AS ocr unit 2 Definitions.
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| 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.
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| Amplitude | is the maximum displacement from the centre of an oscillation
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| 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)
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| displacement | How far and in what direction the body is from its equilibrium position
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| 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
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| Excited | Atom with one or more electrons raised above their ground state positions
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| Excitation energy | Energy require to raise an electron to a position above its ground state
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| Electronvolt | Energy transferred to an electron when it is accelerated through a potential of 1V: equivalent to 1.6 x10-19 J
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| Emission spectra | A line spectrum emitted by a hot gas which is characteristic of the elements it contains. (Coloured lines on a black background)
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| Electromagnetic wave | Transverse combination of oscillating in phase electric and magnetic fields.
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| Energy flux/ intensity | Energy that a wave carries perpendicularly through unit area each second.
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| Equilibrium position | Where resultant force on the oscillating body is zero
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| Frequency | The number of oscillations completed per second
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| Ground state | Atom with its electrons in their lowest energy positions
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| 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).
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| Ionisation | When an electron is completely freed from an atom leaving behind a positive ion.
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| Ionisation energy | Energy required to free an electron from the ground state of an atom.
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| Longitudinal waves | A wave where the displacements are parallel to the direction of propagation
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| Natural Frequency | The frequency at which a free standing system oscillates after it has been displaced and then released
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| Nodes | A point on a stationary wave where the displacement is always zero
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| Out of phase | Completely out of step with each other, π or 180 degrees.
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| Period | The periodic time is the time taken to make one complete oscillation/ rotation
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| In Phase | Two points which are in phase have a complete number of oscillations between them.
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| 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
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| Photon | Small packet of light energy
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| Photoelectric effect | Emission of electrons from a surface when illuminated with electromagnetic radiation of equal frequency or greater than the threshold frequency
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| Plane polarization | Vibrations are confined to a single plane perpendicular to the direction of energy propagation
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| Principle of Superposition | Where waves cross the total displacement is the vector sum of the individual displacements due to each wave at that point.
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| Progressive waves | A disturbance that does transfer energy
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| Quantum | Small packet of energy
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| Saturation | When all the emitted electrons are received by the other electrode and the maximum current flows.
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| Stationary waves | A disturbance that does not transfer energy although it does have energy associated with it.
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| Stopping potential/ voltage | Voltage across a photocell that is just sufficient to stop photoelectrons with the maximum amount of KE reaching the receiving electrode.
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| Threshold frequency | Minimum frequency that will cause photoemission from a material
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| Transverse waves | A wave where the displacements are perpendicular to the direction of propagation
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| Velocity | Rate of change of displacement
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| Unpolarised | Vibrations occur in a large number of planes perpendicular to the direction to energy propagation.
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| Wavefront | Line joining all points across adjacent rays that have exactly the same phase
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| Wavelength | The distance between two consecutive points on a wave that are in phase
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| Work function | Minimum amount of energy needed to release an electron from the surface of a metal
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| 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.
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| Current | Electric current as rate of flow of charge.
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| 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.
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| Kirchoffs First law | Sum of the current entering a junction = sum of the currents leaving a junction
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| Drift speed | The average speed at which charge moves around a circuit . Typical drift velocity =10-5m/s.
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| 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
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| Emf of a cell | Work done /unit charge in pushing the current round the whole circuit (including the internal resistance)
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| Electrical working (Work done) | Work done = Energy transfer.(This is when charge is pushed)
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| Resistance | Resistors oppose the flow of current.Resistance = voltage / current
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| 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.
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| Power dissipation | Power is the rate of working. Power is the rate of energy transfer.
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| Antinodes | A point on a stationary wave that oscillates with the maximum amplitude
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| 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
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| Continuous spectrum | This is emitted by hot solids, hot liquids and hot gases at high pressures. All wavelengths are emitted.
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| cycle | A complete movement of an oscillating or cycle
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| De broglie wavelength | Electrons have a wavelength associated with them with is dependent on the momentum of the particle
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| 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.
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| Internal resistance | This is the resistance to the flow of current inside the power supply itself.
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| 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
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| 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.
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| Thermistor | This sensor changes resistance with temperature. The higher the temperature the lower the resistance.
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| Light dependent resistor | This sensor changes resistance with the level of illumination.The greater the light level the lower the resistance.
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| Ohm’s Law | The current flowing through a metal is directly proportional to the potential difference across it providing the temperature remains constant.
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Created by:
Susan E Casey
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