electricity

• This video is about 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/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
• Resistance is not always equal to 1/gradient of IV characteristic
• 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
• Charge is conserved in circuits, so current in = current out at a 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 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 the 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
• Internal resistances of cells in series

  Add up
• Internal resistances of cells in parallel

  Decrease in effect