Resistance vs length

Use circuit diagrams to set up and check appropriate circuits to investigate factors that affect the resistance of an electrical circuit.

Students investigate how different factors affect resistance. They can investigate the length of a wire at a constant temperature and should also look at combinations of resistors in series and parallel. Their use of circuit equipment and assembling circuits using diagrams is assessed along with their ability to measure potential difference and current and to calculate resistance. Circuit diagrams are provided by the examination board. Zero error can also be discussed during the investigation.

Equipment (per participant)

• Safety glasses
• Bench power supply
• Connecting lead (black)
• Connecting lead (red)
• Crocodile clips
• Constantan wire

• Digital voltmeter
• Digital ammeter
• 2 x BEK resistor (10Ω)
• BEK push switch
• Metre ruler

Method

Activity 1: How resistance is affected by length.

Students should connect the circuit and resistance wire ruler as per the diagram provided by the examination board. The ammeter must be connected in series between the power supply and the resistance wire. The voltmeter should be connected in parallel around the resistance ruler.
Students connect the positive terminal on the ammeter to the positive terminal of the power supply. They should then connect an extra lead to the negative terminal on the ammeter and connect the other end of the lead to the crocodile clip which is positioned at the zero end of the metre ruler. This crocodile clip will stay in the same place throughout the experiment.
Another lead is connected to the ‘travelling’ crocodile clip attached to the Constantan wire and the other end of the lead is connected to the negative terminal on the power supply.
Students should choose a suitable interval to measure (e.g. every 10cm) and position the ‘travelling’ crocodile clip at the first interval. The power supply is switched on at 6 volts and the readings from the ammeter and voltmeter are taken and noted in a suitable table.
Students then move the ‘travelling’ crocodile clip from the zero interval mark to their first interval (e.g. 10cm) and the readings are taken again. The process is repeated for each of the intervals along the metre ruler.
Next, students will calculate the resistance for each of the intervals measured by dividing the reading on the voltmeter (potential difference) by the reading on the ammeter (current). These values are then plotted on a graph. A line of best fit is drawn, and students can use this to explain the relationship between resistance and length.

Activity 2: How the arrangement of resistors in series and in parallel affects resistance.

Using the circuit diagram provided by the examining board, students connect the circuit with two resistors of the same value in series.
The power supply is switched on at 6 volts and when the push switch is closed, the readings on the voltmeter and ammeter are taken.
The power supply is then switched off and students calculate the total resistance of the series circuit.
Students then change their circuit so that the two resistors are positioned in parallel. The power supply is switched back on and when the push switch is closed, the readings on the meters are taken again. Students then calculate the total resistance of the parallel circuit and compare this with the results from the series circuit. They can then assess the effect of adding resistors in series and parallel.

Technician tips

The resistance wire can be set up for the students by taping the constantan wire to the metre rulers in advance of the practical session.
The circuit with the resistance wire ruler should not be left switched on for too long as the length of resistance wire can become hot and will affect the resistance results.