electricity

• Electrical circuit


  A closed loop that contains a power source such as a cell and something for the electrons to flow through like a wire
• Circuit diagram


  • Represents an electrical circuit
• Main circuit components


  • Cell/battery
  • Filament lamp
  • Switch
• Cell


  A simple version of a battery, which is made up of two or more cells
• Filament lamp


  A small light bulb
• Switch


  Can be closed allowing current to continue flowing around the circuit or open which would disrupt the circuit
• Current (I)


  A measure of the flow of electrons around the circuit, similar to the flow of water through a pipe, measured in amperes (amps)
• Potential difference (V)


  The force driving the flow of electrons, provided by the cell or battery, also known as voltage, measured in volts
• Resistance (R)


  Everything that resists or opposes the flow of electrons, similar to a partial blockage in a pipe, measured in ohms
• Electrons flow from the negative terminal to the positive terminal of a cell or battery
• Conventional current flows from the positive terminal to the negative terminal of a cell or battery
• v = ir


  Potential difference or voltage equals current times resistance
• As voltage increases


  Current increases proportionally
• As voltage decreases


  Current decreases proportionally
• Circuits with only wires or resistors


  • Resistance stays constant
  • Current vs potential difference graph is a straight line
• Bigger resistor


  Graph line is less steep
• Smaller resistor


  Graph line is steeper
• Higher currents can cause wires to heat up, so the lines don't always look perfectly straight
• Filament lamps


  • Contain a very thin metal filament
  • As current flows, filament heats up and resistance increases
  • Graph becomes less steep at higher voltages
• Diodes


  • Only allow current to flow in one direction
  • Have very high resistance in reverse direction
• Charge


  A measure of the total current that flowed within a certain period of time
• Charge


  Represented by the letter q, measured in coulombs (C)
• Equation to find charge


  Charge = Current (in amps) x Time (in seconds)
• Charge calculation


  • Kettle draws 12 amps and takes 50 seconds to boil, charge = 12 x 50 = 600 coulombs
• Charge calculation


  • Phone charger transfers 43.2 kilo coulombs over 2 hours, charge = 43,200 coulombs
• Equation to find current

  Current (in amps) = Charge (in coulombs) / Time (in seconds)
• Current calculation


  • Phone charger transfers 43,200 coulombs over 2 hours (7,200 seconds), current = 43,200 / 7,200 = 6 amps
• Circuit diagram


  Representation of the components in an electrical circuit using standardised symbols
• Components needed to provide electric power to a circuit


  • Cell
  • Battery
• Components to control the flow of electricity


  • Switch (closed - allows flow, open - turns off circuit)
• Other components


  • Filament lamps (small bulbs)
  • Fuses (break if too much current)
  • Diodes (allow current flow in one direction)
  • Light emitting diodes (LEDs - emit light when current flows)
• Components to measure things in a circuit


  • Ammeters (measure current, connected in series)
  • Voltmeters (measure potential difference, connected in parallel)
• Resistors


  • Fixed (provide a certain resistance)
  • Variable (resistance can be modified)
• Light dependent resistor (LDR)


  Resistor whose resistance depends on light intensity - low in bright light, high in darkness
• Thermistor


  Resistor whose resistance depends on temperature - resistance falls with higher temperatures
• LDR in bright light

  Low resistance, allows more current to flow
• LDR in darkness


  High resistance, hardly any current flows
• Higher temperature


  Thermistor resistance falls
• Lower temperature


  Thermistor resistance increases
• LDRs and thermistors are useful as temperature and light receptors in various applications