# OCR A-Level Physics/Glossary of Terms

Definitions of keywords and terms that you will need to know.

Contents

## A

Absolute zero
Zero on the thermodynamic temperature scale, or 0 K (kelvin), where a substance has minimum internal energy, and is the coldest possible temperature. It is equal to -273.15 degrees Celsius.
Absorption spectrum
A spectrum of dark lines across the pattern of spectral colours produced when light passes through a gas and the gas absorbs certain frequencies depending on the elements in the gas.
Acceleration
The (instantaneous) rate of change of velocity in respects to time.
Acceleration of free fall (g)
The acceleration of a body falling under gravity (9.81ms-2 on earth).
Ammeter
A device used to measure the electric current in a circuit. It is connected in series with the components.
Amount of substance
A SI quantity, measured in moles (mol).
Ampere
The SI unit for electric current.
Amplitude
The maximum displacement of a wave from its rest/mean position (measured in metres).
Antinode
A point of maximum amplitude along a stationary wave caused by constructive interference.

## B

Braking Distance
Distance travelled from the brakes being applied to the car stopping completely.

## C

Coulomb's Law
The force between two charges is directly proportional to the product of the charges and inversely proportional to the distance between them squared
Couple
Two equal, opposite and parallel forces which create rotational force.

## D

Displacement
The distance in a given direction
Density
Density is the mass of a body per unit volume
Decay Constant
The probability of decay of a nucleus per unit time

## E

Electric field strength (E)
The force that a unit charge would experience at a specified point. Measured in Volts per metre or Newtons per Coulomb
Electric potential (V)
The energy that a unit charge would have at a specified point. Measured in Volts
Energy
The stored ability to do work
Extension (x)
The change in length of an object when a force is applied to it

## F

The emf induced in a conductor is directly proportional to the rate at which the magnetic flux changes.
Force
A force causes a mass to change motion
Frequency
The number of waves that pass a fixed point in a unit of time

## G

Gravitational Field Strength (g)
The force that a unit mass would experience at a specified point. Measured in metres per second per second or Newtons per kilogramme
Gravitational Potential
The energy that a unit mass would have at a specified point. Measured in Joules per kilogramme
Gravitational Potential Energy
the energy an object has due to its relative position above the ground. Found by mass x gravity (or gravitational field strength) x height
or force per unit mass at a set point in a gravitational field

## H

Heat
is a form of energy transfer, also known as 'Thermal Energy'.
Hooke's Law
an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.

## I

Instantaneous acceleration
acceleration at a specific time; slope of tangent to velocity- time graph.
Instantaneous position
position of an object at specific time.
Instantaneous velocity
slope of the tangent to position- time graph.
Internal energy
sum of potential energy and kinetic energy with random motion.

## J

Joule
The SI unit of work done, or energy. One joule is the work done when a force of one newton moves an object one metre.

## K

Kinetic Energy
The energy an object possesses due to its motion, given by KE = 0.5 x mass x velocity²

## L

Lenz's Law
An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

## M

Magnetic Flux

Magnetic flux is the product of magnetic flux density and the projection of area onto a surface at right angles to the magnetic field

## N

Newton
Unit in which force is measured. Symbol "N". One Newton is the force required to give a mass of 1kg an acceleration of 1ms^-2

## O

Ohm's Law

Voltage is directly proportional to current as long as all other physical properties remain constant (V =IR)

## P

Period (T)
The time taken for one complete oscillation. Denoted by 'T'. T=1/f
Power
The rate at which work is done.
Pressure
The load applied to an object per unit surface area.
Potential difference
The work done in moving a unit positive charge from one point to the other. The unit is volt.

## Q

Q or q
Often used as the symbol for charge in equations

## R

Resistivity
The property of a material that measures it resistance to electric current. It is defined as the resistance a wire of the material would have if it had a cross sectional area of one metre square and a length of one meter.
A radian is the angle subtended at the centre of the circle when the arc length is equal in length to the radius.

## S

Scalar
A quantity with magnitude but no direction.
Speed
A scalar quantity, speed = distance / time

NB s can also mean displacement.

Stopping Distance
Stopping distance = Thinking distance + Braking distance
thinking distance (distance traveled while reacting) = time taken to react X velocity
braking distance (distance traveled while braking)
Specific Heat Capacity
The amount of heat energy required to raise the temperature of a substance per unit of mass by 1 degree, without going through a change in state.
Specific Latent Heat
The amount of heat energy required to change the state of 1 kg of the mass of a substance, without a change in temperature.
Fusion
The amount of heat energy required for a phase change of 1 kg of the mass of a substance from solid to liquid or from liquid to solid, without a change in temperature.
Vaporisation
The amount of heat energy required for a phase change of 1 kg of the mass of a substance from liquid to gas or from gas to liquid, without a change in temperature.

## T

Temperature
A SI quantity, measured in kelvin (K).
Tensile force

The forces being applied onto a material (usually a wire) on two opposite sides in order to stretch it. Both forces' values are the same as the tensile force value.

Tensile stress
The tensile force per unit cross-sectional area.
Terminal Velocity
maximum velocity a body can travel. When resistive forces = driving force, acceleration = 0, so it cannot travel any faster.
Thermistor
An electrical component that changes its resistance depending on its temperature.
Thinking distance
The distance travelled from seeing the need to stop to applying the brakes.
Threshold frequency
The lowest frequency of electromagnetic radiation that will result in the emission of photoelectrons from a specified metal surface.
Thrust
A type of force due to an engine (usually forward force).
Time interval (t)
A SI quantity, measured in seconds (s).
Torque / moment
Moment = force x perpendicular distance from the pivot to the line of action of the force
Torque = one of the forces x the distance between them
Transverse Wave
A progressive wave that transfers energy as a result of oscillations/vibrations.
Triangle of forces
If three forces are acting at a point that can be represented by the sides of a triange, the forces are in equilibrium.
Turning forces
More than one forces that if unbalanced will cause a rotation.

## U

Ultimate tensile strength
The maximum tensile force that can be applied to an object before it breaks.
Ultimate tensile stress
The maximum stress that can be applied to an object before it breaks.
Ultraviolet
A form of electromagnetic wave (wavelengths 10-9-3.7x10-7m). It may cause sun tanning. Usually classified into three categeries:UV-A, UV-B and UV-C.
Upthrust
A force experienced due to the pressure difference of the fluid at the top and bottom of the immersed portion of the body.

## V

Vector
A quantity with magnitude and direction.
Velocity
The (instantaneous) rate of change of displacement with respect to time. Velocity is a vector.
Velocity-time graph
A motion graph which shows velocity against time for a given body.
Volt (V)
The unit of potential difference (p.d.) or electromotive force (e.m.f.)

potential difference=energy/charge

Voltmeter
A device used to measure the potential difference across a component. It is connected in parallel across a component.
Volume(V)
A physical quantity representing how much 3D space an object occupies, measured in cubic metres(m3)

## W

Watt(W)
The unit of power.
power = energy / time
Wave
Series of vibrations that transfer energy from one place to another.
Wavelength(λ)
The smallest distance between one point of a wave and the identical point of the next wave, measured in metres (m).
Wave-particle duality
The theory which states that all objects can exhibit both wave and particle properties.
Weight
The gravitational force acting on a body, measured in newtons (N).

weight=mass x gravitational force

Work Done
The energy transferred when an object is moved through a distance by a force. Can be calculated by multiplying the force involved by the distance moved in the direction of the force.

Alternatively, [work done = transfer of energy]. i.e., work is done when energy is transferred from one form to another. [OCR do not accept this definition if asked "Define work done by a force"]

Work function energy (Φ)
The minimum energy that is required for a material to release an electron, measured in joules(J).

## X

X rays
A form of electromagnetic wave (wavelengths:10-12-10-7m). It is used in X-ray photography.

## Y

Young's double slit experiment
An experiment to demonstrate the wave nature of light via superposition and interference.
Young Modulus
Stress per unit Strain, units: Pascals or N/m2