V=IR
Electricity and Magnetism

Switching on a lamp

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

To measure the current through and potential difference across a lamp as it is switched on.

Apparatus and Materials

For each student group

  • 6 V battery
  • Switch
  • Wires
  • 6 V miniature lamp (pea lamp) in holder
  • Datalogging current sensor
  • Voltage sensor
  • If possible a light sensor should also be incorporated into the experiment. You would then also need a black cloth.

Health & Safety and Technical Notes

Read our standard health & safety guidance


The lamp switches on in about 0.1 s so you will need fairly fast data collection.

Procedure

  1. Connect a simple circuit for measuring the current through the lamp and potential difference across it. If you have a light sensor, place it next to the lamp and cover both with a black cloth.
  2. Set the datalogging equipment to record at least 100 measurements per second.
  3. Start the datalogger, close the switch, wait about 2 seconds and open the switch. Stop the datalogger.
  4. If possible, use the datalogger to calculate the power output (V x I) and resistance (V/I) of the lamp as it switches on.

Teaching Notes

There are a number of points to come out of this:

  • Sensible use of dataloggers - here to capture data which changes too fast for humans to record. Students may not think that lamps take any time to switch on but this experiment shows otherwise. They may have seen the new LED traffic lights which have been placed in parallel with normal lamps at some junctions - the LED lights clearly switch on faster.
  • The potential difference will increase instantly - this is controlled by the speed at which the switch is closed. The current will be high initially and then drop to a constant value. This shows the resistance of the lamp increasing as it warms up.
  • The resistance graph, if plotted, will tell this story more clearly. Be careful - the initial current, being zero, will give nonsense values for resistance before the switch is closed, the graph would need to be cut off so that it starts just after the current starts to rise.
  • The power output can be found and compared with the light level. The light output steadily increases (as the lamp gets hotter) but the power peaks with the current and then falls. This can be explained by energy transfers. Initially energy is being transferred to the lamp by an electric current. The filament of the lamp warms up and emits light (and infrared) radiation. Once the lamp reaches constant temperature, work done electrically balances the radiation transfer.
  • This experiment can be presented at a very low level, simply as an introduction to resistance (current drops as lamp heats up). Or it can be used right through to the highest level with power calculations. Data can be gathered in a few seconds. The experiment is readily repeated if there is a glitch.
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