IGCSE Physics Key Definitions: The Ultimate Revision Sheet

Physics is a subject where precise use of terminology matters enormously. In your IGCSE Physics exam, using the correct definition can be the difference between gaining and losing marks. This comprehensive definition sheet covers all the essential terms from the Cambridge IGCSE Physics (0625) syllabus, organised by topic for quick and effective revision.

Save this page, print it, or use it alongside your notes — it is designed to be your essential companion throughout your Physics revision.

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1. General Physics — Measurement and Units

Physical quantity — A property that can be measured and expressed as a number with a unit (e.g., mass in kilograms).

SI units — The internationally agreed system of units used in science, including the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), and mole (mol).

Scalar quantity — A quantity that has magnitude (size) only, such as speed, mass, energy, and temperature.

Vector quantity — A quantity that has both magnitude and direction, such as velocity, force, acceleration, and displacement.

Accuracy — How close a measured value is to the true value of the quantity being measured.

Precision — How close repeated measurements are to each other, regardless of how close they are to the true value.

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2. Motion and Forces

Speed — The distance travelled per unit time. Speed = distance ÷ time (v = d/t).

Velocity — The speed of an object in a given direction; a vector quantity. It is the rate of change of displacement.

Acceleration — The rate of change of velocity. Acceleration = change in velocity ÷ time taken (a = Δv/t). Measured in m/s².

Deceleration — A negative acceleration; a decrease in velocity over time.

Displacement — The distance moved in a specified direction from a reference point; a vector quantity.

Distance-time graph — A graph showing how the distance travelled by an object changes over time. The gradient gives speed.

Velocity-time graph — A graph showing how the velocity of an object changes over time. The gradient gives acceleration, and the area under the graph gives displacement.

Force — A push or pull that can change the shape, speed, or direction of motion of an object. Measured in newtons (N).

Resultant force — The single force that has the same effect as all the individual forces acting on an object combined.

Newton’s first law — An object remains at rest or continues to move at constant velocity unless acted upon by a resultant force.

Newton’s second law — The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass. F = ma.

Newton’s third law — For every action force, there is an equal and opposite reaction force acting on a different object.

Friction — A force that opposes the motion of an object, acting between surfaces in contact or through a fluid (air resistance or drag).

Terminal velocity — The constant maximum velocity reached by a falling object when the upward drag force equals the downward gravitational force, so there is no resultant force and no acceleration.

Momentum — The product of an object’s mass and velocity. Momentum = mass × velocity (p = mv). Measured in kg m/s.

Conservation of momentum — In a closed system, the total momentum before an event (e.g., collision) is equal to the total momentum after the event, provided no external forces act.

Impulse — The change in momentum of an object, equal to the force applied multiplied by the time for which it acts. Impulse = F × t = Δp.

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3. Forces and Matter

Mass — The amount of matter in an object, measured in kilograms (kg). Mass does not change with location.

Weight — The gravitational force acting on an object. Weight = mass × gravitational field strength (W = mg). Measured in newtons (N).

Gravitational field strength (g) — The force per unit mass exerted on an object in a gravitational field. On Earth, g ≈ 9.8 N/kg.

Density — The mass per unit volume of a substance. Density = mass ÷ volume (ρ = m/V). Measured in kg/m³.

Pressure — The force acting per unit area. Pressure = force ÷ area (P = F/A). Measured in pascals (Pa).

Moment — The turning effect of a force about a pivot. Moment = force × perpendicular distance from the pivot (M = F × d). Measured in N m.

Principle of moments — For an object in rotational equilibrium, the sum of clockwise moments about any pivot equals the sum of anticlockwise moments.

Centre of gravity — The point at which the entire weight of an object appears to act.

Hooke’s law — The extension of a spring is directly proportional to the applied force, provided the limit of proportionality is not exceeded. F = k × x.

Limit of proportionality — The point beyond which the extension of a spring is no longer proportional to the applied force.

Elastic deformation — Deformation where the object returns to its original shape and size when the deforming force is removed.

Plastic deformation — Deformation where the object does not return to its original shape when the deforming force is removed.

Spring constant (k) — A measure of the stiffness of a spring, equal to the force per unit extension. Measured in N/m.

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4. Energy, Work, and Power

Energy — The capacity to do work. Measured in joules (J).

Kinetic energy (KE) — The energy an object possesses due to its motion. KE = ½mv².

Gravitational potential energy (GPE) — The energy stored in an object due to its position in a gravitational field. GPE = mgh.

Elastic potential energy — Energy stored in a stretched or compressed elastic object.

Chemical energy — Energy stored in the bonds of chemical compounds, released during chemical reactions.

Thermal energy — The total kinetic energy of the particles in a substance, related to its temperature.

Nuclear energy — Energy stored in the nucleus of an atom, released during nuclear reactions (fission or fusion).

Conservation of energy — Energy cannot be created or destroyed; it can only be transferred from one form to another. The total energy in a closed system remains constant.

Work done — The energy transferred when a force moves an object through a distance in the direction of the force. Work = force × distance (W = F × d). Measured in joules (J).

Power — The rate at which energy is transferred or work is done. Power = work done ÷ time (P = W/t). Measured in watts (W).

Efficiency — The ratio of useful energy output to total energy input, often expressed as a percentage. Efficiency = (useful output ÷ total input) × 100%.

Renewable energy source — An energy source that can be replenished naturally in a relatively short time (e.g., solar, wind, hydroelectric, geothermal, tidal, biomass).

Non-renewable energy source — An energy source that cannot be replaced once used up and will eventually run out (e.g., fossil fuels, nuclear fuel).

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5. Thermal Physics

Temperature — A measure of how hot or cold an object is, related to the average kinetic energy of its particles. Measured in °C or K.

Internal energy — The total kinetic and potential energy of all the particles in a substance.

Specific heat capacity — The amount of energy required to raise the temperature of 1 kg of a substance by 1°C. Q = mcΔT.

Specific latent heat — The amount of energy required to change the state of 1 kg of a substance without changing its temperature. Q = mL.

Latent heat of fusion — The energy required to change 1 kg of a solid to a liquid at its melting point without a change in temperature.

Latent heat of vaporisation — The energy required to change 1 kg of a liquid to a gas at its boiling point without a change in temperature.

Conduction — The transfer of thermal energy through a material by the vibration of particles and, in metals, by free electrons, without the material itself moving.

Convection — The transfer of thermal energy through a fluid (liquid or gas) by the movement of the fluid itself due to differences in density.

Radiation (thermal) — The transfer of thermal energy by electromagnetic waves (infrared radiation), which does not require a medium.

Evaporation — The process by which molecules escape from the surface of a liquid at temperatures below the boiling point, causing cooling.

Boiling — The rapid change of state from liquid to gas throughout the liquid at a fixed temperature (the boiling point).

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6. Waves

Wave — A disturbance that transfers energy from one place to another without transferring matter.

Transverse wave — A wave in which the vibrations are perpendicular to the direction of energy transfer (e.g., light waves, water waves).

Longitudinal wave — A wave in which the vibrations are parallel to the direction of energy transfer (e.g., sound waves).

Amplitude — The maximum displacement of a point on a wave from its rest (equilibrium) position.

Wavelength (λ) — The distance between two consecutive points in phase on a wave (e.g., crest to crest or compression to compression).

Frequency (f) — The number of complete waves passing a point per second. Measured in hertz (Hz).

Period (T) — The time taken for one complete wave to pass a given point. T = 1/f.

Wave speed — The speed at which a wave transfers energy. Wave speed = frequency × wavelength (v = fλ).

Reflection — The bouncing back of a wave when it hits a boundary between two media.

Refraction — The change in direction of a wave as it passes from one medium to another due to a change in speed.

Diffraction — The spreading of a wave as it passes through a gap or around an obstacle. It is most significant when the gap size is similar to the wavelength.

Normal — An imaginary line drawn perpendicular to a surface at the point where a ray meets the surface.

Angle of incidence — The angle between the incident ray and the normal.

Angle of reflection — The angle between the reflected ray and the normal.

Total internal reflection — The complete reflection of light inside a denser medium when the angle of incidence exceeds the critical angle.

Critical angle — The angle of incidence above which total internal reflection occurs when light travels from a denser to a less dense medium.

Electromagnetic spectrum — The continuous range of electromagnetic waves ordered by frequency or wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Sound — A longitudinal wave caused by vibrations, which requires a medium to travel through and cannot travel through a vacuum.

Ultrasound — Sound waves with a frequency greater than 20 000 Hz (above the range of human hearing).

Echo — A reflected sound wave heard after a delay.

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7. Electricity

Electric charge — A property of matter that causes it to experience a force in an electric field. Measured in coulombs (C).

Electric current — The rate of flow of electric charge. Current = charge ÷ time (I = Q/t). Measured in amperes (A).

Potential difference (voltage) — The energy transferred per unit charge between two points in a circuit. V = W/Q. Measured in volts (V).

Resistance — The opposition to the flow of electric current in a circuit. Resistance = voltage ÷ current (R = V/I). Measured in ohms (Ω).

Ohm’s law — The current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant. V = IR.

Electromotive force (e.m.f.) — The energy transferred per unit charge by a source of electrical energy (e.g., a battery). Measured in volts (V).

Series circuit — A circuit in which components are connected end to end in a single loop, so the same current flows through each component.

Parallel circuit — A circuit in which components are connected across each other, providing multiple paths for current to flow.

Direct current (d.c.) — Electric current that flows in one direction only.

Alternating current (a.c.) — Electric current that repeatedly reverses its direction of flow.

Electrical power — The rate at which electrical energy is transferred. P = IV = I²R = V²/R. Measured in watts (W).

Electrical energy — The energy transferred by an electrical component. E = Pt = IVt. Measured in joules (J).

Kilowatt-hour (kWh) — A unit of energy equal to the energy transferred by a 1 kW device operating for 1 hour.

Fuse — A safety device containing a thin wire that melts and breaks the circuit if the current exceeds a specified value.

Circuit breaker — An automatic switch that breaks the circuit when the current exceeds a safe value and can be reset.

Earthing — Connecting the metal case of an appliance to the earth wire so that a large current flows to earth if a fault occurs, blowing the fuse and breaking the circuit.

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8. Magnetism and Electromagnetism

Magnet — An object that produces a magnetic field and attracts magnetic materials such as iron, steel, nickel, and cobalt.

Magnetic field — The region around a magnet where a magnetic force is experienced.

Magnetic field line — A line that shows the direction of the magnetic field. Field lines go from north to south outside the magnet.

Electromagnet — A magnet made by passing an electric current through a coil of wire (solenoid), often with an iron core. It can be switched on and off.

Solenoid — A coil of wire that produces a magnetic field when current flows through it.

Motor effect — A current-carrying conductor placed in a magnetic field experiences a force. The direction of the force is given by Fleming’s left-hand rule.

Fleming’s left-hand rule — A rule to determine the direction of the force on a current-carrying conductor in a magnetic field: the thumb (force), first finger (field), and second finger (current) are held at right angles.

Electromagnetic induction — The generation of an e.m.f. (and hence current in a closed circuit) when a conductor moves through a magnetic field or when the magnetic field through a conductor changes.

Faraday’s law — The induced e.m.f. is proportional to the rate of change of magnetic flux linkage.

Lenz’s law — The direction of an induced current is such that it opposes the change that produces it.

Transformer — A device that changes the voltage of an alternating current using electromagnetic induction. It consists of a primary coil and a secondary coil wound on an iron core.

Step-up transformer — A transformer that increases voltage (more turns on the secondary coil than the primary).

Step-down transformer — A transformer that decreases voltage (fewer turns on the secondary coil than the primary).

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9. Nuclear Physics and Radioactivity

Atom — The smallest particle of an element that retains the chemical properties of that element, consisting of a nucleus (protons and neutrons) surrounded by electrons.

Nucleus — The dense, positively charged centre of an atom, containing protons and neutrons.

Proton — A positively charged subatomic particle found in the nucleus. It has a relative mass of 1 and a charge of +1.

Neutron — A neutral subatomic particle found in the nucleus. It has a relative mass of 1 and no charge.

Electron — A negatively charged subatomic particle that orbits the nucleus. It has negligible mass and a charge of −1.

Atomic number (Z) — The number of protons in the nucleus of an atom.

Mass number (A) — The total number of protons and neutrons in the nucleus of an atom.

Isotope — Atoms of the same element that have the same number of protons but a different number of neutrons.

Radioactive decay — The spontaneous and random emission of radiation from an unstable nucleus.

Alpha particle (α) — A particle consisting of 2 protons and 2 neutrons (a helium nucleus), emitted during alpha decay. It is strongly ionising and has low penetrating power.

Beta particle (β) — A high-speed electron emitted from the nucleus when a neutron changes into a proton during beta decay. It is moderately ionising and moderately penetrating.

Gamma ray (γ) — A high-energy electromagnetic wave emitted from the nucleus. It is weakly ionising and highly penetrating.

Half-life — The time taken for half of the radioactive nuclei in a sample to decay, or for the activity of a source to decrease to half its original value.

Background radiation — Low-level radiation that is always present in the environment, from natural sources (rocks, cosmic rays, radon gas) and some artificial sources.

Nuclear fission — The splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and neutrons. This is the process used in nuclear power stations.

Nuclear fusion — The joining of two small nuclei to form a larger nucleus, releasing a large amount of energy. This is the process that powers stars.

Chain reaction — A self-sustaining series of nuclear fission reactions where neutrons released from one fission event cause further fission in other nuclei.

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How to Use This Definition Sheet for Revision

1. Work through each section to match your syllabus topics and identify any gaps.
2. Test yourself by covering the definitions and recalling them from the term alone.
3. Create flashcards for terms you find difficult to remember.
4. Apply definitions in context — practice past paper questions and use correct terminology in your answers.
5. Teach someone else — explaining a definition in your own words helps consolidate your understanding.

In Physics exams, marks are frequently awarded for the precise use of key terms. Make sure you can state definitions clearly and accurately.

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This definition sheet covers the key terms from the Cambridge IGCSE Physics (0625) syllabus. Always verify against your specific exam board’s syllabus for the most current requirements.

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