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# AS ocr unit 2 Def.

### AS ocr unit 2 Definitions.

Question | Answer |
---|---|

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. |