Physics 5.23 - Radio waves and electrical circuits

Radio waves and circuit oscillations

Higher tier only

This lesson covers Higher-tier Physics 5.23 (1SC0/1PH).

An electrical oscillation is a repeated change in an electrical quantity. In a radio circuit, current repeatedly reverses direction and charges move back and forth in the conductor. The circuit itself does not have to shake mechanically.

From a circuit to a radio wave

A transmitter produces a rapidly alternating current in an aerial. The charges in the aerial accelerate as their motion changes. This creates changing electric and magnetic fields that travel away as a radio wave. The radio wave is electromagnetic radiation and transfers energy across the gap.

An unchanging direct current is not the model required here. A steady current produces steady fields; the oscillating current is what produces the travelling radio wave.

From a radio wave to a receiving circuit

At a receiving aerial, the changing fields of the incoming radio wave exert forces on charges in the conductor. The charges move back and forth, producing an alternating potential difference and an oscillating current in the receiving circuit. The induced oscillation follows the frequency of the incoming wave.

Worked causal example

Trace one signal from a transmitter to a receiver:

1. the transmitter circuit produces an oscillating current;
2. the current makes charges oscillate in the transmitting aerial;
3. the aerial emits a radio wave, which crosses the air gap;
4. the wave reaches the receiving aerial and makes charges there oscillate;
5. the receiving circuit has an induced alternating electrical signal.