3 - Resistance

• What is the method for this practical (wire version)?

  1. Set up a simple circuit and attach a length of wire along a metre ruler using tape. Attach a crocodile clip to one end.
  2. Attach the second crocodile clip 10cm along and record the current and voltage through the wire.
  3. Repeat by moving the crocodile clip 10cm along the wire and each time recording the current and voltage measured.
  4. Calculate the resistance of the wire at each point.
  5. Plot a graph of the length of the wire against the resistance of the wire at that point.
• What is the method for this practical (circuit version)?

  1. Construct a series circuit.
  2. Switch on the power supply & close the switch. Record the voltage and current shown.
  3. Construct a parallel circuit.
  4. Switch on the power supply & close the switch. Record the voltage and current shown.
  5. Calculate the total resistance of each circuit.
• What two types of meters are needed in the circuit and how should they be connected?

  Voltmeter - in parallel to the resistance wire

  Ammeter - in series with the resistance wire
• What equation is used to obtain the resistance from the current and voltage readings?

  R = V / I
• What should the graph of resistance against length look like?

  A straight line going through the origin as the two variables are directly proportional.
• Why may the graph not pass through the origin?

  There may be a zero error due to it being very difficult to attach the crocodile clip exactly at the end of the metre rule.
• Why should the wire used be thin?

  Higher resistance so resistance is large enough to be measured and interpreted.
• Why should a low potential difference be used?

  To prevent high current and the wire heating up.
• If both resistors are identical, which combination of resistors will have the least resistance?

  Parallel.
• What are the safety precautions?

  Do not touch wire as could be very hot

  Low potential difference to avoid overheating
• Relative to the resistance of the resistors, describe the total resistance of the parallel combination.

  Lower than the smallest of the two individual resistances.