(HT only) Radio waves can be produced by oscillations in electrical…

The idea

This is Higher tier. Radio waves are made by making charges oscillate. An alternating current in an aerial means electrons move backwards and forwards along the metal many times a second, and those accelerating charges produce a changing electric and magnetic field that travels away as an electromagnetic wave. The frequency of the radio wave produced is the same as the frequency of the alternating current driving the aerial, so a circuit oscillating at 100 MHz radiates a 100 MHz wave. This is the link between electricity and the electromagnetic spectrum: an oscillating circuit at one end of an aerial and a wave leaving it at 3 × 10⁸ m/s. Longer aerials are generally used to transmit the lower frequencies efficiently.

What earns the marks

• Stating that an alternating current in an aerial makes charges oscillate.
• Saying that the oscillating charges produce a radio wave.
• Stating that the frequency of the radio wave equals the frequency of the oscillation in the circuit.
• Saying that the wave travels away from the aerial at the speed of electromagnetic waves.

Common mistakes and how to put them right

• Saying the current itself travels out of the aerial as the wave.

• Claiming that a direct current in an aerial produces radio waves.

• Confusing the speed of the wave with the frequency of the circuit.

• Saying the aerial emits sound waves which are then converted into radio waves.

Exam tips

• Use the word oscillation for both the circuit and the wave, and state that the two frequencies match.
• This is Higher tier only and usually appears with the absorption statement, so learn the pair together.
• If a frequency is given in MHz, convert it to Hz before any calculation.