Definitions for Module 4 – OCR A Physics
Units
Coulomb
The charge transferred by a current in one second
Volt
per second.
Ohm
The resistance when of potential difference produces
of current
Kilowatt-hour (kWh)
The energy transferred by a dvice in a time of one hour
Electronvolt (eV)
The energy transferred by an electron travelling through a potential difference of
Quantities

Electric current
The flow of charge per unit time
Number density
The number of charged particles per unit volume
Resistance
The ratio of p.d to current;
Resistivity
Power
The rate of doing work;
Potential difference
The energy transferred per unit charge from electrical energy into other forms;
electromotive force
The energy transferred per unit charge into electrical energy from other forms
Internal resistance
Some energy is lost as heat in the battery or cell, it behaves as if it has an internal resistance
Terminal p.d.
The potential difference measured across the terminal of an e.m.f source
Displacement
Distance moved from equilibrium point on a wave
Amplitude
Maximum displacement
Wavelength
The distance between two adjacent peaks and troughs
Period
The time taken for one complete oscillation of a particle
Phase difference
The fraction of a cycle (measured in degrees) between the oscillations of two particles
Path difference
The difference in distances traveled by two waves from coherent sources at a particular point
Frequency
The number of wavelengths passing a point per unit time
Speed of a wave
The distance traveled by a wave per unit time
Work function
The minimum energy required to release an electron from the surface of a metal
Threshold frequency
The minimum frequency of electromagnetic radiation that will release a (photo)electron from the surface of a metal
Concepts

Conventional current
The ‘flow’ of charge from positive to negative terminals of battery
Electron flow
Electrons move from negative to positive terminals of battery
Kirchhoff’s 1st law
The sum of current into a junction equals the sum of current out; charge is conserved
Kirchhoff’s 2nd law
The sum of e.m.f.s around a loop are equal to the sum of p.d.s around the same loop; energy is conserved
Mean drift velocity
The average speed of charged particles along the length of a conductor
Ohm’s law
For a metallic conductor at a constant temperature, the voltage is proportional to the current
Progressive wave
A transfer of energy as a result of oscillations
Stationary wave
A wave which stores energy in pockets
Longitudinal wave
The displacement of particles is parallel (back and forth) to the direction of energy transfer
Transverse wave
The displacement of particles is perpendicular to the direction of energy transfer
Reflection
The bouncing back of a wave from a surface
Refraction
The change in direction of a wave as it crosses an interface between two materials where its speed changes
Diffraction
Wavefronts spread out after passing through a gap or around an obstacle
Intensity
The power per unit area;
Plane polarised wave
A transverse wave that vibrates in one plane only
Superposition
When two or mores waves overlap in space, the resultant displacement is equal to the sum of the individual displacements of each wave
Interference
When two waves superpose at a point and there is a change in overall displacement
Constructive interference
Where two waves meet and reinforce, giving an increased amplitude
Destructive interference
Where two waves meet and cancel to give a reduced amplitude
Coherence
Constant phase difference between waves
Node
Where the amplitude is always zero
Antinode
Where the amplitude of a standing wave takes the maximum possible value
Photon
A packet (or quanta) of energy
Photoelectric effect
The emission of electrons from a metal surface when photons are incident on the surface

Approximate Wavelengths of Electromagnetic Radiation
Radio waves
Micro waves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
NOTE: It is the frequency that define a wave and not the wavelength.
The frequency of an electromagnetic ray is constant no matter the medium it is in. Because the speed of the wave can change, its wavelength can change also.
