The Operation of a Transformer (AQA A Level Physics): Revision Note

The transformer equation

• A transformer is

  A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa

• This is designed to reduce heat energy lost whilst electricity is transmitted down electrical power lines from power stations to the national grid

• A transformer is made up of:

  • a primary coil

  • a secondary coil

  • an iron core

• The primary and secondary coils are wound around the soft iron core

  • The soft iron core is necessary because it focuses and directs the magnetic field from the primary to the secondary coil

  • Soft iron is used because it can easily be magnetised and demagnetised

• In the primary coil, an alternating current producing an alternating voltage is applied

  • This creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage

• A changing magnetic field passes through to the secondary coil through the iron core

  • This results in a changing magnetic flux linkage in the secondary coil and from Faraday's Law, an e.m.f is induced

• An e.m.f produces an alternating output voltage from the secondary coil

• The output alternating voltage is at the same frequency as the input voltage

• The transformer equation is:

$\frac{N_s}{N_p}=\frac{V_s}{V_p}$

• Where:

  • $N_s$ = number of turns in the secondary coil

  • $N_p$ = number of turns in the primary coil

  • $V_s$ = output voltage from the secondary coil (V)

  • $V_p$ = input voltage in the primary coil (V)

• There are two types of transformers:

  • step-up transformer (increases the voltage of the power source) where $N_s>\hspace{0.17em}{N}_p$

  • step-down transformer (decreases the voltage of the power source) where $N_p>N_s$

• A step-up transformer is used between power stations and transmission wires

• A step-down transformer is used between the transmission wires and buildings