Working with simple electrical components
Teaching Guidance for 14-16
It is often up to a teacher and a particular class to decide what equipment to use to introduce electric circuits. There are two general types of equipment used in schools for experimenting with electric circuits:
- Circuit boards (such as the Worcester Circuit Board) are designed with simple components so that the shape of the circuit which is constructed looks like a circuit diagram. This helps students to work from a circuit diagram or draw one themselves as a record of the work they have done. Some teachers find circuit boards can confuse less able students - they don’t realize that parts of the board without anything connected are not part of the circuit. Circuit boards have an advantage in that the connection of the cells in parallel is discouraged.
- Separate components connected by wires. This can be a cheaper solution, but it can also produce a tangle of wires so that the circuit becomes confusing.
Give students simple instructions on how to use the kit. As work progresses, make simple testing devices available, to test whether a cell is flat, a lamp is broken, or a lead not providing a good connection. These are easy to assemble with the item to be tested being the missing component in a simple series circuit consisting of lamp, cell and connecting wires. Learning how to trouble-shoot a circuit probably teaches more than circuits which give the predicted result the first time.
Good maintenance is essential
Time spent in checking the equipment before a lesson will pay dividends in the students’ understanding.
Some agreement must be established within the class so that the brightness of one lamp used with one cell is ‘normal’ brightness. In more complex circuits the brightness of the lamps can then be compared to this standard.
For this to be clear, students need to be given cells which have the same voltage (checked when they are driving a current through a lamp and not on open circuit), and all the lamps in a student’s collection need to produce the same brightness with the same cell. This is quick to do if three cells are connected in series to three rows, each consisting of three lamps, so that all lamps glow with normal brightness. If possible, new cells should be used at the beginning of each year and the old cells used up doing other jobs. The quality control, during production, on simple lamps is not good and even new lamps from the same packet can vary widely.
The difference in brightness of the lamps might be difficult to see in bright sunlight or with laboratory lighting and so the laboratory should be dimmed a little.
What type of cell is best?
The cost of cells has led some teachers to try rechargeable cells, which have their own problems. They have low internal resistance, so, if shorted, allow a large current. And they need to be completely flat before they are recharged. Cheap zinc-chloride cells are best for elementary work. Alkaline-manganese cells may be used where shorted cells are unlikely.
Some teachers even use power supplies. However, power supplies suffer from their internal resistance, just like cells. They may give unexpected, but entirely correct, results when the simple story about electric circuits is being told and internal resistance is being neglected. In order to avoid running down some of the cells and not others during experimenting, students should be issued with switches, or asked to disconnect the circuit when they are doing other things.
The language may vary in different teaching programmes with an insistence on cell for the simple 1.5 volt (approximately) simple cell and battery being reserved for several cells in series. Bulb may also be used instead of lamp.