IGCSE Physics notes: every 0625 topic
These notes summarise every section of the Cambridge IGCSE Physics (0625) syllabus: the key ideas, the equations and the facts examiners ask for most. Use them to revise, then test yourself with past papers.
On this page
1. Motion, forces and energy
- Measurement: use rulers, measuring cylinders and clocks; measure many oscillations or thicknesses and divide to improve precision. Scalars have size only; vectors have size and direction.
- Motion: speed = distance ÷ time. On a distance-time graph the gradient is speed; on a speed-time graph the gradient is acceleration and the area under the line is distance.
- Mass, weight and density: weight W = mg, with g about 9.8 N/kg. Density ρ = m / V; objects less dense than a liquid float in it.
- Forces: a resultant force changes speed or direction (F = ma). Springs obey Hooke's law up to the limit of proportionality. Friction and air resistance oppose motion; at terminal velocity the forces balance.
- Turning effects: moment = force × perpendicular distance. In equilibrium, clockwise moments equal anticlockwise moments and the resultant force is zero.
- Momentum: p = mv; momentum is conserved in collisions; impulse = FΔt = Δ(mv).
- Energy, work and power: energy is stored as kinetic, gravitational potential, chemical, elastic, nuclear, electrostatic and internal energy, and transferred by forces, electrical currents, heating and waves. Work = force × distance; power = energy ÷ time; efficiency = useful output ÷ total input.
- Energy resources: compare fossil fuels, nuclear, wind, water, geothermal, solar and biofuels by renewability, availability, reliability, scale and environmental impact. The Sun is the source of most of Earth's energy resources.
- Pressure: p = F / A; pressure in a liquid increases with depth and density (Δp = ρgΔh).
2. Thermal physics
- Particle model: solids have fixed, closely packed particles that vibrate; liquids have close particles that move around each other; gas particles are far apart and move randomly at high speed.
- Gases: gas pressure comes from particles colliding with the walls. At constant temperature, pV is constant. Absolute zero is −273 °C, or 0 K.
- Thermal expansion: solids expand least, gases most; this matters in bridges, railway tracks and thermometers.
- Specific heat capacity: the energy needed to raise 1 kg by 1 °C. ΔE = mcΔθ.
- Changes of state: melting and boiling happen at fixed temperatures while energy is absorbed; evaporation happens at any temperature from the surface and cools the liquid.
- Transfer of thermal energy: conduction (best in metals), convection (in fluids, as warmer, less dense fluid rises) and infrared radiation (dull black surfaces are the best emitters and absorbers).
3. Waves
- General properties: waves transfer energy without transferring matter. Transverse waves vibrate at right angles to the direction of travel; longitudinal waves vibrate along it. v = fλ. Waves reflect, refract and diffract.
- Light: angle of incidence equals angle of reflection; light bends towards the normal entering a denser medium. Refractive index n = sin i / sin r. Total internal reflection happens above the critical angle, used in optical fibres. Converging lenses form real or virtual images; white light disperses into a spectrum in a prism.
- Electromagnetic spectrum: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays, all travelling at 3 × 108 m/s in a vacuum, with uses and dangers for each.
- Sound: a longitudinal wave that needs a medium. Human hearing is about 20 Hz to 20 kHz; ultrasound is above that. Louder means larger amplitude; higher pitch means higher frequency. Echoes are used to measure the speed of sound and distances.
4. Electricity and magnetism
- Magnetism: like poles repel, unlike attract. Field lines go from north to south. Soft iron magnetises and demagnetises easily; steel keeps its magnetism.
- Charge and current: current is the rate of flow of charge, I = Q / t, measured with an ammeter in series. Conventional current flows from + to −.
- Potential difference and resistance: measured with a voltmeter in parallel. R = V / I. Resistance of a wire increases with length and decreases with cross-sectional area.
- Electrical power and energy: P = IV and E = IVt; household energy is paid for in kilowatt-hours.
- Circuits: in series, current is the same everywhere and voltages add; in parallel, voltage is the same across each branch and currents add. Thermistors, LDRs, diodes and potential dividers control circuits.
- Electrical safety: fuses and circuit breakers protect against overload; earthing protects against electric shock from metal cases.
- Electromagnetic effects: a changing magnetic field induces an e.m.f. (generators); a current in a magnetic field feels a force (motors); transformers change alternating voltages, which is why electricity is transmitted at high voltage to reduce power loss.
5. Nuclear physics
- The atom: a small, dense, positive nucleus of protons and neutrons, with electrons around it. The alpha-scattering experiment gave evidence for the nucleus.
- Nuclides: proton number Z and nucleon number A. Isotopes have the same Z but a different number of neutrons.
- Radioactivity: alpha (helium nuclei, stopped by paper), beta (electrons, stopped by a few millimetres of aluminium) and gamma (electromagnetic waves, reduced by thick lead). Decay is random and spontaneous.
- Half-life: the time for half the nuclei in a sample to decay, or for the count rate to halve. Background radiation must be subtracted from measurements.
- Safety: reduce exposure time, increase distance and use shielding.
- Fission and fusion: fission splits large nuclei in reactors; fusion joins small nuclei in stars.
6. Space physics
- The Earth: rotation on its axis gives day and night; its orbit around the Sun takes about 365 days; the tilt of its axis gives the seasons. The Moon orbits the Earth in about one month.
- The Solar System: the Sun, eight planets, dwarf planets, moons, asteroids and comets. Gravity keeps bodies in orbit, and orbital speed v = 2πr / T.
- Stars: the Sun is a medium-sized star powered by fusion of hydrogen into helium. Stars form from clouds of dust and gas and evolve differently depending on their mass.
- The Universe: galaxies are moving away from us, shown by redshift, which supports the Big Bang theory. The Hubble constant H0 = v / d, and the age of the Universe is about 1 / H0.
Using the notes on a typical question
A 2.0 kg block is heated by a 50 W heater for 4.0 minutes and its temperature rises by 12 °C. Find its specific heat capacity.
- Energy supplied: E = Pt = 50 × 240 = 12 000 J.
- c = ΔE / (mΔθ) = 12 000 / (2.0 × 12) = 500 J/(kg °C).
- Real answers come out lower than the true value if energy is lost to the surroundings, a point examiners often ask about.
A half-life question
A source has a count rate of 800 counts per minute, after subtracting background. Its half-life is 2 hours. What is the count rate after 6 hours?
- 6 hours is three half-lives: 800 → 400 → 200 → 100.
- Count rate = 100 counts per minute.
How to use these notes
- Read one section, then close the page and write down every equation and definition you remember.
- Check against our IGCSE Physics equations list.
- Answer past paper questions on that section from the 0625 past papers.
- Add anything you got wrong to a mistakes list, and revisit it weekly.
Common mistakes
- Confusing mass (kg) with weight (N).
- Saying evaporation happens only at the boiling point.
- Mixing up series and parallel rules for current and voltage.
- Forgetting to subtract background radiation before half-life calculations.
Practice questions
Try each one before opening the answer.
Q1. A car travels 150 m in 12 s. What is its average speed?
Show answer
150 / 12 = 12.5 m/s.
Q2. Which type of radiation is most ionising?
Show answer
Alpha.
Q3. Why is electricity transmitted at high voltage?
Show answer
To reduce the current, and so reduce power losses in the cables (P = I²R).
Q4. What is 27 °C in kelvin?
Show answer
27 + 273 = 300 K.
Frequently asked questions
Are these notes enough to get an A*?
They cover the syllabus content, but top grades come from practising past paper questions and learning the exact wording in mark schemes.
Do these notes cover Extended content?
Yes, they include the main Supplement ideas. Check your syllabus for which points are Extended only.
Is there an equation sheet in the exam?
No. Cambridge 0625 expects you to recall the equations.
Key takeaways
- Six sections: motion and energy, thermal, waves, electricity and magnetism, nuclear, space.
- Learn each equation with its units.
- Definitions must be precise.
- Practise each section with past papers.
- No equation sheet in the exam.