Transformers (Edexcel IGCSE Physics)

• A transformer is a device used to change the value of an alternating potential difference or current
• This is achieved using the generator effect

Structure of a transformer

• A basic transformer consists of:
  • A primary coil
  • A secondary coil
  • An iron core

• Iron is used because it is easily magnetised

Structure of a transformer

How a transformer works

• An alternating current is supplied to the primary coil
• The current is continually changing direction
  • This means it will produce a changing magnetic field around the primary coil

• The iron core is easily magnetised, so the changing magnetic field passes through it
• As a result, there is now a changing magnetic field inside the secondary coil
  • This changing field cuts through the secondary coil and induces a potential difference

• As the magnetic field is continually changing the potential difference induced will be alternating
  • The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil

• If the secondary coil is part of a complete circuit it will cause an alternating current to flow

• A transformer can change the size of an alternating voltage
• They also have a number of other roles, such as:
  • To increase the potential difference of electricity before it is transmitted across the national grid
  • To lower the high voltage electricity used in power lines to the lower voltages used in houses
  • Used in adapters to lower mains voltage to the lower voltages used by many electronic devices

• A step-up transformer increases the potential difference of a power source.
  • A step-up transformer has more turns on the secondary coil than on the primary coil

• A step-down transformer decreases the potential difference of a power source.
  • A step-down transformer has fewer turns on the secondary coil than on the primary coil

Transformers in electricity transmission

What does a step up transformer do?

• When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
• The electrical energy is transferred at high voltages from power stations
• It is then transferred at lower voltages in each locality for domestic uses
• The voltage must be stepped up by a step-up transformer
  • These are placed after the power station

Why are step down transformers used?

• For the domestic use of electricity, the voltage must be much lower
• This is done by stepping down by the voltage using a step-down transformer
  • These are placed before buildings

Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables using transformers