Recall that radio waves can be produced by, or can themselves induce,…

The idea

Radio waves sit at the low-frequency, long-wavelength end of the electromagnetic spectrum. First, an oscillating current in a circuit produces radio waves: electrons accelerating back and forth in a conductor radiate electromagnetic energy, and an aerial is designed to make this efficient. Second, the reverse happens: a passing radio wave can induce an oscillating current in a circuit, which is how a receiving aerial and tuned circuit pick up a signal. The frequency of the emitted or induced oscillation matches the frequency of the radio wave. A useful method is to picture a transmitter driving a current up and down an aerial and a receiver where the wave drives a small current in the aerial circuit.

What earns the marks

• States that an oscillating current in an electrical circuit produces radio waves.
• States that radio waves can induce an oscillating current in an electrical circuit.
• Links the frequency of the induced oscillation to the frequency of the incoming radio wave.
• Identifies the aerial or tuned circuit as the part of the circuit that emits or detects the radio waves.

Common mistakes and how to put them right

• Claims that radio waves are sound waves.

  Put it right by stating that radio waves are electromagnetic waves that can travel through a vacuum.

• Asserts that a steady direct current produces radio waves.

  Put it right by stating that the current must oscillate.

• Says a radio wave can only be absorbed and never induces a current.

  Put it right by stating that a radio wave can induce an oscillating current in a circuit.

• Treats the process as one-way only.

  Put it right by stating that the link works in both directions.

Exam tips

• Read the question carefully to see whether it asks about producing radio waves or about radio waves inducing a current, because the direction matters.
• Remember the word oscillation: a steady current does not radiate radio waves.
• Link the frequency of the circuit oscillation to the frequency of the radio wave when a question asks about tuning.