Physics 3 - Electromagnetism - Transformers and The National Grid

When we use mains device, the electricity is supplied to us through the National Grid from distant power stations. The higher the grid p.d., the greater the efficiency of transferring electrical power through the grid.

This is why transformers are used to step up the p.d. from a power station to the grid p.d. and to step down the grid p.d. to the mains voltage. The grid p.d. is at least 132,000 V. So what difference would it make if the grid p.d. was much lower? Much more current would be needed to deliver the same amount of power. The grid cables would therefore heat up more and waste more power.

The transformer equation:

The secondary p.d. of a transformer depends on the primary p.d. and the number of turns on each coil. We can use the following equation to calculate any one of these factors if we know the other ones.

p.d. across primary, Vp/p.d. across secondary, Vs = number of turns on primary, Np/number of turns on secondary, Ns

- For a step up transformer, the number of secondary turns (Ns) is greater than the number of primary turns (Np). Therefore the Vs is greater than the Vp.

- For a step down transformer, the number of secondary turns (Ns) is less than the number of primary turns (Np). Therefore the Vs is less than Vp.

Transformer Efficiency:

Transformers are almost 100% efficient. When a device is connected to the secondary coil, almost all the electrical power supplied to the transformer is delivered to the device.

- Power supplied to the transformer = primary current, Ip x primary p.d., Vp

- Power delivered by the transformer = secondary current, Is x secondary p.d., Vs

Therefore, for 100% efficiency:

- primary current x primary p.d. = secondary current x secondary p.d.

(IpVp = IsVs)