Electricity

• 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 and measure potential difference

  3. Plot I-V graph
• 1. V characteristic of an ohmic conductor

  • Linear graph passing through the origin
• Ohm's law states that current is proportional to potential difference for an ohmic conductor</b>
• 1. V characteristic of a filament lamp

  • Non-linear, resistance not equal to 1/gradient
• 1. V characteristic of a diode

  • Allows current flow in one direction only
• Resistivity (ρ)


  Material property that determines resistance, depends on length and cross-sectional area
• As temperature increases


  Resistance of a conductor increases
• Semiconductors


  • Resistance decreases as temperature increases, due to more charge carriers being liberated
• Superconductors


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


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


  Energy transferred to charge carriers per unit charge
• Potential difference


  Work done per unit charge
• EMF


  Energy transferred to the circuit by a battery
• Potential difference


  Energy transferred out by a component
• Around any closed loop in a circuit, the sum of the EMFs is equal to the sum of the potential differences
• Series circuit


  • Same current everywhere
• Parallel circuit


  • Current splits at a junction
• In a series circuit

  Total resistance = sum of individual resistors
• In a parallel circuit

  Total resistance = 1 / (1/r1 + 1/r2 + ...)
• Potential divider circuit


  Splits the 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 current multiplied by (external resistance + internal resistance)
• Terminal potential difference


  Less than EMF due to internal resistance
• Cells in series


  Internal resistances add up
• Cells in parallel


  Combined internal resistance decreases
• Power = current x potential difference = current^2 x resistance = potential difference^2 / resistance
• Power = energy / time
• Total energy transferred = power x time