Transformers (Cambridge International IGCSE Physics)

Structure of a transformer

Spec point — 4.5.1e · Describe the construction of a simple transformer with a soft iron core, as used for voltage transformations

• A transformer is a device used to change the size of an alternating potential difference or current

  • This is achieved using the generator effect

• A basic transformer consists of:

  • a primary coil
  • a secondary coil
  • a soft iron core

• Iron is used because it is easily magnetised

A simple transformer is made up of a primary coil and a secondary coil wound on a soft iron core

Step-up & step-down transformers

Spec point — 4.5.2c · Use the terms primary, secondary, step-up and step-down

• A transformer consists of a primary and secondary coil:

  • The primary coil is the first coil
  • The secondary coil is the second coil

Step-up transformer

• A step-up transformer:

  • increases the potential difference of a power source (V_ s greater than V_ p)
  • has more turns on the secondary coil than on the primary coil (N_ s greater than N_ p)

A step-up transformer has more turns on the secondary coil, which increases the size of the potential difference

Step-down transformer

• A step-down transformer:

  • decreases the potential difference of a power source (V_ s less than V_ p)
  • has fewer turns on the secondary coil than on the primary coil (N_ s less than N_ p)

A step-down transformer has fewer turns on the secondary coil, which decreases the size of the potential difference

Operation of a transformer

Spec point — Explain the principle of operation of a simple iron-cored transformer

Extended Tier Only

• 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 an e.m.f. (potential difference)

• As the magnetic field is continually changing, the induced e.m.f. will be alternating

  • The alternating e.m.f. 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