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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).

The variables in this experiment are:

• 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.

The control variables are kept the same by:

• not changing the wire during the experiment (material, length and cross section area of the wire stay the same);
• keeping the current small (temperature of the wire does not change);
• opening the switching between readings (temperature of the wire does not change).

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

Equation

or:

Prediction

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

Justification for the prediction

The greater the length of wire the greater the opposition to the flow of current.

This will result in greater resistance.

Safety

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.

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