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13.3 Communications

GCSE Physics · AQA · Chapter 13: Electromagnetic Waves

A free GCSE Physics lesson for AQA (8463) from OBA School: choosing waves for communication, carrier waves, mobile phone safety and optical fibres. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how the wavelength of a wave affects its range, spreading and the information it can carry
  • Explain why microwaves are used for satellite links and radio waves of different wavelengths for broadcasting
  • Evaluate evidence about whether mobile phones are safe to use
  • (Higher tier) explain what a carrier wave is and how radio waves are produced and absorbed by oscillating charges in circuits
  • Describe how optical fibres carry signals and their advantages

Key words

  • Signal: Information (sound, pictures, data) sent from one place to another.
  • Carrier wave: (Higher tier) A wave that carries a signal. The signal is added by varying (modulating) the wave’s amplitude or frequency.
  • Modulation: (Higher tier) Changing a carrier wave so that it matches the pattern of the signal.
  • Aerial: A metal rod or wire that emits radio waves (transmitter) or absorbs them (receiver).
  • Optical fibre: A very thin glass fibre that carries pulses of light or infrared by total internal reflection.
  • SAR: Specific absorption rate: the energy per second absorbed per kilogram of the head from a phone, in W/kg.

Choosing the right wave

Radio waves, microwaves, infrared and visible light are all used to carry signals. Which one is chosen depends on how much information has to be carried and how far it has to go. The radio and microwave part of the spectrum is shared out in bands, because the wavelength changes how a wave behaves.

The shorter the wavelength:

• the more information the waves can carry each second

• the shorter the range (the atmosphere absorbs more of the waves)

• the less the waves spread out, so the beam stays narrow and the signal stays strong.

  • Microwaves; Typical wavelength about 1 mm – 10 cm; Used for satellite TV and phone links; Reason pass through the atmosphere to satellites; spread out very little
  • Short radio waves; Typical wavelength less than 1 m; Used for TV broadcasting from masts; Reason carry more information than longer radio waves
  • Medium radio waves; Typical wavelength 1 m – 100 m; Used for local radio, emergency services; Reason range limited to the area around the transmitter
  • Long radio waves; Typical wavelength more than 100 m; Used for national and international radio; Reason very long range: they follow the curve of the Earth

Worked example: time delay on a satellite link

A TV satellite orbits 36 000 km above the ground. A signal goes up from a ground station and back down to a dish. Calculate the time taken.
distance = 2 × 36 000 km = 72 000 km = 7.2 × 10⁷ m
time = distance ÷ speed = 7.2 × 10⁷ ÷ 3.0 × 10⁸ = 0.24 s

Answer: 0.24 s

Are mobile phones safe?

A mobile phone sends and receives microwaves (or short radio waves) to and from the nearest mast. When the phone is held against the head, some of this energy is absorbed by the head. Some scientists have suggested this might affect the brain. Children may be more at risk because their skulls are thinner and their nervous systems are still developing.

So far, studies have found no clear evidence of harm, but mobile phones have only been in wide use for a few decades. Health advice is therefore cautious: use a hands-free kit or speaker, keep calls short, and limit use by children. More research is needed, especially long-term studies.

  • Model K; SAR at the head (W/kg) 0.35
  • Model L; SAR at the head (W/kg) 0.92
  • Model M; SAR at the head (W/kg) 1.48

Phones sold in the UK must have a SAR below 2.0 W/kg. In the table, all three pass; model K transfers the least energy per second to the head, so it is the lowest-risk choice.

Exam tip: When you evaluate a study, ask: Was the sample large and varied? Was there a control group (non-users)? Did it last long enough for slow illnesses such as cancer to appear? Who paid for it – could there be bias? Were people’s answers just based on memory?

Carrier waves and radio signals (Higher tier)

A carrier wave is a wave used to carry a signal. In a radio station:

  • A microphone turns sound into a varying electric current – the audio signal.
  • An oscillator produces an alternating current (a.c.) of a fixed high frequency – the carrier wave.
  • In the transmitter the audio signal modulates (varies) the amplitude or frequency of the carrier.
  • The modulated a.c. flows in the transmitter aerial. The oscillating charges (electrons) in the aerial emit radio waves with the same frequency as the a.c.
  • When the radio waves are absorbed by a receiver aerial, they make the electrons in it oscillate, inducing an alternating current with the same frequency as the radio waves.
  • The receiver separates the audio signal from the carrier and sends it to a loudspeaker.

Optical fibres

An optical fibre is a very thin strand of glass. Pulses of light or infrared travel along it. Each time a ray reaches the edge of the fibre it is reflected back in (total internal reflection), so the signal stays inside even when the fibre bends.

Compared with radio waves and microwaves, optical fibres:

  • carry much more information, because light and infrared have much shorter wavelengths and so more pulses can be sent each second
  • are more secure, because the signal stays inside the fibre and cannot be picked up by anyone nearby
  • are not affected by electrical interference from other devices.

Common mistakes

  • Saying long radio waves are used for TV because they carry more information. It is the shorter wavelengths that carry more information.
  • Forgetting that a satellite signal travels up and down – double the height before working out the time.
  • Stating that mobile phones are proven to cause cancer, or proven to be safe. The evidence so far is not conclusive.
  • (Higher tier) Saying the receiver aerial produces a direct current. It is an alternating current at the same frequency as the radio waves.
  • Writing that light escapes from the sides of an optical fibre. It is reflected back inside every time.

Revision questions

Try these, then check your answers below each one.

Question 1. As the wavelength of radio waves used for communication gets shorter, which of these happens? (Choose from: The waves can carry more information, The waves have a longer range, The waves spread out more, The waves travel faster.)

Answer: The waves can carry more information

Working: Shorter wavelengths carry more information, have a shorter range (more absorption by the atmosphere) and spread out less. Speed stays the same.

Question 2. A local radio station broadcasts at 95.0 MHz. The speed of radio waves is 3.0 × 10⁸ m/s. Calculate the wavelength to 3 significant figures.

Answer: 3.16 m

Working: λ = 3.0 × 10⁸ ÷ 9.50 × 10⁷ = 3.16 m – in the 1 m to 100 m band used for local radio.

Question 3. (Higher tier) True or false? When radio waves are absorbed by a receiver aerial, they induce an alternating current with the same frequency as the radio waves.

Answer: True

Working: True. The radio waves make the electrons in the aerial oscillate, which is an alternating current at the frequency of the waves.

Related lessons

Continue in the full course

This lesson is section 13.3 (AGCPH1303) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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