# IGCSE Definitions

This document contains the common definitions that may be asked in the Cambridge IGCSE examinations.

**IGCSE Definition List (2016)**

**General Physics**

**Speed**

Speed is the distance travelled by an object per unit time.

**Acceleration**

Acceleration is the rate of change of an object’s velocity.

**Resultant** **force**

Resultant force is the single force that has the same effect as two or more forces.

**Weight**

Weight is the product of the mass and the acceleration of free fall.

**Density**

Density is mass per unit volume.

**Force**

Force is the product of the mass and the acceleration of the body.

**Newton**

Newton the is the force required to give a mass of 1 kg an acceleration of 1 m s^{–2}.

**Momentum**

Momentum is the product of the mass and the velocity of the body.

**Impulse**

Impulse is the change in momentum of a body.

**Principle of conservation of momentum**

The principle of conservation of momentum states that the total momentum before a collision equals to the total momentum after the collision.

**Conditions for equilibrium**

For an object to be in equilibrium, it must satisfy two conditions:

- there is no net forces on the body,
**AND** - there is no net moment on the body.

**Moment of a force**

The moment of a force is the product of the force and the perpendicular distance from the pivot to the force.

**Principle of moment**

The principle of moment states that for an object to be in equilibrium, the total clockwise moments about a point is the same as the total anticlockwise moments about the same point.

**Hooke’s Law**

Hooke’s law states that the extension of a spring is proportional to the load applied to it,

**Pressure**

Pressure is the force acting per unit area at right angles to a surface.

**Pascal**

Pascal is the SI unit of pressure, equivalent to one newton of force per square metre.

**Principle of conservation of energy**

In any energy conversion, the total amount of energy before and after the conversion is constant.

**Efficiency**

The efficiency of an energy conversion is the fraction of the energy that ends up in the desired form.

**Work done**

Work done is the product of the force and the distance moved by the force in the direction of the force.

**Joule**

One joule of energy os the energy transferred by a force of 1 newton when it moves through a distance of 1 m.

**Power**

Power is the rate at which energy is transferred.

**Watt**

Watt is the power when 1 J of work is done in 1 s.

**Thermal Physics**

**Specific heat capacity**

Specific heat capacity is the energy required per kilogram and per degree Celsius to raise the temperature of a substance.

**Specific latent heat of vaporisation**

Specific latent heat of vaporisation is the energy per kilogram required to cause a substance to change state from liquid to gas at its boiling point.

**Specific latent heat of fusion**

Specific latent heat of fusion is the energy per kilogram required to cause a substance to change state from solid to liquid at its melting point.

**Physics of Waves **

**Law of reflection**

Law of reflection states that in a reflection, the angle of incidence equals to the angle of reflection.

**Refractive index**

Refractive index of a medium is the ratio of the speed of light a vacuum to the speed of light in the medium.

**Snell’s law**

Snell’s law states that when a wave travels from vacuum to the medium, the refractive index of the medium is the ratio of sine of the incidence angle to the sine of the refraction angle.

**Total Internal Reflection**

Total internal reflection occurs at a boundary when a light travels with an angle of incidence greater than the critical angle from a denser to a less dense medium.

**Real Image**

A real image is an image which light falls on it and it can be formed on a screen.

**Virtual Image**

A virtual image is an image which light does not fall on it and it cannot be formed on a screen.

**Frequency**

Frequency is the number of waves per second passing a point.

**Wavelength**

Wavelength is the distance between adjacent crests of a wave.

**Longitudinal waves**

A longitudinal wave is a wave which the particles move back and forth parallel to the direction of the wave velocity.

**Transverse waves**

A transverse wave is a wave which the particles move back and forth perpendicular to the direction of the wave velocity.

**Amplitude**

The amplitude of a wave is the maximum distance that the surface of the wave is displaced.

**Sequence of electromagnetic radiation**

**Electricity and Magnetism**

**Current**

Current is the rate at which electric charge passes a point in a circuit.

**Resistance**

Resistance is the ratio of the potential difference across a resistor to the current flowing through it.

**Electromotive force**

Electromotive force is the amount of energy needed to push one coulomb charge round a circuit.

**Potential difference**

Potential difference is the amount of energy released from a resistor when one coulomb charge passes through it.

**Volt(unit)**

One volt is the SI unit of potential difference when 1 joule of energy is produced when 1 C charge passes through a point.

**Transducers**

An input transducer produces a voltage due to a change in the environmental conditions.

**Law of magnetism**

Like poles repel, unlike poles attract.

**Light-dependent resistor**

LDR is a resistor which the resistance depends on the intensity of light.

**Thermistor**

Thermistor is a resistor which the resistance depends on its temperature.

**Diode**

Diode is a component that allows current to flow in one direction only.

**Logic gate**

A logic gate is a device that receives one or more electrical input signals, and produces an output signal that depends on those input signals.

**AND**

**OR **

**NOT**

**NAND **

**NOR **

**Atomic physics**

**Alpha particle**

Alpha particle is a helium nuclei and it has 2 protons and 2 neutrons.

**Beta particle**

Beta particle is a fast moving electron.

**Gamma ray**

Gamma ray is part of the family of electromagnetic waves.