P4 Waves and Radioactivity
Wave properties
All waves transfer energy without transferring matter.
Transverse waves: vibration is perpendicular to wave direction. Examples: all EM waves, water waves. Longitudinal waves: vibration is parallel to wave direction. Example: sound.
Key terms:
- Amplitude A: maximum displacement from equilibrium. Related to energy.
- Wavelength (λ): distance of one complete cycle (peak to peak).
- Frequency (f): number of complete cycles per second (Hz).
- Period (T): time for one complete cycle; T = 1/f.
Wave equation:
v = fλ
Speed (m/s) = frequency (Hz) × wavelength (m).
The electromagnetic spectrum
All EM waves travel at 3 × 10⁸ m/s in a vacuum. In order of increasing frequency (decreasing wavelength):
Radio → Microwaves → Infrared → Visible → UV → X-rays → Gamma rays
| Wave | Typical use |
|---|---|
| Radio | Broadcasting, communication |
| Microwaves | Satellite communication, cooking |
| Infrared | Remote controls, thermal imaging |
| Visible | Sight, photography |
| UV | Sterilisation, tanning |
| X-rays | Medical imaging |
| Gamma | Cancer treatment, sterilising food |
Higher frequency → higher energy → more ionising (and more hazardous).
Reflection and refraction
Reflection: angle of incidence = angle of reflection (both measured from normal).
Refraction: wave changes direction when it changes speed (entering a new medium). Light slows down and bends towards the normal when entering a denser medium.
n = sin i / sin r (Snell's law — Higher)
Total internal reflection (Higher): when angle of incidence exceeds the critical angle inside a denser medium, light reflects internally. Used in optical fibres and endoscopes.
PAG P4.1: Investigate reflection and refraction using ray boxes, mirrors, and glass blocks.
Nuclear model and radioactivity
Rutherford scattering established the nuclear model: atoms have a tiny, dense, positive nucleus surrounded by electrons.
The nucleus contains protons (+) and neutrons (neutral). Atomic number (Z) = number of protons; mass number A = protons + neutrons.
Isotopes: same element (same Z) but different number of neutrons. Some are unstable → radioactive decay.
Types of radiation
| Radiation | Nature | Charge | Mass | Range in air | Stopped by |
|---|---|---|---|---|---|
| Alpha (α) | Helium nucleus (2p, 2n) | +2 | 4 | ~5 cm | Paper / skin |
| Beta (β) | Fast electron | −1 | ~0 | ~1 m | 3 mm aluminium |
| Gamma (γ) | EM radiation | 0 | 0 | Very large | Several cm lead / thick concrete |
Ionising power: α > β > γ. Penetrating power: γ > β > α.
Decay equations
Alpha: ᴬ_Z X → ᴬ⁻⁴_{Z−2} Y + ⁴₂He
Beta (β⁻): ᴬ_Z X → ᴬ_{Z+1} Y + ⁰_{−1}e
Gamma: no change in A or Z; nucleus just loses energy.
Half-life
Half-life (t½): time for the count rate (or number of undecayed nuclei) to halve.
After n half-lives, activity/count = Initial × (½)ⁿ.
PAG P4.2: Simulate radioactive decay using dice/coins; plot N vs time; measure half-life from graph.
Nuclear fission and fusion
Fission: a large nucleus (e.g. uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing 2–3 neutrons (chain reaction) and large amounts of energy.
In a nuclear reactor, a moderator (water or graphite) slows neutrons to increase fission probability. Control rods (boron or cadmium) absorb neutrons to control the rate of reaction.
Fusion: two light nuclei (e.g. hydrogen isotopes) combine to form a heavier nucleus, releasing enormous amounts of energy. Requires very high temperature (>10⁷ K) and pressure — achieved in stars.
Star lifecycle: nebula → protostar → main sequence → (red giant or supergiant) → (white dwarf, neutron star, or black hole).
Balance of forces in a star: gravitational collapse balanced by radiation pressure (outward force from fusion energy).
⚠Common mistakes
- Wave equation: confusing v = fλ with v = f/λ.
- Half-life: calculating the number of half-lives wrong — always check how many t½ fit into the total time.
- Alpha, beta, gamma penetration: "aluminium stops alpha" — wrong, paper/skin stops alpha; aluminium stops beta.
- Fission vs fusion: fission = splitting (U-235); fusion = joining (H isotopes in stars).
- Decay equations: forgetting to balance both A and Z on both sides.
AI-generated · claude-opus-4-7 · v3-ocr-physics