a high voltage transmission system for getting electricity to where it is needed

a high voltage cable often seen between electricity pylons

a device that can change lower alternating voltages to higher ones

a device that can change higher alternating voltages to lower ones

the transfer of energy into the surroundings by heating

The National Grid

Combined science

Danger from electric shock

Short circuits and earth wires

Power of an appliance or electric circuit

Calculating with electric charge

Energy resources

In the Circuit components unit, pupils learnt how the resistance of filament lamps and diodes change with current and about the properties and applications of light dependent resistors and thermistors.

This unit covers UK domestic a.c. supply, mains wiring safety, energy changes in electrical systems, power ratings, and energy transfer. It explains the National Grid, transformers, energy sources, and calculations for energy changes and power transmission efficiency.

Mains electricity

Starting with the power station, sort the following parts of the National Grid in the order in which they are connected together.

A 400 kV transmission line delivers 400 MW of power. The current through the transmission line is {{}} A.

A transmission line has a resistance of 3.0 Ω per 100 km and a current of 1000 A through it. How much energy is dissipated by 250 km of this transmission line each second?

A transformer, with 100% efficiency, increases voltage from 10 kV to 40 kV. If the current through the primary coil is 60 A, the current through the secondary coil is {{}} A. 

High power requires high voltage because power depends only on voltage.

If transmission lines used 230 V to deliver a very high power, they could melt.

Rate of energy dissipation in electrical cables by heating depends on current.

Using a higher voltage allows more power to be delivered using less current.

Which of the following statements about the current and voltage of transmission lines are correct?  

A transformer in a mobile phone charger decreases voltage from 230 V to 10 V. The current in the secondary coil is 2.0 A. If the transformer is 100% efficient, what is the current in the primary coil?

Which of the following are correct equations that include power?

The potential difference across a lamp is 230 V and the current through the lamp is 0.20 A. The energy transferred by the lamp in one second is {{}} J.

Which of the following equations correctly relates current, resistance and potential difference for an electrical component?

All electrical appliances, such as laptops and vacuum cleaners, transfer energy into the surroundings by heating. This process is called …

Efficiency = useful output power ÷ total input power

Efficiency = total input energy transfer ÷ useful output energy transfer

It is the fraction of energy supplied that is usefully transferred.

Which of the following statements about efficiency are correct?

A p.d. is induced across a conductor when the current through it changes.

A p.d. is induced across a conductor when the magnetic field around it changes.

A magnetic field is induced around a conductor when the current in it changes.

A magnetic field is induced around a conductor when the p.d. across it changes.

A current-carrying conductor experiences a force when it is in a magnetic field.

Which of the following statements describes the generator effect?

The National Grid

Lesson outcome