1. COPY this please!

2. Topic p14 TEST!
P14 Electromagnetic Effects
Monday 14th December

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

4. 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.)

5. Today’s lesson - Transformers

6. Transformers – Revenge of the Fallen

7. Transformers
A transformer consists of 3 main parts

8. 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

9. 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.

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

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

12. 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

13. 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

14. 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.

15. 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

16. No energy for nothing
However, no new energy is made

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

18. 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?

19. Why do we need transformers?

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

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

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

23. Transmission of Electricity
Step-up
Step-down

24. 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.

25. Some questions!

26. 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

27. 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

28. 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

29. 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.?

30. Transformers.

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

32. Let’s try some questions!