IGCSE Physics equations you need to know
Cambridge IGCSE Physics (0625) does not give you an equation sheet. The syllabus tells you which equations to recall and use, and they are all here, topic by topic, with units and worked examples.
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No equation sheet: learn them
The 0625 syllabus lists the equations you must recall and use. They are not printed in the exam papers, so you need to know each one, its symbols and its units. Some appear only in the Supplement content, which only Extended candidates study. The syllabus marks which ones are Supplement, so check it for your exam year; Extended candidates need every equation below.
1. Motion, forces and energy
- Speed: v = s / t, and average speed = total distance ÷ total time
- Acceleration: a = Δv / Δt
- Weight: W = mg, so g = W / m (g is about 9.8 N/kg)
- Density: ρ = m / V
- Spring constant: k = F / x
- Resultant force: F = ma
- Momentum: p = mv, and impulse = FΔt = Δ(mv)
- Moment of a force = force × perpendicular distance from the pivot
- Kinetic energy: Ek = ½mv²
- Change in gravitational potential energy: ΔEp = mgΔh
- Work done: W = Fd = ΔE
- Power: P = W / t = ΔE / t
- Efficiency = useful energy output ÷ total energy input × 100% (or the same with power)
- Pressure: p = F / A, and pressure in a liquid Δp = ρgΔh
2. Thermal physics
- Temperature conversion: T (in K) = θ (in °C) + 273
- Gas at constant temperature: pV = constant
- Specific heat capacity: c = ΔE / (mΔθ)
3. Waves
- Wave speed: v = fλ
- Refractive index: n = sin i / sin r
- Critical angle: n = 1 / sin c
4. Electricity and magnetism
- Current: I = Q / t
- Electromotive force and potential difference: E = W / Q and V = W / Q
- Resistance: R = V / I
- Electrical power: P = IV, and electrical energy E = IVt
- Resistors in series: R = R1 + R2; in parallel: 1/R = 1/R1 + 1/R2
- Potential divider: R1 / R2 = V1 / V2
- Transformer: Vp / Vs = Np / Ns, and for 100% efficiency IpVp = IsVs
- Power loss in cables: P = I²R
5 and 6. Nuclear and space physics
- Half-life calculations: after each half-life, the activity or number of undecayed nuclei halves
- Orbital speed: v = 2πr / T
- The Hubble constant: H0 = v / d, and the age of the Universe is about 1 / H0
Two equations in one question
A 1200 kg car accelerates from 5.0 m/s to 15 m/s in 4.0 s. Find the acceleration and the resultant force.
- a = Δv / Δt = (15 − 5.0) / 4.0 = 2.5 m/s².
- F = ma = 1200 × 2.5 = 3000 N.
- Show both equations: Cambridge mark schemes award marks for the correct equation even if a later number slips.
Electricity: power and energy
A 2.0 kW heater runs on 230 V for 15 minutes. Find the current and the energy transferred.
- I = P / V = 2000 / 230 = 8.7 A.
- E = Pt = 2000 × 15 × 60 = 1.8 × 106 J.
- Convert minutes to seconds and kilowatts to watts before substituting.
Units you must know
Many marks are lost on units rather than physics. These are the ones that come up most in 0625 papers:
- Force: newton (N). Mass: kilogram (kg). Weight is a force, so it is in N, not kg.
- Energy and work: joule (J). Power: watt (W), which is joules per second.
- Pressure: pascal (Pa), which is N/m².
- Density: kg/m³ or g/cm³. Note that 1 g/cm³ = 1000 kg/m³.
- Momentum: kg m/s. Specific heat capacity: J/(kg °C).
- Charge: coulomb (C). Current: ampere (A). Potential difference: volt (V). Resistance: ohm (Ω).
- Frequency: hertz (Hz). Temperature: °C or kelvin (K).
How to learn them
- Learn each equation in words as well as symbols, for example “pressure equals force divided by area”.
- Learn the unit of every quantity; many marks are lost on units.
- Practise rearranging each equation for every variable.
- Test yourself with past papers, writing the equation each time before substituting.
Common mistakes
- Expecting an equation sheet in Cambridge 0625: there is none.
- Forgetting to convert km to m, minutes to s, or g to kg.
- Mixing up mass and weight in W = mg.
- Using sin of the angle to the surface instead of the angle to the normal in Snell's law.
Practice questions
Try each one before opening the answer.
Q1. A block of mass 540 g has volume 200 cm³. Find its density in g/cm³.
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Q2. A wave has wavelength 0.50 m and speed 340 m/s. Find its frequency.
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Q3. A transformer has 100 primary turns and 2000 secondary turns. The input is 12 V. Find the output voltage.
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Q4. Light enters glass at 40° to the normal and refracts to 25°. Find n.
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Frequently asked questions
Is there an equation sheet in IGCSE Physics?
Not for Cambridge 0625. You must recall the equations listed in the syllabus.
Do Core candidates need every equation?
No. Core papers are based on the Core content only, so Core candidates need only the Core equations; the syllabus marks the Supplement ones. Extended candidates need them all.
Which equations come up most?
Speed, density, F = ma, kinetic and potential energy, power, V = IR, P = IV and v = fλ appear in almost every series.
Key takeaways
- No equation sheet in Cambridge 0625.
- Learn each equation with symbols, words and units.
- Extended candidates also need the Supplement equations.
- Always convert units before substituting.
- Write the equation first for method marks.