Electricity

• The current is the flow of electrons around a circuit.
• Electric charge can be measured using an ammeter.
• Current flows from positive to negative, but electrons actually move in the opposite direction (from negative to positive).
• Power = Current x Potential Difference
• Electric power is measured in watts, where one joule of work done every second equals one watt.
• Current


  Flow of electrical charge
• Potential Difference


  Driving force that pushes charge round
• Resistance


  Anything that slows down charge flow
• Current in circuits with no source of potential difference, there is no current
• Current through a component depends on two factors: the component's resistance and the potential difference across the component
• Charge in circuits


  Size of the current is the rate of flow of charge
• Ohm's Law


  V=IR
• Circuit Symbols


  • Battery
  • Switch open
  • Switch closed
  • Resistor
  • Variable resistor
  • Filament lamp
  • Diode
  • LDR (Light Dependent Resistor)
  • Thermistor
  • LED (Light Emitting Diode)
  • Fuse
• Ohmic conductor


  • Current is directly proportional to potential difference, so resistance doesn't change
• Components with changing resistance

  • Filament lamp: Current increases, so temperature increases, so resistance increases
  • LDRs and Thermistors: Resistance depends on light intensity and temperature respectively
• Diode


  • High resistance in one direction, only allows current to flow in the other direction
• 1. V characteristics of components with changing resistance are non-linear
• Series circuits


  Current is the same everywhere, total resistance is the sum of individual resistances
• Parallel circuits


  Total current is the sum of currents through each branch, potential difference is the same across each branch, total resistance is less than the smallest resistance
• Alternating current (AC) constantly changes direction, direct current (DC) always flows in the same direction
• Facts about UK mains supply

  • Frequency of 50 Hz
  • Voltage around 230 V
  • Three-core cable with live, neutral and earth wires
• Earth wire


  Provides a path for current in case of a fault, stops appliance casing becoming live
• Electric shocks can cause injury or death, even if a plug socket is turned off there is still danger
• Any connection between the live wire and the earth can be dangerous, e.g. it could cause a fire
• Power


  Energy transferred per second
• National Grid


  System of cables and transformers that connect power stations to consumers, transfers power at high potential difference and low current for efficiency