Radio waves come from oscillating circuits
Definition
Oscillation
A repeated variation about a central value.
- An alternating potential difference makes electrons in a transmitting circuit oscillate, so the current repeatedly changes size and direction.
- An accelerating charge produces changing electric and magnetic fields, which travel away from the circuit as an electromagnetic wave.
- The radio wave has the same frequency as the oscillation that produces it, so a faster electrical oscillation produces a higher-frequency radio wave.
- A transmitting aerial transfers energy from the oscillating circuit to the electromagnetic field.
Radio waves induce electrical oscillations
Definition
Induced oscillation
An oscillation produced in a circuit by the changing electric and magnetic fields of an incoming electromagnetic wave.
- When a radio wave reaches a receiving aerial, its changing electric field exerts forces on electrons in the conductor.
- The electrons oscillate at the frequency of the incoming wave, producing a small alternating potential difference and current in the receiving circuit.
- A tuned circuit responds most strongly when its natural frequency matches the radio-wave frequency, allowing one signal to be selected from many.
- The signal can then be amplified and decoded to reproduce the transmitted information.
Example
Matching transmitter and receiver
- A transmitter circuit oscillates at 95.8 MHz95.8\,\text{MHz}95.8MHz, so it emits radio waves with frequency 95.8 MHz95.8\,\text{MHz}95.8MHz.
- A receiving aerial develops an alternating signal at the same frequency.
- Tuning the receiver to 95.8 MHz95.8\,\text{MHz}95.8MHz makes that station's signal produce the strongest response.
Exam technique
Explain the complete chain
- Link oscillating electrons in the transmitter to the production of a radio wave.
- State that the incoming wave makes electrons in the receiving circuit oscillate.
- Use the word frequency to connect the transmitter, electromagnetic wave and receiver.
Common Mistake
Do not state that the radio wave carries electrons from the transmitter to the receiver; energy and information travel in the electromagnetic wave while electrons oscillate locally in each circuit.
Communication depends on modulation
- Information is carried by deliberately varying a property of a high-frequency carrier wave, a process called modulation.
- The receiver extracts these variations to recover sound, images or digital data.
- The detailed electronics of modulation are not required here, but the carrier still originates from electrical oscillations.
Self review
- How does an oscillating current produce a radio wave?
- What determines the frequency of the radio wave?
- What happens to electrons in a receiving aerial?
- Why can a tuned receiver select one station?
