resistance

• Required practical

  At least 15% of the marks in your exam will come from questions relating to required practicals
• Factors affecting the resistance of electrical circuits

  • Length of a wire
• Equipment used

  • Battery
  • Ammeter
  • Voltmeter
  • Length of wire
• Measuring resistance in the circuit

  1. Use ammeter to measure current

  2. Use voltmeter to measure potential difference

  3. Calculate resistance using R=V/I
• Increasing length of wire

  Increases resistance
• Graph of resistance vs length

  • Straight line passing through zero
  • Indicates resistance is directly proportional to length
• There is a small resistance when the length of the wire is zero
• Zero error


  A reading on a measuring instrument when the value should be zero, a systematic error
• Zero error is caused by the equipment, e.g. difficulty getting crocodile clip at zero position, resistance from contact between clip and wire</b>
• Zero error needs to be subtracted from all resistance readings
• Heating effects can increase resistance as temperature of wire increases
• Reducing heating effects

  1. Use low potential difference to keep current low

  2. Only turn on current when taking readings, turn off between readings
• Variable resistor

  Contains a long piece of wire in a coil, using a slider to change the length of wire the current runs through
• Variable resistors are used to control potential difference across a component by changing resistance