Electricity

• Resistance equations


  • R = R1 + R2 + ... (series)
  • 1/R = 1/R1 + 1/R2 + ... (parallel)
  • P = I^2R
  • P = IV
  • P = I^2/R
• Ohm's law


  For an ohmic conductor, current is directly proportional to the potential difference across it given that physical conditions are constant
• Ohmic conductors


  • Metal wire
  • Ohmic filament lamp
• Non-ohmic conductors


  • Semiconductor diode
• Resistivity


  A measure of how easily a material allows charge carriers to flow through it
• Determining the resistivity of a wire

  1. Attach crocodile clip to wire and measure length

  2. Close switch and measure current and potential difference

  3. Calculate resistance

  4. Repeat at different lengths

  5. Plot graph of resistance against length
• Power


  The rate of transfer of energy
• Electrical energy


  Energy = current x potential difference x time
• EMF


  The energy supplied by a source per unit of charge
• Internal resistance


  The resistance within the source that opposes the flow of current
• EMF


  E = V + Ir
• Kirchhoff's first law: The total current entering a junction equals the total current leaving the junction
• Kirchhoff's second law: The total EMF around a series circuit equals the sum of the potential differences across each component
• Series circuits


  • Same current at all points
  • EMF is split between components
  • Voltage splits proportionally to resistance
• Parallel circuits


  • Current is split into each junction
  • Same potential difference across all components
• Potential divider


  A circuit with a voltage source and a couple of resistors that divides the voltage
• Potentiometer


  A variable resistor that allows you to continuously adjust the output voltage
• Variable resistors


  • LDR (light dependent resistor)
  • Thermistor