GCSE Physics Electricity

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  • Created by: RyEC
  • Created on: 19-02-18 16:01
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  • Physics - Electricity
    • Static Electricity
      • This is electricity that does not move - it builds up
      • Like charges repel
      • Unlike charges attract
      • Uses include: -Paint/crop spraying; Photocopier; smoke/dust precipitators
      • Static Charge results from the loss or gain of electrons by friction
        • Loss = positive
        • Gain = negative
      • Dangers: aeroplane refuelling; lightning; shocks
      • Earthing safely discharges surplus charge from an insulator
      • Conductors allow charge to flow; insulators do not
    • Charge flow
      • Measured in coulombs (C)
      • 1A of current = 1C of charge passing a point per second
      • Charge = current x time
        • Q = It
    • Current and circuits
      • Conventional current flows from positive to negative
        • Electrons flow from negative to positive
      • The potential difference (voltage) is define by the energy transferred per unit charge
        • Voltage = energy/charge
          • V = W/Q
          • Measured in volts (V)
            • 1V = 1J/C
      • The current is the same in all parts of a series circuit
        • The current will divide among the branches of a parallel circuit
          • The total voltage in a series circuit is shared/split across its components
            • The voltages across each branch in a parallel circuit are equal
      • Ohm's law is that the current flowing through a metallic conductor (or resistor), held at constant temperature, is directly proportional to the potential difference across
        • Voltage = current x resistance
          • V = IR
        • If heat increases so too does resistance
          • The length of the wire is directly proportional to resistance if temperature, material and CSA are constant
            • If CSA increases then resistance will decrease
              • Different materials have different resistances (depends on # of delocalised electrons)
                • The length of the wire is directly proportional to resistance if temperature, material and CSA are constant
                  • If CSA increases then resistance will decrease
                    • Different materials have different resistances (depends on # of delocalised electrons)
        • In series: Total resistance = R1+R2+R3+...
          • In parallel: 1/Rt = 1/R1+1/R2+...
        • Power
          • Electrical power = current x voltage
            • P=IV
          • Energy transferred = power x time
            • P=Et
      • Household Electricity
        • Energy(kWh) = Power(kW) x time(h)
        • Neutral wire - bLue to the Left
        • Earth wire - middle, yellow+green
        • Live wire - bRown to the Right
        • A fuse contains a metal wire that will melt if current exceeds a stated value
          • Choose the fuse that is closest to but above the standard operating current
          • Double insulation can be used as an alternative to earthing
        • Types of current
          • a.c. - alternating current (from mains), changes direction regularly
            • Used in National Grid for transformers
              • Vs/Vp=Ns/Np
                • Transformer equation:
              • Step-up transformer: increases voltage; decreases current; more turns on secondary coil than primary
              • Step-down transformer: decreases voltage; increases current; more turns on primary coil than secondary
              • Power in = power out and P=IV so if voltage increases then current decreases
          • d.c. - direct current (from battery), always flows in one direction
        • Electromagnetic induction

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