Unit 1 - Electricity

Unit 1 - Electricity

1. Circuit Diagrams

   • Series & Parallel
   • Symbols

2. Current & Potential Difference

   • Current is the rate of flow of charge
     • I = Q / t
   • Pd is the work done in moving a unit charge between two points
     • V = W / Q

3. Resistance

   • The hindrance to the flow of charge
   • R = V / I
   • Ohmic Conductors
     • Obey Ohm's law
     • I is directly proportional to V
       • If temperature is constant

4. I-V Characteristics

   • I-V Graphs
     • The lower the gradient, the higher the resistance
     • Metallic Conductors
     • Filament Lamps
   • Semiconductors
     • Quite good conductors
       • But not as good as metals due to fewer charge carriers
       • However, if energy is supplied to a semiconductor (e.g. a rise in temperature), more charge carriers can be released and the resistance decreaes
     • Components
       • Thermistors
       • Diodes
       • LDRs

5. Resistivity & Superconductors

   • Resistivity is the resistance of a 1m length with a 1m^2 cross-sectional area
     • ρ = RA / L
   • You can lower the resistivity of some metals by cooling them down
     • Reach a transition temperature and resistivity disappears entirely - superconductors
       • Without any resistance, no electrical energy is wasted as heat, so none is lost
       • This transition temperature is below 10 kelvin (-263°C) for most 'normal' conductors
         • It is tricky and very expensive to reach such temperatures
       • Superconductors can be used for cables with no loss of power, really strong electromagnets, and high-speed circuits

6. Power & Electrical Energy

   • Power Equations
     • P = E / t
     • P = VI
     • P = V^2 / R
     • P = I^2R
   • Energy Equations
     • E = VIt
     • E = (V^2 / R)t
     • E = I^2Rt

7. E.m.f & Internal Resistance

   • E.m.f is the amount of electrical energy a battery produces and transfers to each coulomb of charge
     • You can use a V-I graph to calculate ℰ and r
       • The y-intercept (c) is ℰ and the gradient (m) is -r
   • Equations
     • ℰ = E / Q
     • ℰ = I(R + r)
       • ℰ = V + v
         • V = ℰ - v
           • V = ℰ - Ir

8. Conservation of Energy & Charge

   • Kirchhoff's Laws
     • First Law
       • The total current entering a junction = the total current leaving it
     • Second Law
       • The total e.m.f. around a series circuit = the sum of the p.d.s across each component
         • ℰ = ΣIR
   • In circuits
     • Parallel
       • 1 / R total = (1 / R1) + (1 / R2) + (1 / R3)...
     • Series
       • R total = R1 + R2 + R3...
       • ℰ total = ℰ1 + ℰ2 + ℰ3...

9. The Potential Divider

   • At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
     • You can use potential dividers to supply a p.d., V out, between zero and the p.d. across the power supply.
       • V out = (R2 / R1 + R2)Vs
       • Uses
         • Light sensors
         • Temperature sensors
         • Potentiometers

10. Alternating Current

    • An AC or AV is one that changes with time
      • Oscilloscopes
        • An AC source gives a regular repeating waveform
        • 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
        • Equations
          • f = 1 / T
          • V rms = V0 / sqrt2
          • P = V rms * I rms
          • R = V rms / I rms
            • R = V0 / I0