AQA AS Physics Definitions

Specific charge

the charge to mass ratio or charge in coulombs divided by the mass in kilograms

Isotope

a nucleus with the same number of protons but different number of neutrons

Hadron

a particle that experiences the strong nuclear force. Composite particles made of either qqq (baryons) or q anti-q (baryon)

Antimatter

Particles with the same mass but opposite charge

Photoelectric effect

the emission of electrons from metal surfaces by light of an appropriate frequency

Photon

a discrete packet/quantum of electromagnetic energy

Work function

the minimum energy required for an electron to escape from the surface of the metal

Threshold frequency

the minimum frequency of a photon required to release electrons from the surface of a metal

Electron volt

the energy given to an electron when it passes through a potential difference of 1 V

Energy Level

Allowed/discrete energy of an atom or electron in an atom

Ionisation energy of an atom

the minimum energy required to remove an electron from an atom in its ground state

Excitation energy

the energy required to move an electron from a lower energy level to a higher energy level

Line spectra

the characteristic wavelengths of light produced by individual excited atoms

Longitudinal waves

direction of oscillation is parallel to the direction of wave travel. Wave travels with series compressions and rarefactions.

Transverse waves

direction of oscillation is perpendicular to the direction of wave travel with peaks and troughs.

Electromagnetic wave

a transverse wave, consisting of a changing magnetic field at right angles to a changing electric field

Amplitude

the maximum displacement from the equilibrium position

Frequency

the number of oscillations per second

Wavelength

the shortest distance between two consecutive particles in a wave which are in phase

Wave speed

the speed of energy transfer through the medium

Diffraction

Diffraction is the spreading out of a wave as it passes through an aperture/around an obstacle.

Polarised waves

electromagnetic waves where the oscillations are in one direction only that is 90° to the direction of wave travel

Unpolarised waves

electromagnetic waves where the oscillations are in many directions

Refractive index

the absolute refractive index of the substance is the speed of light in a vacuum divided by the speed of light in the substance, n = Vc/Vs

Critical angle

the angle of incidence on a boundary between two media when the angle of refraction is 90°

Total internal reflection

total internal reflection occurs when an EM wave goes from a more dense to less dense medium and the angle of incidence is greater than critical angle. All the light is reflected within the substance

Step index fibre

there is a sudden change in refractive index between the core and the cladding

Path difference

the difference in distance one wave has to travel to reach a point compared to another wave

Superposition

when two or more waves of the same type meet at a point the resultant displacement of will be the sum of the individual displacements

Stationary wave

a stationary wave is formed by two waves of the same frequency and similar amplitude travelling in opposite directions. There is no net energy transfer.

Node

a point on stationary wave with zero amplitude

Antinode

a point on stationary wave with maximum amplitude

Fundamental frequency

the lowest frequency of a wave which will produce a stationary wave

Coherence

coherent waves have a constant phase relationship and the same frequency

Monochromatic

waves of only one frequency

Fringe spacing

the distance between two consecutive maxima or minima in an interference pattern

Electric current

the number of coulombs of charge passing a point per second (I=Q/t)

Potential difference

the work done per unit charge when charges move from one point in the circuit to another (V=E/Q)

Resistance

the ratio of the potential difference across a component to the current through it

Ohmic conductor or resistor

the ratio of potential difference to current remains constant. Resistance is constant. It follows Ohm's Law (R=V/I)

Ohm's law

the current through component is proportional to the potential difference across it (R=V/I)

Superconductivity

A material that has zero resistivity below a certain temperature (the critical temperature, Tc)

Critical temperature, Tc

the temperature at or below which the resistivity of a superconductor becomes zero

Kirchhoff's first law

the sum of the currents into the junction is zero. The current in any one series loop is constant

Kirchhoff's second law

in any closed loop the sum of the supply voltage equals the sum of the potential differences

EMF, ε

energy provided (transferred from chemical to electrical) by the cell per unit charge

Internal resistance, r

the resistance inside a battery, cell or power supply

Lost volts

the voltage drop across the internal resistance of the power supply.

Terminal PD, V

energy per unit charge available to be transferred into other forms by the circuit. It can be found by measuring the voltage across a cell when the current is zero

Scalar

a quantity which has magnitude only

Vector

a quantity which has magnitude and direction

Equilibrium

an object is in equilibrium when the resultant force on it is zero and the resultant moment is zero

Moment/torque

the moment/torque/turning force about point is the force multiplied by the perpendicular distance from the point to the line of action of the force

Principle of moments

for an object in equilibrium the sum of the clockwise moments equals the sum of the anticlockwise moments

Couple

two equal and opposite forces separated by a distance D between the lines of action of the forces (this produces rotational motion only)

Centre of mass

the point at which the whole mass of the object appears to act

Displacement

the distance an object has moved in a particular direction

Speed

rate of change of distance

Velocity

rate of change of displacement

Acceleration

the rate of change of velocity

Parabolic path

the shape of the path of a particle moving with a component of motion at right angles to a constant resultant force (constant acceleration)

Newton's first law

an object remains at rest or moves with a constant velocity providing no resultant external force is applied

Newton's second law

for a constant mass the acceleration of the object is proportional to the resultant force applied to it

Newton's third law

all forces occur in pairs of same size and type but opposite direction and on opposite objects

Work

work done equals force multiplied by the distance moved in the direction of the force

Power

power equals work done per unit time

Principle of conservation of energy

energy cannot be created or destroyed it can only be changed from one form to another

Elastic Collision

an interaction where kinetic energy is conserved

Inelastic Collision

an interaction where kinetic energy is not conserved

Density

the mass per unit volume

Hooke's law

the extension of a spring or material is proportional to the tensile force applied (this applies up to the limit of proportionality of the material). The line must start at the origin and be straight

Elastic behaviour

will return to its original shape after deforming forces are removed

Plastic behaviour

will not return to its original shape after deforming forces are removed

Brittle

a brittle material will not undergo plastic deformation before fracture. It does not absorb lots of energy before fracture.

stress, σ

tensile force per unit cross-sectional area of the material

strain, ε

extension per unit length

Young's modulus, E

stress divided by strain

Fracture

this is when the material breaks. Materials are compared by their ultimate tensile stress, this is the tensile stress needed to break the material (strength of material)

Potential Divider

A combination of resistors in series that splits voltage

Tensile Deformation

Changing the shape of a sample by an increase in extension (stretching)

Compressive Deformation

Changing the shape of a sample by a decrease in extension (squashing)