12.3 Transmission of electrical power

12.3.1 Outline the reasons for power losses in transmission lines and real transformers.


Power is lost in transmission lines due to resistance (which causes the wires to heat up). Power losses also occur in real (or non-ideal) transformers due to the following reasons:

• Resistance of the windings of a transformer result in the transformer heating up.
• Eddy currents are unwanted currents induced in the iron core. The currents can be reduced by laminating the core into individually electrically insulated thin strips.
• Hysteresis losses causes the iron core to warm up due to the continued cycle of changes to its magnetism.
• Flux losses are caused by magnetic leakage. A transformer is only 100% efficient if all of the magnetic flux that is produced by the primary links with the secondary.

12.3.2 Explain the use of high-voltage step-up and step-down transformers in the transmission of electrical power.

If large amounts of power are being distributed, the current I will be very high. As the wires don't (and can't) have 0 resistance, they dissipate some power. Power dissipated is P = I^2 x R. If I is large then I^2 and therefore P will be  very large. Over a long distance, the power dissipated would be significant. The solution is to choose to transmit the power at a very high potential difference, such that only a small current needs to flow. A very high potential difference is much more efficient, but much more dangerous to the user. A step-up transformer is used to increase the voltage at the transmission stage and a step-down transformer is used to decrease it for use by the end user.


12.3.4 Suggest how extra-low-frequency electromagnetic fields, such as those created by electrical appliances and power lines, induce currents within a human body.


Electrical power lines carry alternating currents, which means they produce changing extra-low frequency electromagnetic fields. These changing fields are theoretically able to induce currents within any conductor nearby, including human bodies. The photons sent out by these electromagnetic fields therefore induce small currents in the human body, however these are too low to ionise.


12.3.5 Discuss some of the possible risks involved in living and working near high-voltage power lines.

• Electrical power lines on pylons are not insulated along their length and therefore are very dangerous if they become unattached from the pylons.
• There is also some statistical evidence to suggest that children who live near power lines are more likely to get leukemia. However, this relationship has not been properly proven and is not completely understood.