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AS ocr unit 2 Def.

AS ocr unit 2 Definitions.

Absorption spectra A spectrum produced by light passing through a gas, which has dark lines where the gas has absorbed light, characteristic of the elements in the gas.
Amplitude is the maximum displacement from the centre of an oscillation
Diffraction The spreading out of waves when they pass through an opening or round an obstacle (into regions where they would not be expected if light travels only in straight lines)
displacement How far and in what direction the body is from its equilibrium position
Electromagnetic spectrum gamma 10-13-10-15 m 2,Xrays 10-8-10-13 m 5,Ultraviolet 4x10-7-10-8m 1,Visible 3.9 x10-7-7.8x10-7 m 0,Infrared 7.9x10-7-10-3m, 4Microwave 10-3-10-1m 2,Radio 10-1-105m 4
Excited Atom with one or more electrons raised above their ground state positions
Excitation energy Energy require to raise an electron to a position above its ground state
Electronvolt Energy transferred to an electron when it is accelerated through a potential of 1V: equivalent to 1.6 x10-19 J
Emission spectra A line spectrum emitted by a hot gas which is characteristic of the elements it contains. (Coloured lines on a black background)
Electromagnetic wave Transverse combination of oscillating in phase electric and magnetic fields.
Energy flux/ intensity Energy that a wave carries perpendicularly through unit area each second.
Equilibrium position Where resultant force on the oscillating body is zero
Frequency The number of oscillations completed per second
Ground state Atom with its electrons in their lowest energy positions
Interference In a region where wave trains from coherent sources cross, superposition occurs giving reinforcement of the waves at some points and cancellation at others (constructive or destructive interference).
Ionisation When an electron is completely freed from an atom leaving behind a positive ion.
Ionisation energy Energy required to free an electron from the ground state of an atom.
Longitudinal waves A wave where the displacements are parallel to the direction of propagation
Natural Frequency The frequency at which a free standing system oscillates after it has been displaced and then released
Nodes A point on a stationary wave where the displacement is always zero
Out of phase Completely out of step with each other, π or 180 degrees.
Period The periodic time is the time taken to make one complete oscillation/ rotation
In Phase Two points which are in phase have a complete number of oscillations between them.
Phase difference between two points in an oscillation is a measure of what fraction of a complete oscillation has been completed between the two points
Photon Small packet of light energy
Photoelectric effect Emission of electrons from a surface when illuminated with electromagnetic radiation of equal frequency or greater than the threshold frequency
Plane polarization Vibrations are confined to a single plane perpendicular to the direction of energy propagation
Principle of Superposition Where waves cross the total displacement is the vector sum of the individual displacements due to each wave at that point.
Progressive waves A disturbance that does transfer energy
Quantum Small packet of energy
Saturation When all the emitted electrons are received by the other electrode and the maximum current flows.
Stationary waves A disturbance that does not transfer energy although it does have energy associated with it.
Stopping potential/ voltage Voltage across a photocell that is just sufficient to stop photoelectrons with the maximum amount of KE reaching the receiving electrode.
Threshold frequency Minimum frequency that will cause photoemission from a material
Transverse waves A wave where the displacements are perpendicular to the direction of propagation
Velocity Rate of change of displacement
Unpolarised Vibrations occur in a large number of planes perpendicular to the direction to energy propagation.
Wavefront Line joining all points across adjacent rays that have exactly the same phase
Wavelength The distance between two consecutive points on a wave that are in phase
Work function Minimum amount of energy needed to release an electron from the surface of a metal
Charge A coulomb is the amount of charge that flows when there is a current of one ampere for one second. Symbol Q.1C is the charge carried by 6.25 x 1018 electrons.Measured by a coulombmeter.
Current Electric current as rate of flow of charge.
Law of conservation of charge No charge is lost in a circuit or at any junction in a circuit.This is a consequent of Kirchoff’s first law.
Kirchoffs First law Sum of the current entering a junction = sum of the currents leaving a junction
Drift speed The average speed at which charge moves around a circuit . Typical drift velocity =10-5m/s.
Electrical potential difference Potential difference or voltage between two is the work done per unit charge travelling between the two points.P.d. = work done/ charge or Potential difference (voltage) as power/current
Emf of a cell Work done /unit charge in pushing the current round the whole circuit (including the internal resistance)
Electrical working (Work done) Work done = Energy transfer.(This is when charge is pushed)
Resistance Resistors oppose the flow of current.Resistance = voltage / current
Resistivity You must write out the eqn Resitivity = resistance x cross sectional area/ length .It is a constant for a particular material of a wire.
Power dissipation Power is the rate of working. Power is the rate of energy transfer.
Antinodes A point on a stationary wave that oscillates with the maximum amplitude
Coherence If sources of wave are coherent there must be a constant phase difference between them and therefore they must have the same frequency. They must have also approximately the same amplitude
Continuous spectrum This is emitted by hot solids, hot liquids and hot gases at high pressures. All wavelengths are emitted.
cycle A complete movement of an oscillating or cycle
De broglie wavelength Electrons have a wavelength associated with them with is dependent on the momentum of the particle
Conservation of energy in circuits Circuit e.m.f. = ΣIR.This come from Kirchoff’s second law-Around any closed loop, the sum of the e.m.f.s is equal to the sum of the p.d.s.
Internal resistance This is the resistance to the flow of current inside the power supply itself.
Series circuits The current passes through one component , then through the next and then through the next. The current is the same at all points in the circuit. The voltage is split between the components VT = V1 +V2
Parallel circuits The current from the cell splits at a junction. IT =I1 +I2.The current travels through branches. The voltage is the same across each branch.
Thermistor This sensor changes resistance with temperature. The higher the temperature the lower the resistance.
Light dependent resistor This sensor changes resistance with the level of illumination.The greater the light level the lower the resistance.
Ohm’s Law The current flowing through a metal is directly proportional to the potential difference across it providing the temperature remains constant.
Created by: Susan E Casey



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