OCR A - A-Level Physics Definitions

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).

Antiphase

Any odd integer of pi radians apart (for points in waves)

Attenuation

The gradual loss of flux intensity through a medium.

Archimede's Principle

For an object submerged in a fluid, the upthrust is equal to the weight of that the object displaces.

Average Speed (v)

Rate of change in distance (m) calculated over a completed journey, with respect to time. Measured in ms^-1.

Antinode

A point of maximum amplitude along a stationary wave caused by constructive interference.

Braking Distance

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

Charge (Q)

How much positive or negative the charge carriers are per unit volume. Measured in coulombs.

Chemical Energy (Ec)

Energy contained within the chemical bonds between atoms and molecules.

Coefficient of Friction (μ\mu)

Ratio between the frictional force and normal contact force acting on an object.

Conservation of Energy Law

Total energy in a closed system of interacting objects remains constant - can be transferred into alternate forms.

Conservation of Momentum Law

For a closed system of interacting objects, the total momentum in a specified direction remains constant considering no external resultant force acts on it.

Conventional current

The concept of current as the transfer of positive charge. I.e. from ve+ to ve- in a circuit.

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

Conductor

A material that allows the transfer of energy through it (e.g. electrical).

Compression

Moving region in which the medium is more dense or of higher pressure than the surrounding.

Critical Angle (θ\theta )

Angle of incidence at which from that point on, no more refraction will occur - only total internal reflection. Measured in degrees.

Critical Temperature

The temperature in which the electrical resistance of a material falls to zero.

Current (I)

Rate of flow of charge. Measured in amperes, A.

Couple

Two equal, opposite and parallel forces which create rotational force.

Displacement

The distance in a given direction

Diffraction

Property of a wave in which when it passes through a gap, it spreads out.

Density

Density is the mass of a body per unit volume

Drag

The resistive force on an object when it travels through a fluid.

Drift velocity (v)

The average velocity of electrons travelling within a material. Measured in ms^-1.

Ductility

The ability of a material to plastically deform under tensile forces without fracturing.

Decay Constant

The probability of decay of a nucleus per unit time

Elastic energy (Ee)

Energy stored within an object due to an elastic change in its shape (i.e. stretch or compression). Measured in joules.

Elastic deformation

A change in objects shape in which it will return to its original shape once the force is removed.

Electrical energy

Energy of electrical charges due to their position in an electric field. Measured in joules.

Electric field strength (E)

The force that a unit positive 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

Elementary Charge (e)

Amount of charge carried by a single proton/electron (1.6x10^-19 C) - must consider ve+/ve-.

Energy

The stored ability to do work

Electromagnetic Radiation

A form of progressive wave with perpendicularly oscillating magnetic and electric fields.

Electromotive Force (e.m.f)

Energy available to be transferred from other forms (e.g. chemical) into electrical, per unit charge.

Equilibrium position

The resting position for particles in a medium.

Extension (x)

The change in length of an object when a force is applied to it

Faraday's Law

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.

Frictional Force

Force arising when two objects rub against each other.

Frequency

The number of oscillations per second. Measured in hertz, Hz.

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 kilogram

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 in a gravitational field. Found by mass x gravity (or gravitational field strength) x height or force per unit mass at a set point in a gravitational field. Measured in joules.

Hardness

Ability of an object to resist surface scratching and pressure.

Heat

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.

Inertia

Property of matter by which it continues in its existing state, rest, or uniform motion, unless acted on by an external resultant force.

Inelastic Collision

A collision in which some kinetic energy is transferred to other forms (e.g. heat).

In-phase

Points in waves in which have no phase difference.

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

Velocity over an infinitesimal interval of time or displacement.

Intensity (I)

Describes how much energy per unit time is received per unit area from a wave. Measured in Wm^-2.

Internal resistance (r)

Resistance at the source of e.m.f (e.g. cell) due to its construction. Energy is lost as heat etc which reduces the terminal p.d.

Interference

The superposition of waves when they interact, leaving a resultant amplitude.

Impetus

The force or energy of which a body moves with.

Impulse (Δ\Delta p)

Described as the change in momentum. Calculated by the product of force and change in time. Measured in Ns.

Internal energy

Sum of random potential and kinetic energies of atoms within a system. Measured in joules.

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.

Kirchhoff's 1st Law

The sum of current entering a junction in a circuit, equals the total leaving it - conservation of charge law.

Kirchhoff's 2nd Law

The sum of e.m.f's equals the sum of p.d's across components in a closed loop of a circuit - conservation of energy law.

Kinetic Energy

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

Longitudinal wave

Form of progressive wave that oscillates parallel to the direction of energy transfer.

Lenz's Law

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

Maxima

Point of constructive interference due to path difference being whole integer of wavelengths (from coherent waves).

Minima

Point of destructive interference due to path difference being an odd number of half wavelengths (from coherent waves).

Malleability

Ability to plastically deform under compressive stress.

Moment (M)

Product of the force applied and perpendicular distance between force and pivot point or axis of rotation. Measured in Nm.

Momentum (p)

Vector quantity describing the 'inertia of movement'. Product of velocity and mass of an object. Measured in kgms^-1.

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

Node

A point of minimum amplitude of a stationary wave due to deconstructive interference.

Normal contact force

The force that arises when an object rests on another. Measured in N.

Newtons 1st Law of motion

States that a body will remain at rest or at constant velocity unless and external resultant force acts on it.

Newtons 2nd Law of motion

The resultant force acting on an object is directly proportional to the rate in change of momentum with respect to time, and is in the same direction.

Newtons 3rd Law of motion

For interacting objects, they will exert equal and opposite forces on each other.

Nuclear energy (En)

The energy stored within the nuclei of atoms. Measured in joules.

Number density (n)

The number of free charge carriers within a material per unit volume. Unit: m^-3.

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

Oscillation

A regular variation in displacement of particles in a medium.

Out of Phase

Points in waves with phase difference.

Ohm's Law

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

Phase difference (Φ\Phi )

Describes the difference in positions of points on a wave, or multiple waves. Measured in radians or degrees.

Photoelectric Effect

The emission of photoelectrons from a material into a vacuum due to the absorption from EM radiation. (above threshold frequency).

Photovoltaic Effect

Generation of a current/p.d. form the absorption of EM radiation.

Photoconductive Effect

A material becomes more electrically conductive from the absorption of EM radiation.

Period (T)

The time taken for one complete oscillation. Denoted by 'T'. T\=1/f

Perfectly Elastic collision

A collision in which no kinetic is lost.

Power

The rate at which work is done. Measured in Watts.

Polarisation

Action of restricting the oscillations of a transverse wave - only oscillate in one plane.

Pressure

The load applied to an object per unit surface area. Measured in Pa.

Principle of superposition

When two or more waves meet, the resultant displacement for a single point is equal to the sum of individual displacements for the waves at that point.