Electricity

• Electrical current is the flow (movement) of electric charges. For an electric charge to flow there has to be a source of potential difference and a closed (or complete) circuit.
• current is measured in amps
• a conductor is a material that allows a charge to pass through
• an electrical insulator doesn't carry a current
• potential difference is measured in volts
• if the resistance increases the current decreases
• if the potential difference increases the current increases
• Resistance is a measure of how difficult it is for current to flow through a component.
• the longer the wire the higher the resistance
• Ohmic Resistor graph

  
• Filament lamp graph

  
• Ohms law is that the current through an ohmic conductor at constant temperature is directly proportional to the potential difference across the resistor.
• resistor

  
• variable resistor

  
• diode

  
• diode graph

  
• The current through a diode only flows in one direction. In the reverse direction, the resistance is very high.
• thermistor

  
• thermistor:

  • increasing temperature decreases resistance
  • used in central heating
• LDR

  
• LDR

  • increasing light intensity decreases resistance
• series:

  • current is same
  • potential difference splits
• parallel:

  • current splits
  • potential difference is the same
• in a series the total resistance is calculated by adding up the resistance of each individual resistor
• In a parallel circuit, the current through the energy source is equal to the sum of the currents in the separate branches.
• plug sockets are AC
• mains electricity has a frequency of 50 Hz and 230 V
• neutral wire is blue
• neutral wire

  • blue
  • close to earth wire
  • completes the circuits and carries current away from the appliance
• the live wire is brown
• live wire

  • brown
  • carries 230 V
  • provides an alternating potential difference from the mains electricity supply
• the earth wire is green and yellow
• earth wire

  • green and yellow
  • 0 V
  • only carries a current if there is a fault
  • it is a safety wire to stop the appliance becoming live
• power stations

  • create an electric current
  • larger power stations are more efficient
  • steam turbines are more efficient at higher temperature
  • produce electricity at 25000 V
• step-up transformers

  • increase the potential difference to 400000 V
  • this reduces the current to reduce heat loss
  • increases efficiency
• step-down transformer

  • decreases potential difference to 230 V
• National grid

  1. power stations produce electricity at 25000 V
  2. step-up transformers then increase to 400000 V
  3. step-down transformers decrease it to 230 V for domestic use
• fuses contain a thin wire that melts if the current is too high, which breaks a circuit and stops the flow of current
• circuit breakers can be used multiple times whereas a use needs to be replaced every time
• earth wire

  • low resistance wire that connects the metal case to the ground
  • if the casing is touched by a wire inside the appliance, the current will flow through the earth wire
  • because the resistance is low alot of current will flow through the wire , breaking the fuse and stopping the flow of current