Electricity~

• Current is the flow of electrical charge.
• Current can only flow if there is a potential difference.
• The unit for current is the ampere (amps/A)
• The unit for potential difference is volts (V).
• The unit for resistance is ohms (Ω).
• The greater the resistance across a component, the smaller the current.
• Cell

  
• Battery

  
• Switch

  
• Filament lamp

  
• Fuse

  
• LED

  
• Resistor

  
• Variable resistor

  
• Ammeter

  
• Voltmeter

  
• Diode

  
• Light dependent resistor

  
• Thermistor

  
• Factors like wire length, temperature and series vs parallel can affect resistance.
• Ammeters are connected in series.
• Voltmeters are connected in parallel around the component you are investigating.
• In an ohmic conductor, the current is directly proportional to potential difference.
• Resistance increases with temperature (which is why filament lamps are not ohmic conductors).
• Diodes let current flow one way and have a very high resistance in the opposite direction.
• Ohmic conductor

  
• Filament lamp

  
• Diode

  
• In an LDR, as the light intensity increases, resistance falls.
• LDRs can be used as automatic night lights or in burglar detectors.
• In a thermistor, as temperature increases, resistance falls.
• Thermistors are used as temperature detectors e.g. in a thermostat.
• You can use LDRs and thermistors in sensing circuits.
• In series circuits:

  • potential difference is shared across components
  • current is the same everywhere
  • resistance adds up
• The bigger a component's resistance, the bigger it's share of the potential difference.
• If you connect cells in series, their potential difference adds up.
• In parallel circuits:

  • potential difference is the same across all components
  • current is shared between branches
  • total resistance = 1/resistor 1 + 1/resistor 2 ...etc.
• In ac supplies, the current is constantly changing direction. This is what the mains supply is.
• In dc supplies, the current only flows in one direction. This is the current produced by a battery.
• The mains supply in the UK is 230V and 50Hz.