Electrical circuits

• Circuit diagrams


  • Switch (open)
  • Switch (closed)
  • Fuse
  • Thermistor
  • Lamp
  • Cell
  • Ammeter
  • Ground
  • Voltmeter
  • LDR
  • Resistor
  • Variable Resistor
  • Rectifier
• Components are represented by symbols
• Wires are represented by straight lines. There should be no breaks in the wires or this would show a broken circuit and no current would flow
• Diodes


  Components that only allow current to flow through it one direction
• Rectifier


  A component that turns alternating current (this is current that repeatedly changes direction) into a direct current (1 direction only)
• Diodes act as rectifiers

  They block the alternating current when it changes direction
• Diodes


  Have a very high level of current on one side and an extremely low level of current (preferably 0) on the other
• Thermistors


  A component that can be triggered by temperature
• Thermistors


  • Their resistance changes depending on the temperature
  • When the temperature increases, the resistance increases
  • Often used within heaters, when a certain temperature is reached then the thermistor will turn the heater off
• LDRs


  A component that can be triggered by light intensity
• LDRs


  • Their resistance changes with light intensity
  • When light intensity increases, the resistance increases
  • Used to turn lights on when it gets dark or when a camera phone needs to use flash when taking a photo
• Resistance


  A measure of how difficult it is for current to flow through a component, measured in Ohms
• When resistance is increased

  The current size/volume will decrease
• When potential difference is increased


  The current will increase
• Potential Difference


  Energy used between 2 points in a circuit, either side of a component
• Measuring Resistance


  1. Use Ohm's Law

  2. Use an ammeter in series

  3. Use a voltmeter in parallel
• Current-Voltage Graph for a Resistor

  • The straight line obeys Ohm's law
  • The slope of the graph is equal to 1/resistance
  • The constant slope (straight line graph) tells us that the resistance is constant
• Current-Voltage Graph for a Filament Lamp

  • The gradient (slope) decreases at larger potential differences
  • When the current through a filament (thin wire) lamp increases, the temperature of the lamp increases
  • This increases the resistance of the lamp and causes the graph to curve
• Electrical Work


  In a circuit, the energy source (battery or power supply) makes a current flow
• Charge Flow


  • Charge will flow from the battery and power supply (energy source) and through the rest of the circuit and its components like lamps or heaters
  • The low current transfers energy from the energy source into its components
  • This transferred energy into the components triggers it to work
• Power


  The rate of energy transfer to a component, equal to the current through the component multiplied by the potential difference across the component
• Total Energy Transfer


  Equal to the power multiplied by the time a current flows for
• Ohmic Conductors


  Conductors that obey Ohm's Law
• Constant Resistance


  • The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor
  • This means that the resistance remains constant as the current changes
• Power Label


  Tells you the power rating of an appliance
• Calculating Current from Power Label

  1. Power = potential difference X current

  2. Power = current squared X resistance

  3. 230 V is the potential difference across the mains power supply in the UK
• Calculating Energy Transferred


  1. Energy transferred = power X time

  2. Energy transferred = charge X potential difference
• National Grid


  A network of cables and transformers that connect power stations with houses and shops in the UK
• Power Stations


  • Create an electric current, which is an energy carrier
  • Large power stations are more efficient than smaller power stations as they burn lots of fuel at very high temperatures, creating very hot steam that can turn the steam turbines
  • Electricity is produced at a voltage of 25,000 volts
  • Step-up transformers then increase the potential difference to 400,000 Volts, reducing the current and reducing heat loss during transport
  • Step-down transformers are then used to make the potential difference 230 volts when it enters homes as this is safest
• Fuses


  They contain a thin wire that melts if the current flowing through an appliance is too high, breaking the circuit and stopping the flow of electric current, preventing the appliance overheating and a dangerous situation such as a fire
• Circuit Breakers


  Devices that switch off the flow of current if it is too high, and are reusable unlike fuses
• Fuse Ratings


  Tell us the level where a fuse will begin to melt, and the fuse chosen should have a value just above the normal current that an electrical appliance uses