Problem circuit

Class practical

An extra wire can shed light on the problem.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Lamps with holders, 2
• Leads, 4 mm, 6

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Experiments on shorting cells should be restricted to used cells which are not quite flat. The old ammonium chloride cells would polarize under short-circuit conditions and the current would then drop so that overheating and explosion did not occur.

Read our standard health & safety guidance

Procedure

1. Set up the circuit shown. Why do the lamps not light?

2. What will happen if an additional link is added as shown below?

Teaching Notes

• Two lamps are set up with two cells whose polarities face opposite ways: the lamp does not light. The idea that opposing cells cancel each other out is clear to most students.
• The second part of this experiment is designed to make students think about individual loops within a circuit. Students should try to predict the outcome before trying out the circuit in practice. The effect of adding a connection from a position between the cells to a point between the lamps, and so dividing the circuit into two circuits, is quite startling.
• Other effects of shorting out other components can be discussed.

This experiment was safety-tested in April 2006