This post contains all the important definitions that you need for GCE O Level Physics. (Equivalent to American High School Diploma) If you do not recognise any of the terms listed here, you should proceed to review the respective topic (Click on each of the sub-headers). For a list of the important equations for O Level Physics, please visit O Level Formula List.

Please drop a comment below if any particular important definitions are missing in this post.

## Table of Contents

## Measurements

A **base quantity (or basic quantity)** is chosen and arbitrarily defined, rather than being derived from a combination of other physical quantities.

A **derived quantity** is defined based on a combination of base quantities and has a derived unit that is the exponent, product or quotient of these base units.

**Scalar quantities** are quantities in which the magnitude is stated, but the direction is either not applicable or not specified.

**Vector quantities** are quantities in which both the magnitude and the direction must be stated.

## Kinematics

**Distance** travelled by an object is the length of path taken.

**Displacement** is the shortest distance from the initial to the final position of an object.

**Speed** is the distance moved per unit time.

**Velocity** (v) of an object is the rate of change of displacement with respect to time.

**Acceleration** of an object is the rate of change of velocity with respect to time.

## Forces and Turning effect of Force

**Newton’s First Law** states that an object will continue in its state of rest or uniform motion in a straight line as long as there is no net force acting on the body.

**Newton’s Second Law** states that when a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force.

**Newton’s Third Law** states that if object A exerts a force on object B, then object B will also exert an equal and opposite force on object A

The **moment of a force (torque)** is defined as the turning effect of the force about a pivot and is the product of the force and the perpendicular distance from the line of action of the force to the pivot.

The **centre of mass** of a body of matter is an imaginary point at which the entire mass of the body seems to act.

The **centre of gravity** of a body of matter is an imaginary point at which the entire weight of the body seems to act.

## Mass, Weight & Density

**Mass** is defined as the amount of matter in an object

**Weight** is defined as the gravitational force acting on an object

**Inertia** is defined as the reluctance on an object to change its state of rest or motion, due to its mass.

A **Gravitational Field** is a region in which a mass experiences a force due to gravitational attraction

**Gravitational Field Strength** is defined as the gravitational force acting per unit mass** .**

**Density** ($\rho$) is defined as the mass of a substance per unit volume.

**Terminal velocity** is the highest **velocity** attainable by an object in free fall.

## Pressure

**Pressure** is defined as the perpendicular force acting on unit area of a surface or the force per unit area.

**Boyle’s Law** states that the volume of a fixed mass of gas at constant temperature is inversely proportional to the pressure applied to the gas.

## Work, Energy & Power

**The Principle of conservation of energy** states that energy cannot be created nor destroyed but can be converted from one form to another and the total amount of energy of a enclosed system remains constant.

**Kinetic Energy**, $E_{k}$ is the energy a body possesses by virtue of its motion.

**Gravitational Potential Energy** is defined as the amount of work done in order to raise the body to the height *h* from a reference level.

**Power** is defined as the rate of work done OR;

**Power** is defined as the rate of energy converted with respect to time.

## Thermal Physics

**Melting** is the process in which energy absorbed by a substance results in a change of state from solid to liquid, without a change in temperature.

**Solidification** is the process in which energy taken away from a substance results in a change of state from liquid to solid, without a change in temperature.

**Boiling** is the process in which the energy absorbed by a substance changes it from liquid state to gaseous state, without a change in temperature.

**Condensation** is the process in which energy taken away from substance changes it from gaseous state to liquid state, without a change in temperature.

**Heat Capacity**, C,of a body is defined as the amount of heat (Q) required to raise its temperature (θ) by one degree, without going through a change of state.

**Specific heat capacity**, c, of a body is defined as the amount of heat (Q) required to raise the temperature (θ) of a unit mass of it by one degree, without going through a change in state.

**Specific latent heat of fusion** of a substance is defined as the amount of heat required to change a unit mass of the substance from solid to liquid state, without any change in the temperature.

**Specific latent heat of vapourization **of a substance is defined as the amount of heat required to change unit mass of the substance from liquid state to gas state without a temperature change.

**Conduction** is the transfer of thermal energy from one place to another without any flow of the medium.

**Convection** is the transfer of thermal energy from one place to another by means of convection currents in a fluid (gas or liquid), due to a difference in density

**Radiation** is the transfer of thermal energy from one place to another by means of electromagnetic radiation, without the need of an intervening material medium.

## Waves, Reflection & Refraction of light, Converging Lens

**Amplitude** is the maximum displacement from the rest or central position, in either directions.

**Frequency** (f) is defined as the number of complete waves produced per unit time.

**Wavelength** (λ) is the distance between corresponding points of two consecutive waves.

**Speed** of the wave propagation is defined as the distance travelled by a wave per unit time.

**Period** (T) is defined as the time taken to produce one complete wave.

**First law of reflection ** states that the incident ray, the reflected ray and the normal to the surface all lie in the **same** plane.

**Second law of reflection** states that the angle of incidence is equal to the angle of reflection.

**Refraction** of light is the change in direction (bending of light rays) when it passes from one optically transparent medium to another.

**First law of refraction** states that the incident ray, the refracted ray and the normal to the interface all lie in the same plane.

**Second law of refraction** states that for two given media, the ratio $\frac{\sin{i}}{\sin{r}} = \, \text{constant}$, where $i$ is the angle of incidence and $r$ is the angle of refraction.

## Electricity and D.C. Circuits

**Electric Current** is defined as the rate of flow of charges.

**Electromotive Force (e.m.f)** of a source is defined as the the work done by the source in driving a unit charge around a complete circuit.

**Potential difference** across a component is defined as the work done to drive a unit charge through the component.

**Ohm’s law** states that the current flowing through a metallic conductor is directly proportional to the potential difference across it, provided that the physical conditions remain constant.

## Electromagnetic Force and Electromagnetic Induction

**Faraday’s Law of Electromagnetic induction** is the process in which an electromotive force (emf) is induced in a closed circuit due to changes in the magnetic field around the circuit.

**Lenz’s law** states that the direction of the induced e.m.f. and hence the induced current in a closed circuit is always such that it opposes the change in producing it.

## Radioactivity & the nuclear atom

**Isotopes** are different atoms of an element which have the same number of protons, but a different number of neutrons from each other.

**Radioactive decay** refers to the process in which α-particles and β-particles are emitted by an unstable nuclei (contains too many neutrons or protons) of an element in order to form a more stable nuclei of another element.

The **half-life** of a sample of a radioactive isotope is defined as the time taken for half the original unstable radioactive nuclei to decay.