Properties of electromagnetic waves 2 — AQA GCSE Physics
Test yourself on Properties of electromagnetic waves 2 with AQA GCSE practice questions.
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Properties of electromagnetic waves 2 explained
This is Higher tier. Radio waves are made by making charges oscillate.
Read the full explanation
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.
Your focus
- Describe how an alternating current in an aerial produces a radio wave.
- State the relationship between the frequency of the circuit oscillation and the frequency of the wave emitted.
- Explain why a steady direct current in an aerial does not produce a radio wave.
Properties of electromagnetic waves 2 exam tips
Marking Points
- one mark for stating that an alternating current in an aerial makes charges oscillate
- one mark for saying that the oscillating charges produce a radio wave
- one mark for stating that the frequency of the radio wave equals the frequency of the oscillation in the circuit
- one mark for saying that the wave travels away from the aerial at the speed of electromagnetic waves
Examiner 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.
Common Mistakes
- 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