(P) Resistance & V=IR

• This practical investigates the factors affecting resistance
• The resistance of a circuit can depend on a number of factors, like whether components are in series or parallel, or the length of wire used in the circuit
• The Ammeter: Measure the current (in amps) flowing through the test wire. The ammeter must always be placed in series in a circuit

  
• The voltmeter: Measures the potential difference (p.d.) across the test wire (in volts). The voltmeter must always be placed in parallel in a circuit (not aroundany other bit of the circuit e.g. battery)

  
• 1. Attach a crocodile clip to the wire level with 0 cm on the ruler

  
• 2. Attach the second crocodile clip to the wire, e.g. 10 cm away from the 1st clip. Write down the length of the wire between the clips

  
• 3. Close the switch, then record the current through the wire and the p.d. across it

  
• 4. Open the switch, then move the second crocodile clip, e.g. another 10 cm, along the wire. Close the switch again, then record the new length, current and p.d.

  
• 5. Repeat this for a number of different lengths of the test wire

  
• 6. Use your measurements of current and p.d. to calculate the resistance for each length of wire, using R=V/I (from V=IR)

  
• 7. Plot a graph of resistance against wire length and draw a line of best fit

  
• 8. Your graph should be a straight line through the origin, meaning resistance is directly proportional to length - the longer the wire, the greater the resistance

  
• 9. If your graph doesn't go through the origin, it could be because the 1st clip isn't attached exactly at 0 cm, so all your length readings are a bit out. This is a systematic error

  
• A thin wire will give you the best results. Make sure it's as straight as possible so your length measurements are accurate
• The wire may heat up during the experiment, which will affect its resistance. Leave the switch open for a bit between readings to let the circuit cool down