Physics 3 - Electromagnetism - Transformers

A typical power station generator produces an alternating potential difference of about 25,000 volts. Mains electricity to homes and offices is at 230 volts.

When you plug a heater into the mains, electricity reaches you from a power station via a network of cables called The National Grid. The alternating p.d. of the cables (the grid voltage) is typically 132,000.

A transformer is used at each stage to change the alternating p.d. We also use transformers in low-voltage supply units to step the mains p.d. down from 230 V.

How they work:

A transformer has two coils of insulated wire, both wound round the same iron core. When an alternating current passes through the primary coil, an alternating p.d. is induced into the secondary coil. This happens because:

- Alternating current passing through the primary coil produces an alternating magnetic field

- The lines of the alternating magnetic field pass through the secondary coil and induce an alternating p.d. in it

If a lamp is connected across the secondary coil, the induced p.d. causes a current in the secondary circuit. So the lamp lights up. Electrical energy is therefore transferred from the primary coil to the secondary coil. This happens even though they are not electrically connected in the same circuit.

Transformers only work with alternating current. With a direct current, there is no changing magnetic field so the secondary voltage is 0. The core of the transformer 'guides' the field lines in a loop through the coils. But the field must be changing to induce a p.d. in the secondary coil. 

In a practical transformer, the primary and secondary coils are both wound round the same part of the core. The core is layered (laminated) to cut out induced currents in the iron layers. If it were not laminated, the efficiency of the transformer would be greatly reduced.