AQA A Level Physics

Revision Notes

7.10.4 The Operation of a Transformer

The Transformer Equation

  • A transformers is

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

  • This is 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
    • Laminated soft iron core
  • The primary and secondary coils are wound around the laminated soft iron core
  • The soft iron core is necessary to strengthen the magnetic field and reduces eddy currents induced in the iron core
  • At the primary coil, alternating current produces an alternating voltage is applied
  • This sets up a changing magnetic field inside the iron core and therefore a changing magnetic flux linkage
  • A changing magnetic field also passes through to the secondary coil
  • This results in a changing magnetic flux linkage in the secondary coil and from Faraday’s Law, an e.m.f is induced
  • This produces an alternating output voltage from the wire wrapped around the secondary coil
  • The transformer equation is defined as:

Ideal Transformer Equation equation 1

  • Where:
    • Vp = voltage in the primary coil (V)
    • Vs = output voltage from the secondary coil (V)
    • Np = number of turns in the primary coil
    • Ns = number of turns in the secondary coil
  • For an ideal transformer, there is no electrical energy lost and it is 100% efficient
  • This means the power in the primary coil equals the power in the second coil

VpIp = VsIs

  • Where:
    • Ip = current in the primary coil (A)
    • Is = output current from the secondary coil (A)
  • This means the ideal transformer equation can be written as:

Ideal Transformer Equation equation 2

  • The two types of transformers are:
    • Step-up transformer (increases the voltage of the power source) where Ns > Np
    • Step-down transformer (decreases the voltage of the power source) where Np > Ns

Worked Example

A step-down transformer turns a primary voltage of 0.5 kV into a secondary voltage of 100 V.

Calculate the number of turns needed in the secondary coil if the primary coil contains 3000 turns of wire.

Step 1: Ideal transformer equation with the number of turns of coil N and voltage V

Ideal Transformer Equation Worked Example equation 1

Step 2: Rearrange for number of turns in the secondary coil Ns

Ideal Transformer Equation Worked Example equation 2

Step 3: Substitute in values

Ideal Transformer Equation Worked Example equation 3

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