# Edexcel Physics AS unit 2

All the definitions you need to know for your Physics Unit 2

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• Created by: Kayleigh
• Created on: 28-04-12 14:56

## Waves definitions

Displacement (s): The distance moved by a wave carrier/ particle from the mean position (m, metres)

Amplitdude (a): The Maximum displacement from the mean position (m, metres)

Period (T): the time taken for one complete oscillation (s, seconds)

Frequency (f): the number of whole cycles (oscilations) per second (Hz, Hertz)

Wavelength (λ): the distance beyween two identical points on a wave (m, metres)

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## Waves definitions continued

Transverse: a wave where the displacement of the particles is perpendicular to the direction  of propogation

Longitudinal: a wave where the displacement of the particles is parrelel to the direction the of propogation

Superposition: The net displacement of two or more waves is equal to the sum of the individual waves

Coherence: The characteristic of light when all the waves have the same frequency and are in phase

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## Waves

Polarised light: In plane polarised light the oscillations take place in only one plane at a right angle to the direction of propagation

Doppler effect: an increase (or decrease) in the frequency of sound, light or other waves as the source and observer move towards (or away from) each other- e.g the sudden change in pitch noticable in a passing siren and the red shift of the universe seen by astronomers

Pulse Echo Technique: Pulses of waves emitted from a source where the time taken for the reflective signal to return is recorded using  2 x distance= Velocity x time the distance is calculated

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## Electricity Definitions

Circuit: an arrangement of components that allows a current to flow

Current: The rate of flow of charge around an electrical circuit

Parallel: An arrangement of components along side each other on separate branches of the circuit

Series: An arrangement of components one after another on the same branch of circuit

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## Electicity Definitions continued

Resistance: A property of something that describes how easily electric currents can flow through it (Ohms Ω)

Potential Difference: The difference in energy levels of charge carriers at different points in a circuit (volts/energy per coulomb)

Ampere (A): The unit of electric current

Ohm (Ω): the unit of resistance

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## Electric equations

I = Q / t            when I= current A, Q= charge (coulombs) & t= time (seconds)

V = W / Q            when V = volts, W= Work Done (joules) & Q= charge in coulombs

R= V/I            when R=resistance (Ω), v= p.d (volts) & I=current (amps)

Series

Total Voltage= Sum of all voltages

Total Resistance= sum of all resistances

Parallel

1/total resistance = 1/sum of all resistances

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## Resistivity

Resistance: how easily current flows in a component

Resistivity:how easily a current flows in a material

Temp Coefficient of Resistivity

+ve coefficient of temp: as temp increases resistance increases  (applies to all metallic conductors)

-ve coefficient of temp: as temp increases resistance decreases (applies to many thermistors, semi-conductors e.g.glass)

In a thermistor as temperature increases the number of charge carriers that are free to carry current increases

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## I=nAqv

I = nAqv

when I= current(A), n= no. density (per m3), A= cross sectional area (metre2), q= charge (coulombs) & v= drift velocity (metres per second)

Drift Velocity: how fast the electrons move in the direction of the current

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## Diffraction

• When part of a wave is obstructed the rest of the wave spreads out into the 'shadow region'
• a narrower gap causes more spreading
• a wider gap causes less diffraction
• when the width of the gap is similar to the wavelength the wave fronts are almost circular
• when more than 1 wave is diffracted they can superpose causing areas of constructive and deconstructive interference
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