Revision notes for AQA GCSE Physics Properties of electromagnetic waves 2. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Properties of electromagnetic waves 2
What you'll learn
- How radio waves can be produced and detected using oscillating electrical circuits.
- How changes in atoms and nuclei can generate or absorb electromagnetic waves.
- Why ultraviolet, X-rays and gamma rays can be hazardous to body tissue.
- How to use radiation dose data in sieverts and millisieverts to make sensible conclusions.
Quick recap: electromagnetic waves
Electromagnetic waves are waves that can transfer energy through empty space. They include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Definition
Electromagnetic wave
An electromagnetic wave is a transverse wave that transfers energy by oscillating electric and magnetic fields. Transverse means the vibrations are at right angles to the direction the wave travels.
In a vacuum, all electromagnetic waves travel at about 3.0×108 m/s3.0 \times 10^8 \text{ m/s}3.0×108 m/s. What changes across the spectrum is mainly the frequency and wavelength.
Definition
Frequency and wavelength
- Frequency, fff, is the number of complete waves or oscillations each second. It is measured in hertz, Hz.
- Wavelength, λ\lambdaλ, is the distance from one point on a wave to the same point on the next wave, such as crest to crest. It is measured in metres, m.
Higher frequency electromagnetic waves have shorter wavelengths. This matters because the high-frequency end of the spectrum is where the most hazardous waves in this section are found.
Radio waves and oscillating circuits
This bit is Higher Tier only, so do not panic if it feels a little more abstract.
Radio waves can be produced by oscillations in electrical circuits. An oscillation is a repeated back-and-forth change. In a circuit, this can mean charges moving backwards and forwards.
Definition
Oscillation and alternating current
- An oscillation is a repeated to-and-fro change about a central position or value.
- An alternating current (a.c.) is a current that repeatedly changes direction, so the charges in the circuit oscillate.
In a transmitting aerial, an alternating current makes electrons move up and down. These oscillating charges produce radio waves that travel away from the aerial.
When a radio wave is absorbed, its energy is transferred to the material it hits. If a radio wave is absorbed by a receiving aerial, it can make electrons in the aerial oscillate. This induces an alternating current in the circuit with the same frequency as the radio wave.
Key Idea
Radio waves and circuits
A radio wave can be made by an oscillating electrical circuit, and an incoming radio wave can induce oscillations in another circuit at the same frequency.
Common Mistake
Forgetting the frequency match
Do not just write “the radio wave makes a current”. The important detail is that the induced alternating current has the same frequency as the absorbed radio wave.
Electromagnetic waves from atoms and nuclei
An atom is a tiny particle of matter. It has a central nucleus, containing protons and neutrons, with electrons around it.
Changes inside atoms and nuclei can cause electromagnetic waves to be generated or absorbed. Generated means produced. Absorbed means the wave’s energy is taken in by the atom or nucleus.
For GCSE, you do not need a detailed quantum explanation here. The key idea is that when energy changes happen inside atoms or nuclei, electromagnetic radiation may be emitted or absorbed across a wide range of frequencies.
Definition
Gamma rays
Gamma rays are high-frequency electromagnetic waves that originate from changes in the nucleus of an atom.
The electromagnetic spectrum below shows how the waves fit together, and highlights the main hazards from this section.
Key Idea
Origin of gamma rays
If a question says the radiation comes from a change in the nucleus, the electromagnetic wave to think of is a gamma ray.
Hazards of ultraviolet, X-rays and gamma rays
Electromagnetic waves are very useful, but some types can harm human body tissue. Tissue means a group of cells working together, such as skin tissue.
The hazard depends on two things:
- the type of radiation
- the size of the dose
Definition
Ionising radiation
Ionising radiation has enough energy to remove electrons from atoms or molecules, forming ions. In living cells, this can damage DNA, which may cause gene mutations and cancer.
A gene is a section of DNA that carries instructions for a characteristic. A mutation is a change in a gene.
| Radiation | Main GCSE hazard | Important detail |
|---|---|---|
| Ultraviolet | Can cause skin to age prematurely and increase the risk of skin cancer | Often linked with sunlight and sunbeds |
| X-rays | Can cause gene mutations and cancer | X-rays are ionising radiation |
| Gamma rays | Can cause gene mutations and cancer | Gamma rays are ionising radiation and come from nuclear changes |
Key Idea
Risk, not certainty
Radiation exposure usually increases the risk of harm. It does not mean that one exposure will definitely cause cancer.
Radiation dose
Radiation dose is used to describe the risk of harm from exposure to radiation.
Definition
Radiation dose
Radiation dose is a measure of the risk of harm resulting from exposure of the body to radiation. It is measured in sieverts, Sv, and often in millisieverts, mSv.
You are not expected to recall the unit name from memory, but exam questions may give you doses in Sv or mSv. You should be able to use the conversion:
1000 mSv=1 Sv1000 \text{ mSv} = 1 \text{ Sv}1000 mSv=1 SvSo:
- to convert from mSv to Sv, divide by 1000
- to convert from Sv to mSv, multiply by 1000
Example
Converting radiation dose
A dose is recorded as 350 mSv. Convert it into sieverts.
- Use the conversion 1000 mSv=1 Sv1000 \text{ mSv} = 1 \text{ Sv}1000 mSv=1 Sv, so converting from mSv to Sv means dividing by 1000.
- Calculate 350 mSv÷1000=0.350 Sv350 \text{ mSv} \div 1000 = 0.350 \text{ Sv}350 mSv÷1000=0.350 Sv.
- Simplify the value: 0.350 Sv=0.35 Sv0.350 \text{ Sv} = 0.35 \text{ Sv}0.350 Sv=0.35 Sv, so the dose is less than 1 Sv.
Common Mistake
Mixing up mSv and Sv
500 mSv is not 500 Sv. Because milli means one-thousandth, 500 mSv is 0.5 Sv.
Drawing conclusions from radiation data
Questions may give you data about radiation exposure and ask you to make a conclusion. This is not just recall: you need to use the numbers and connect them to the possible effects.
A strong conclusion usually does three things:
- compares the doses using the data
- mentions the type of radiation
- states the consequence carefully, using “increases the risk” rather than “will definitely cause”
Tip
Using data in your conclusion
Use the pattern: “As the dose increases, the risk increases…” Then quote one comparison from the data and name the possible consequence, such as gene mutation or cancer.
Example
Drawing a conclusion from radiation data
A student is given these exposure data:
- Exposure A: X-rays, 0.1 mSv
- Exposure B: X-rays, 5.0 mSv
- Exposure C: gamma rays, 20 mSv
Which exposure has the greatest risk of gene mutation or cancer?
- Compare the dose values: 20 mSv is greater than 5.0 mSv and 0.1 mSv, so exposure C gives the largest dose.
- Check the radiation type: X-rays and gamma rays are ionising, so all three exposures can increase the risk of gene mutations and cancer.
- Quantify the comparison: 20÷5.0=420 \div 5.0 = 420÷5.0=4, so C gives four times the dose of B; 20÷0.1=20020 \div 0.1 = 20020÷0.1=200, so C gives 200 times the dose of A.
- Make a careful conclusion: exposure C has the greatest risk in this data, but the data shows increased risk, not a guarantee that cancer will occur.
Common Mistake
Overstating the danger
Do not write “X-rays always cause cancer”. A better GCSE answer is: “X-rays are ionising and can cause gene mutations, increasing the risk of cancer.”
Exam technique
In the exam
- For Higher Tier radio questions, link oscillating charges in a circuit to producing radio waves, and absorption of radio waves to an induced a.c. of the same frequency.
- For origins, remember that changes in atoms or nuclei can generate or absorb electromagnetic waves, but gamma rays originate from the nucleus.
- For hazards, match the effect to the radiation: ultraviolet can age skin and increase skin cancer risk; X-rays and gamma rays are ionising and can cause mutations and cancer.
- For dose questions, convert between mSv and Sv if needed, compare the data directly, and write “increases the risk” rather than “definitely causes”.
Self review
Check yourself
- Why does a receiving aerial produce an alternating current with the same frequency as the incoming radio wave?
- What is the difference between the main hazard of ultraviolet and the main hazard of X-rays or gamma rays?
- Convert 750 mSv into Sv, then compare it with a dose of 2 Sv.
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