 ## Definitions for Module 4 – OCR A Physics

### Units

Coulomb

The charge transferred by a $1 \text{Ampere}$ current in one second

Volt $1 \text{Joule}$ per second.

Ohm

The resistance when $1 \text{Volt}$ of potential difference produces $1 \text{Ampere}$ of current

Kilowatt-hour (kWh)

The energy transferred by a $1 \ kW$ dvice in a time of one hour

Electronvolt (eV)

The energy transferred by an electron travelling through a potential difference of $1 \ V$

### 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; $\text{Resistance} = \frac{Potential difference}{Current}$

Resistivity $\text{Resistivity} = \frac{Resistance \cdot Cross-sectional Area}{Length}$

Power

The rate of doing work; $Power = \frac{Work done}{time}$

Potential difference

The energy transferred per unit charge from electrical energy into other forms; $\text{Potential difference} = \frac{Work done}{Charge}$

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; $Intensity = \frac{Power}{Cross-sectional area}$

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 $\lambda = 1.0 \times 10^{-1} \ m \rightarrow 3 \times 10^{3} \ m$

Micro waves $\lambda = 1.0 \times 10^{-3} \ m \rightarrow 1.0 \time 10^{-1} \ m$

Infrared $\lambda = 7.1 \times 10^{-7} \ m \rightarrow 1.0 \time 10^{-3} \ m$

Visible light $\lambda = 4.0 \times 10^{-7} \ m \rightarrow 7.0 \time 10^{-7} \ m$

Ultraviolet $\lambda = 1.0 \times 10^{-8} \ m \rightarrow 3.9 \time 10^{-7} \ m$

X-rays $\lambda = 1.0 \times 10^{-12} \ m \rightarrow 1.0 \time 10^{-8} \ m$

Gamma rays $\lambda = 10^{-15} \ m \rightarrow 1.0 \time 10^{-11} \ m$

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.