Lesson Goals
How to Use This Lesson
- Read each section in order.
- Watch the videos placed directly after the related topic.
- Complete the interactive checks.
- Finish with the self-check quiz and reflection.
P5 Nuclear Physics
P5.1 The nucleus and P5.2 Radioactivity
P5.1 The Nucleus
The nucleus is the tiny, dense centre of an atom. It contains protons and neutrons. Electrons are found outside the nucleus.
- Atomic number (Z) = number of protons
- Nucleon / mass number (A) = protons + neutrons
- Neutrons = A − Z
Nuclide notation: A / Z X for example, carbon-14 can be written as 14 / 6 C.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Supplement Ideas
- The proton number decides the identity of the element.
- The relative charge of the nucleus depends on the number of protons present.
- Fission is the splitting of a large nucleus into smaller nuclei.
- Fusion is the joining of small nuclei to form a larger nucleus.
Nuclear Physics Videos: Part 1
Video 1: The Nucleus
Video 2: Atomic Structure and Nuclear Ideas
P5.2.1 Detection of Radioactivity
Ionising radiation is radiation that can remove electrons from atoms and turn them into ions.
Background radiation is the low level of radiation always present around us.
Main sources of background radiation
- radon gas in the air
- rocks and buildings
- food and drink
- cosmic rays from space
Radioactivity can be measured using a detector connected to a counter. Count rate may be given in counts per second or counts per minute.
P5.2.2 Three Types of Emission
| Type | Nature | Ionising | Penetration |
|---|---|---|---|
| Alpha (α) | helium nucleus | strong | weak |
| Beta (β) | fast electron | medium | medium |
| Gamma (γ) | electromagnetic wave | weak | strong |
P5.2.3 Radioactive Decay
Radioactive decay is a spontaneous and random change in an unstable nucleus.
- During alpha decay, the nucleus loses 2 protons and 2 neutrons.
- During beta decay, a neutron changes into a proton and an electron.
- Gamma emission often happens after alpha or beta decay when the nucleus loses extra energy.
P5.2.4 Half-Life
The half-life of a radioactive isotope is the time taken for half the nuclei in a sample to decay.
- After 1 half-life, 1/2 remains
- After 2 half-lives, 1/4 remains
- After 3 half-lives, 1/8 remains
You may use tables or decay curves to answer half-life questions.
P5.2.5 Applications of Radioactivity and Safety Precautions
Applications
- smoke alarms
- irradiating food to kill bacteria
- sterilising equipment using gamma rays
- measuring and controlling thickness of materials
- diagnosis and treatment of cancer using gamma rays
Safety
Ionising radiation can harm living things by causing cell death, mutations and cancer.
Radioactive materials must be handled safely using:
- Time – reduce exposure time
- Distance – stay as far away as possible
- Shielding – use suitable shielding such as lead or concrete
Nuclear Physics Videos: Detection, Decay and Half-Life
Video 3: Radioactive Decay and Half-Life
Nuclear Physics Videos: Applications and Safety
Video 4: Why Radiation Is Harmful
Video 5: Using Radiation in Medicine
Quick Check: Key Nuclear Terms
Half-Life Practice
A sample starts with 160 undecayed nuclei. After 3 half-lives, how many undecayed nuclei remain?
Drag-and-Drop Matching
Drag each label into the correct box.
Alpha radiation
Gamma radiation
Half-life
Atomic number
P6 Space Physics
P6.1 The Solar System and P6.2 Stars and the Universe
P6.1 The Solar System
The Solar System contains:
- one star: the Sun
- the eight planets in order from the Sun
- minor planets including dwarf planets such as Pluto and asteroids in the asteroid belt
- moons that orbit planets
P6.2.1 The Sun as a Star
- The Sun is the closest star to Earth.
- Astronomical distances can be measured in light-years.
- The Sun contains most of the mass of the Solar System, which explains why planets orbit the Sun.
- The force that keeps an object in orbit around the Sun is due to the Sun’s gravitational attraction.
- The Sun is a medium-sized star made mostly of hydrogen and helium.
- It radiates energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.
Supplement
where r is the radius of the orbit and T is the orbital period.
- Orbital speed can be defined using the equation above.
- The strength of the Sun’s gravitational field decreases with distance.
- Orbital speeds of planets decrease as distance from the Sun increases.
- Stars are powered by nuclear reactions. In stable stars, hydrogen fuses into helium.
Space Physics Videos: Part 1
Video 6: The Sun and the Solar System
P6.2.2 Life Cycle of Stars
- Stars are formed as protostars from interstellar clouds of gas and dust due to gravitational attraction.
- Small mass star: red giant → white dwarf + planetary nebula
- Large mass star: red supergiant → supernova → neutron star
- Very large mass star: red supergiant → supernova → black hole
Supplement
The nebula from a supernova may form new stars with orbiting planets.
Interactive Star Life Cycle Diagram
Follow the pathways from a nebula to the final stages of different stars.
Space Physics Videos: Part 2
Video 7: Life Cycle of Stars
Video 8: Supernova
Video 9: Black Holes
Self-Check Quiz
1. What particles are found in the nucleus?
2. Which type of radiation has the greatest penetrating ability?
3. Which statement about radioactive decay is correct?
4. Background radiation can come from:
5. What is the correct order of the first four planets from the Sun?
6. What powers stable stars?
7. Which final stage can form from a very large mass star?
Reflection
Click to reflect on your learning
1. Which part of nuclear physics do you understand best?
2. What is one difference between alpha, beta and gamma radiation?
3. Why is shielding important when handling radioactive materials?
4. How does the life cycle of a small star differ from that of a very large star?
5. Can you remember the order of the planets from the Sun?