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: Investigating How Resistance is Affected by Length
Equipment Setup:
- Circuit Components: Set up the circuit with a resistance wire and metre ruler as provided by the examination board.
- Connections:
- Connect the ammeter in series between the power supply and resistance wire.
- Connect the voltmeter in parallel around the resistance ruler.
- Terminal Connections:
- Connect the positive terminal of the ammeter to the positive terminal of the power supply.
- Attach a lead from the negative terminal of the ammeter to a fixed crocodile clip at the 0 cm mark of the metre ruler. This clip remains in place for the entire experiment.
- Connect a second lead to a ‘travelling’ crocodile clip attached to the Constantan wire and attach the other end to the negative terminal on the power supply.
Procedure:
- Setting Intervals:
- Decide on a measurement interval (e.g., every 10 cm).
- Position the ‘travelling’ crocodile clip at the first interval mark on the ruler.
- Taking Measurements:
- Switch on the power supply at 6 volts.
- Record the readings from both the ammeter and voltmeter in a table.
- Repeating Measurements:
- Move the ‘travelling’ crocodile clip to the next interval (e.g., 10 cm) along the ruler.
- Take and record new readings from the meters.
- Repeat this step for each selected interval along the metre ruler.
Calculations and Analysis:
- Calculate Resistance:
- For each interval, calculate resistance using Resistance = Voltage / Current (using voltmeter and ammeter readings).
- Graph Results:
- Plot resistance values on a graph against the length of the wire.
- Draw a line of best fit to observe the relationship between resistance and length.
Activity 2: Observing Resistance Changes with Series and Parallel Resistors
Equipment Setup:
- Circuit Components: Using the diagram provided by the examining board, assemble the circuit with two resistors of equal value connected in series.
Procedure:
- Series Circuit:
- Turn on the power supply at 6 volts.
- Close the push switch and record readings from the voltmeter and ammeter.
- Switch off the power supply.
- Calculating Total Resistance:
- Use the readings to calculate the total resistance of the series circuit.
- Parallel Circuit:
- Rearrange the circuit so the two resistors are connected in parallel.
- Turn on the power supply at 6 volts.
- Close the push switch and record the readings from both meters.
- Calculating and Comparing Resistance:
- Calculate the total resistance of the parallel circuit.
- Compare the resistance values for the series and parallel circuits to understand the impact of different resistor arrangements.
Analysis:
- Assess how adding resistors in series and parallel affects the overall resistance in the circuit.
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.
To do this practical you will need
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