Investigating the resistance of a metallic conductor

A guide to carrying out a practical to investigate resistance

To investigate experimentally how the resistance of a metallic conductor at constant temperature depends on length and obtain sufficient values to plot a graph of resistance (y-axis) and length (x-axis).

Variables

The main variables in a science experiment are the independent variable, the dependent variable and the control variables.

The Independent Variable is what we change or control in the experiment.

The Dependent Variable is what we are testing and will be measured in the experiment.

The Control Variables are what we keep the same during the experiment to make sure it’s a fair test.

In this experiment the:

  • Independent Variable is the length of wire.
  • Dependent Variable is the resistance of the wire.
  • Control Variables are the material, the cross section area and the temperature of the wire. These are kept the same by not changing the wire during the experiment, by keeping the current small and opening the switching between readings.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the length of wire.

Equation

Resistance R = \frac{voltage~V}{current~I}

Prediction

As the length of wire increases, the resistance will increase.

Justification for the prediction

The greater the length of wire the greater the number of collisions between the free electrons and metal ions.

This will result in greater resistance.

Safety

HazardConsequenceControl measures
WaterElectric shockDo not set up the experiment near taps, sinks etc.
Wire gets hotMinor burnsDo not handle the wire. Switch off between readings.

Apparatus

1m length of constantan wire, a metre rule, a low voltage power pack, a voltmeter, an ammeter, connecting leads, a switch, 2 crocodile clips, Sellotape.

1m length of constantan wire, a metre rule, a low voltage power pack, a voltmeter, an ammeter, connecting leads, a switch, 2 crocodile clips, Sellotape.

Method

  1. Set up the circuit, as shown above. Attach the flying lead at the 20 cm mark so that the length of wire the current flows through is 20 cm. Record this length in a suitable table.
  2. Adjust the power pack until the current on the Ammeter is 0.4 A. Record the current in the table.
  3. Read the corresponding value of voltage across the wire on the voltmeter and record in the table.
  4. Switch the switch off to prevent the temperature of the wire rising.
  5. Switch on again and repeat the reading of voltage. Record in the table. Switch off and calculate the average voltage.
  6. Calculate the resistance of this length of wire and record in the table.
  7. Switch on again. Ensure that the current is still 0.4 A and repeat current and voltage reading for lengths of 40 cm, 50 cm 60 cm 80 cm and 100 cm.
  8. Calculate the resistance for each length, remembering to switch off between each reading.

Error

The temperature of the wire must be kept constant.

Whenever a current flows through a conductor there is a heating effect.

Electrical energy is converted to heat energy.

To ensure the temperature of the wire does not increase, switch off between readings and keep the current as low as possible.

Read the ammeter and voltmeter accurately by reading the scale from directly above the pointer or use digital instruments.

Results

Length l in cmCurrent I in AReading 1 (Voltage V)Reading 2 (Voltage V)Average (Voltage V)Resistance R in Ω
20
40
50
60
80
100