5.23-5.24 - EM radiation from circuits and atoms

Radiation as a two-way energy transfer

Imagine a spacecraft sending a radio signal to Earth. There is no conducting wire between the spacecraft and the receiving station, so electric charge cannot flow through a complete circuit from one to the other. Instead, the transmitter transfers energy into a radio wave, and the wave transfers energy across space to the receiver.

This is one example of a wider two-way idea:

• a physical system can generate electromagnetic radiation, transferring energy out;
• electromagnetic radiation can be absorbed by a system, transferring energy in and causing a change.

The system might be an electrical circuit, an atom or a nucleus. The detailed response differs, but electromagnetic radiation always carries energy between the source and the absorber. It does not carry the source's atoms or the circuit's electrons across the gap.

Higher tier: Radio waves and circuit oscillations

This part covers Higher-tier statement 5.23.

In a transmitting circuit, an alternating voltage makes charge move back and forth. This repeated back-and-forth change is an electrical oscillation. When the transmitting aerial is part of the circuit, the oscillating charges produce radio waves and energy is transferred away from the aerial.

At a receiving aerial, the process works in the other direction. The incoming radio wave acts on charges in the conductor and makes them oscillate, producing a small alternating voltage or current in the receiving circuit. Induce means cause here: the radio wave induces an electrical oscillation without a conducting connection to the transmitter.

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The complete sequence is:

oscillations in transmitting circuit -> radio wave -> oscillations in receiving circuit

The radio wave is not an electric current flowing through space. It is electromagnetic radiation transferring energy. The current exists in each conductor, and receiver electronics can then process and amplify the small induced signal.

Changes that generate radiation

The rest of the lesson is shared Foundation-and-Higher content from statement 5.24.