Unit 1 - Electricity

Circuit Diagrams
1. Series & Parallel
2. Symbols
Current & Potential Difference
1. Current is the rate of flow of charge
  1. I = Q / t
2. Pd is the work done in moving a unit charge between two points
  1. V = W / Q
Resistance
1. The hindrance to the flow of charge
2. R = V / I
3. Ohmic Conductors
  1. Obey Ohm's law
  2. I is directly proportional to V
    1. If temperature is constant
I-V Characteristics
1. I-V Graphs
  1. The lower the gradient, the higher the resistance
  2. Metallic Conductors
  3. Filament Lamps
2. Semiconductors
  1. Quite good conductors
    1. But not as good as metals due to fewer charge carriers
    2. However, if energy is supplied to a semiconductor (e.g. a rise in temperature), more charge carriers can be released and the resistance decreaes
  2. Components
    1. Thermistors
    2. Diodes
    3. LDRs
Resistivity & Superconductors
1. Resistivity is the resistance of a 1m length with a 1m^2 cross-sectional area
  1. ρ = RA / L
2. You can lower the resistivity of some metals by cooling them down
  1. Reach a transition temperature and resistivity disappears entirely - superconductors
    1. Without any resistance, no electrical energy is wasted as heat, so none is lost
    2. This transition temperature is below 10 kelvin (-263°C) for most 'normal' conductors
      1. It is tricky and very expensive to reach such temperatures
    3. Superconductors can be used for cables with no loss of power, really strong electromagnets, and high-speed circuits
Power & Electrical Energy
1. Power Equations
  1. P = E / t
  2. P = VI
  3. P = V^2 / R
  4. P = I^2R
2. Energy Equations
  1. E = VIt
  2. E = (V^2 / R)t
  3. E = I^2Rt
E.m.f & Internal Resistance
1. E.m.f is the amount of electrical energy a battery produces and transfers to each coulomb of charge
  1. You can use a V-I graph to calculate ℰ and r
    1. The y-intercept (c) is ℰ and the gradient (m) is -r
2. Equations
  1. ℰ = E / Q
  2. ℰ = I(R + r)
    1. ℰ = V + v
      1. V = ℰ - v
        V = ℰ - Ir
Conservation of Energy & Charge
1. Kirchhoff's Laws
  1. First Law
    1. The total current entering a junction = the total current leaving it
  2. Second Law
    1. The total e.m.f. around a series circuit = the sum of the p.d.s across each component
      1. ℰ = ΣIR
2. In circuits
  1. Parallel
    1. 1 / R total = (1 / R1) + (1 / R2) + (1 / R3)...
  2. Series
    1. R total = R1 + R2 + R3...
    2. ℰ total = ℰ1 + ℰ2 + ℰ3...
The Potential Divider
1. At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
  1. You can use potential dividers to supply a p.d., V out, between zero and the p.d. across the power supply.
    1. V out = (R2 / R1 + R2)Vs
    2. Uses
      1. Light sensors
      2. Temperature sensors
      3. Potentiometers
Alternating Current
1. An AC or AV is one that changes with time
  1. Oscilloscopes
    1. An AC source gives a regular repeating waveform
    2. A DC source is always at the same voltage, so you get a horizontal line. You'd get a dot on the voltage axis if the time base was turned off
    3. Equations
      1. f = 1 / T
      2. V rms = V0 / sqrt2
      3. P = V rms * I rms
      4. R = V rms / I rms
        R = V0 / I0

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Unit 1 - Electricity

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	Created by Callum McClintock over 10 years ago