Electricity AS

• There are three key quantities: V, I, and R
• Current (I)


  The rate of flow of charge particles
• Conventional current


  Flows from positive to negative, but the actual charge carriers (electrons) move from negative to positive
• Potential difference (V)


  The energy transferred per unit charge
• Resistance (R)


  The ratio of potential difference across a component to the current in that component
• Investigating component characteristics


  1. Set up circuit with ammeter and voltmeter

  2. Vary current/potential difference

  3. Measure current and potential difference
• IV characteristics


  • Relationship between current (I) and potential difference (V)
  • For ohmic conductors, a straight line through the origin
  • For non-ohmic conductors, a non-linear relationship
• Ohm's law states that current is proportional to potential difference, provided physical conditions are constant
• Resistance


  Not always equal to 1 over the gradient of the IV characteristic
• IV characteristics


  • Resistor: straight line through origin
  • Filament lamp: non-linear
  • Diode: allows current flow in one direction only
• Resistivity (ρ)


  A material property that determines the resistance of a conductor based on its length and cross-sectional area
• As temperature increases


  Resistance of most materials increases
• Semiconductors


  As temperature increases, resistance decreases due to more charge carriers being liberated
• Superconductors


  Below a critical temperature, resistance drops to zero
• Kirchhoff's first law: the sum of currents into a junction equals the sum of currents out of the junction
• EMF (ε)


  The energy per unit charge transferred to the circuit by a source (e.g. battery)
• Around any closed loop in a circuit, the sum of the EMFs equals the sum of the potential differences
• EMF


  Energy transferred to the circuit by a battery
• Potential difference


  Energy transferred within a component
• Around any closed loop in the circuit


  The sum of the EMFs is equal to the sum of the potential differences
• Series circuit


  • Current is the same everywhere in the circuit
• Parallel circuit


  • Current is split at a junction
• In a parallel circuit


  The potential difference is the same across each component
• Total resistance in series circuit


  Sum of individual resistors
• Total resistance in parallel circuit

  1 over (1/r1 + 1/r2 + ...)
• Power


  Rate of energy transfer = IV = I^2R = V^2/R
• Total energy transferred


  Power x time = IVt
• Potential divider circuit


  • Splits potential difference between two resistors
• Potential divider components


  • Thermistor
  • Light dependent resistor
  • Variable resistor
• Internal resistance


  Resistance within a cell or power supply
• EMF


  Equal to terminal potential difference + current x internal resistance
• As current increases


  Terminal potential difference decreases
• Cells in series


  Internal resistances add up
• Cells in parallel


  Combined internal resistance decreases