Practical: investigating resistance

• Carried out to see how RESISTANCE (R) in an electrical circuit is influenced by factors like the LENGTH OF THE WIRE & the COMBINATION OF RESISTORS in the circuit.
• INVESTIGATING RESISTANCE & LENGTH OF WIRE:

  • Connect a crocodile clip to the wire at 0 cm on the ruler.
  • Connect a second crocodile clip at a set length on the wire being investigated, e.g. 5 cm.
  • Note the current & potential difference
  • Move the second clip to a new length, e.g., 10 cm & record again.
  • Repeat for various lengths & record all measurements.
  • Use the formula V = I × R to calculate RESISTANCE.
  • Plot a graph of resistance against wire length and draw a LINE OF BEST FIT.

  
• INVESTIGATING RESISTANCE & LENGTH OF WIRE:

  Ensure that the circuit is DISCONNECTED between readings, as the wire would warm up and affect the resistance readings.

  
• The number of resistors & their arrangement can affect the total resistance in the circuit.

  
• To investigate how resistors in SERIES & in PARALLEL affect the overall resistance, you can set up the following 3 circuits:

  • one resistor
  • resistors in series
  • resistors in parallel

  
• INVESTIGATING COMBINATIONS OF RESISTORS IN SERIES & PARALLEL method:

  • For the FIRST circuit, note down the ammeter & voltmeter values & use the formula to work out the resistance of resistor 1.
  • Repeat for RESISTOR 2.
  • Connect the SECOND CIRCUIT & work out the resistance.
  • Connect the THIRD CIRCUIT & work out the resistance.

  
• INVESTIGATING COMBINATIONS OF RESISTORS IN SERIES & PARALLEL results in series:

  • The resistance of the combined resistors in SERIES is equal to the sum of the two individual resistances.
  • Because the electrons flow through just ONE path through both resistors, so the current does too.

  
• INVESTIGATING COMBINATIONS OF RESISTORS IN SERIES & PARALLEL results in parallel:

  • The resistance of the combined resistors in PARALLEL is LESS than the sum of the two individual resistances.
  • Because the electrons are SPLIT between the different paths (or 'loops') but the resistors still have the same potential difference across them.

  
• The line is close to more points, as the points are not in a straight line.

  
• • potential difference across R decreased.
  • current in R decreased.

  
• The temperature of the resistor had increased, so the resistance of the resistor has increased, so the current is lower.

  
• • connect one additional resistor in parallel with resistor R.
  • measure the current with the ammeter
  • measure the p.d. with the voltmeter
  • calculate resistance using R = V/ I
  • repeat with increasing number of resistors.

  
• • length measured with a ruler
  • length varied by moving crocodile clips
  • current measured with an ammeter
  • potential difference measured with a voltmeter
  • calculate resistance for each length
  • record current & p.d. for a large number of different lengths
  • repeat readings of current & p.d. for each length to remove anomalies
  • ensure values of current are low to minimise heating of wire
  • ensure circuit is disconnected between readings

  
• The variable resistor changes the resistance of the circuit to keep the current the same, so the temperature of the wire is kept constant.