4.Main Content
experiments
Simple electric circuits
Class experiment
This series of experiments enables students to gain confidence in setting up their own circuits.
Refer to work done in primary school science. If students have done this experiment before, use it to allow them to show what they already know.
Apparatus and materials
For each student group:
- Cells, 1.5 V, with holders, 3
- Lamps with holders, 3
- Leads, 4 mm, 6
Technical notes
Alternatively, you could use circuit boards.
Safety
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.
Read our standard health & safety guidance
Procedure
a Start with 1 cell, 2 lamps with holders, 6 leads. Additional cells, lamps, and holders should be available - they will be needed later in the experiment.
b Connect a cell to a lamp to make it light.
c Try the cell the other way round.
d Try the circuit with a different shape.
e Try the cell with two lamps.
f Repeat everything done already with one lamp but using an extra cell. Try the cells facing the same way, and facing opposite ways (as shown below).
g Repeat everything with two lamps and two cells. The illustration below shows two cells connected in opposite senses so that their voltages (if equal) will cancel out.
h Try again with two lamps and three cells.
i Try again with three lamps and two cells.
j Try again with three lamps and three cells.
k Summarize your findings.
Teaching notes
1 Points which should emerge are:
• the need for a continuous circuit to make the lamp light
• the shape of the circuit does not matter if there is only one single loop (this is important if Worcester circuit boards are used)
• with more than one cell then the relative directions of the cells matter
• the lamps in a series circuit are all the same brightness
• more lamps in a series circuit means dimmer lamps
• the same number of cells and lamps in a series circuit means that each lamp is at normal brightness
• one cell with two lamps connected in parallel with it shows that both lamps have the same brightness (if 'high power, or low internal resistance cells are used)
Two lamps in series with a cell will run for longer than two similar lamps in parallel with a cell, though students may not discover this during one lesson.
2 Students should be encouraged to find things out for themselves. You will need to make suggestions but they should be in words and as circuit diagrams.
3 Students can connect and disconnect their circuits simply by disconnecting one of the cell leads. Alternatively, simple toggle switches and/or bell pushes can be provided.
4 Should the students complain that they cannot see the light from the lamp, they should shield the lamp with the hand and look very carefully.
5 There are many simple experiments which students can design and carry out on their own, so behaving as a 'scientist for the day'. The work in the laboratory is in the nature of constructive play with a large share of the planning done by the students themselves. If the teacher draws circuits and the student follows them obediently then the work will proceed quickly and efficiently but students will miss a lot of the fun of doing their own experiments and will have built up very little lasting knowledge, which comes from making that knowledge their own by struggling to find solutions to their problems.
It is possible to allow students to proceed at their own pace getting their equipment from a cafeteria of components and every now and again using a class discussion for students to show what they have achieved. Of course encouragement and sensitive questioning is needed to bring out the best in pupils. Provide them with a route map to show what they have achieved and where they are going. Students need to think why they need extra components and not just randomly fill the board with lighted lamps.