current-voltage relationships

In this experiment the resistance of a lamp, resistor or diode is measured as the current is varied. Voltage and current sensors make the measurements while the software plots the results in real-time. After the experiment, the resistance and the power can be plotted against the potential difference.

What you need

Rheostat, dry-cells, lamp, Si / Ge diodes, resistors (for example 18 and 36 ohm), interface, voltage and current sensors.

Connect up the circuit. Connect the current sensor to the first socket on the interface and the voltage sensor to the second socket. If the sensors are adjustable, set a 1A range on the current sensor and a 2V range on the voltage sensor.
Run your sensing software. You may need to do a short test run to establish a suitable voltage for the power supply. Your system may recognise the sensors automatically, otherwise you will need to set them up yourself.
Note that you will not be recording against time as you usually do, just one variable against another. Get the software to plot the current against potential difference, as the experiment proceeds.
Start recording. Move the rheostat slider to change the current in small steps. See that the software plots readings as you do this. If you cannot see any points, your readings may simply be out of range.
You might then plot: the resistance (V / I) against potential difference or the power (V x I) against potential difference.


  • What does the graph tell you about the change in current?
  • What does the graph tell you about the change in potential difference?
  • What is the relationship between current and potential difference?
  • Use the software to calculate the resistance (V / I). Plot this resistance against the potential difference.
  • Use the software to calculate power (V x I). Plot this against potential difference.

Teacher question

Your software can plot the current and potential difference against time. Is this better than the approach suggested above?

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