1. & 13 transformers

2. What do they do?
Fundamentally a transformer is able to step the voltage up or down from the powered coil to the unpowered coil, ie change the voltage coming out of the transformer!
How do they work?
There is an effect of electromagnetism
known as mutual induction.
Two or more coils of wire placed so that the changing magnetic field created by one induces a voltage in the other.
If we have two inductive coils and we energise one coil with AC, we will create an AC voltage in the other coil. When used as such, this device is known as a transformer:

3. Why should AC be used rather than DC?
As useful as transformers are, they only work with AC. Since the phenomenon of mutual induction relies on changing magnetic fields, and direct current (DC) can only produce steady magnetic fields, transformers simply will not work with direct current.
Of course, direct current may be interrupted (pulsed) through the primary winding of a transformer to create a changing magnetic field (as is done in automotive ignition systems to produce high-voltage spark plug power from a low-voltage DC battery), but pulsed DC is not that different from AC.
Perhaps more than any other reason, this is why AC finds such widespread application in power systems.

4. Why have an Iron core?
The laminated iron core simply redirects the magnetic field from the primary coil through the secondary.
What is the relationship between the Voltage produced and the current?
If the secondary coil is powering a load, the current through the secondary coil is just the opposite ie If the Voltage increases the current decreases

5. Relationship between Voltage and Number of turns
In any transformer the number of turns has an effect on the voltages produced.
You will remember making electromagnets in Key Stage 3 where the number of turns of wire affected the strength of the magnet.
If more turns were used you made a stronger magnet. Therefore the amount of coils of wire around a transformer could affect the output!?

6. Output proportional to turns
The strength of the magnetic field is proportional to the input voltage and the number of turns around the the primary coil.
Therefore, the output voltage must be proportional to the strength of the changing magnetic field and the number of turns on the secondary coil.

7. A typical arrangement of a transformer is shown below.
The relationship between the number of turns on the primary and secondary is mirrored in the input and output voltages.

8. This relationship has a very close mechanical analogy, using torque and speed to represent voltage and current, respectively:

9. If the winding ratio is reversed so that the primary coil has less turns than the secondary coil, the transformer "steps up" the voltage from the source level to a higher level at the load:

10. Relationship between turns and voltages
The Voltages in the primary and secondary coils are linked in the same way as the number of coils in the primary and secondary. They have a constant ratio as shown below……
No. of turns on primary = Voltage on primary
No. of turns on secondary Voltage on secondary

11. Step up and Step down transformers on power lines.
Industrial Applications
You need to research the application of transformers with regard to the large scale transmission of electricity.
This means you have to explain the use of
Step up and Step down transformers on power lines.
You need to discuss the effect of these transformers on the voltages at all stages and the reasons why in terms of energy loss in the power lines.

12. The transformer's ability to step AC voltage up or down with ease gives AC an advantage unmatched by DC in the realm of power distribution.
When transmitting electrical power over long distances, it is far more efficient to do so with stepped-up voltages and stepped-down currents causing lower resistive power losses, then step the voltage back down and the current back up for industry, business, or consumer use.

13. Transformer technology has made long-range electric power distribution practical. Without the ability to efficiently step voltage up and down, it would be cost-prohibitive to construct power systems for anything but close-range (within a few miles at most) use.