Practical Activity for 14-16
An opportunity to measure the electric current and introduce the ampere unit. There is no need to define an ampere.
Apparatus and Materials
For each student group
- Cells, 1.5 V, with holders, 2
- Lamps with holders, 3
- Ammeter (0-1 amp), DC, preferably moving-coil
- Leads, 4 mm, 6
- Digital and analogue ammeters with varying ranges (optional)
- Digital multimeter with multiple current ranges (optional)
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.
- Set up a circuit in which a cell, a lamp and an ammeter are connected in series.
- To record what you observe, draw a circuit diagram. Beside the lamp, note its brightness. Beside the ammeter, note its reading.
- Set up a second circuit with two lamps connected in series with the cell and ammeter. Record your observations.
- Repeat this with the two lamps connected in parallel with each other (side-to-side).
- Repeat these observations using two cells in place of one.
- How does the reading on the ammeter relate to the brightness of the lamps?
- Investigate how the reading on the ammeter depends on its position in the circuit.
- With two lamps in series, less light is produced and the ammeter will show that the current is less. This is where
high powercells are needed. Otherwise the result is spoilt by internal resistance.
- When two lamps are connected in parallel, twice as much light will be produced (two lamps of equal brightness), and the ammeter will show that twice as much current is flowing.
- Note that the results of this experiment may not match up to an idealized view of current flow. There are two reasons for this:
- when two lamps are connected in series, the voltage across them is halved but the current is likely to be more than half the previous value (because the lamp's resistance is lower when it is cooler)
- the voltage provided by a cell is likely to be less when it is making a bigger current flow, as a consequence of the internal resistance of the cell.
- Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of ammeter. By demonstrating that there are ammeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.
- In some experiments, students will not get a single, fixed reading on their ammeter but will get a constantly fluctuating value. Deciding what the 'right' reading is provides an excellent opportunity to discuss:
- the relative merits of analogue and digital meters
- uncertainty in measurements
- how to select a meter with an appropriate range and sensitivity.
- Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.
This experiment was safety-checked in December 2006