Investigating series and parallel circuits

Class practical

Introduces two ideas: current is shared between parallel paths in a circuit, and brightness of a lamp depends on current.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamp with holders, 5
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 8
• Variable resistor or rheostat e.g. 3 W 25 ohms

Health & Safety and Technical Notes

Read our standard health & safety guidance

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.

Procedure

1. Connect the ammeter between the lamps and vary the variable resistor. Move the ammeter so that it is between one lamp and a cell and repeat the experiment. Explain what you observe.

2. Set up circuits like those shown. Describe and explain the patterns you observe in the brightnesses of the lamps.

Teaching Notes

• Once students have an understanding of resistance, practice in using resistors in circuits and attempting to explain their results is good reinforcement.
• Step 1: The variable resistor is connected in parallel with one or other lamp. It allows part of the current to by-pass that lamp, so that the other lamp will be brighter.
• Step 2: Explanations of these circuits should be in terms of current-sharing when lamps are in parallel. When lamps are in series, they have the same current, so they are of equal brightness.
• By the end of this series of experiments, students should be confident in understanding series and parallel circuits which include both fixed and variable resistors.

This experiment was safety-tested in April 2006