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
• Ohm's law


  Current is proportional to potential difference, provided physical conditions are constant
• Resistance


  • Depends on length (proportional) and cross-sectional area (inversely proportional)
  • Resistivity is a material property
• 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 transferred to the circuit per unit charge by a source (e.g. battery)
• Around any closed loop in a circuit, the sum of the EMFs is equal to the sum of the potential differences
• EMF


  Energy transferred to the circuit by a battery
• Potential difference


  Work done per unit charge, energy transferred within 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
  • Current in = current out
• Parallel circuit


  • Current splits at a junction
  • Potential difference is the same across each branch
• Calculating total resistance in series circuit


  Add up individual resistor values
• Calculating total resistance in parallel circuit

  Take reciprocal of sum of reciprocals of individual resistor values
• Power


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


  Power x time = IVt
• Potential divider circuit


  • Splits potential difference between two resistors
  • Can use for sensing circuits like thermistors or LDRs
• Internal resistance


  • Resistance within a cell or power supply
  • Causes terminal potential difference to be less than EMF
• Cells in series


  Internal resistances add up
• Cells in parallel


  Combined internal resistance decreases