Transformers (Cambridge (CIE) IGCSE Physics): Revision Note

Structure of a transformer

• 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

Step-up & step-down transformers

• 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 $\left(V_s>V_p\right)$

  • has more turns on the secondary coil than on the primary coil $\left(N_s>N_p\right)$

Step-down transformer

• A step-down transformer: 

  • decreases the potential difference of a power source $\left(V_s<V_p\right)$

  • has fewer turns on the secondary coil than on the primary coil $\left(N_s<N_p\right)$

Operation of a 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