up 1,000 50 240 12 15,000 5,000 33,000 11,000

Primary coil secondary coil a.c. output a.c. input Iron core There are 4 turns of coil in the primary coil and 32 turns in the secondary coil. The output voltage is 4000 Volts. Is this a stepping up or stepping down transformer? Calculate the input voltage. Is the output voltage a.c or d.c.?

Transformers. http://www.youtube.com/watch?v=VPxdl1zpcC8

Can you stick the sheets in please? Do now! Can you stick the sheets in please?

Let’s try some questions!