Electricity_Concept_4
Electrical Circuits
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Subject facts
Why you need to know these facts.
Vocabulary Amazing facts
Common Misconceptions
Questions
Teaching Ideas
Subject Facts
Matching Components
Before constructing an electrical circuit, you need the right components. It is important to use batteries and bulbs that, as far as possible, have similar voltage ratings. The 1.5V battery (either an AA or a C type) and 1.5V bulb are a good starting point. The voltages of batteries are quite easy to read, but those of bulbs are more difficult. On the metal casing just below the glass bubble, you should be able to read the voltage (this may require a magnifying glass). The metal terminals on the battery allow you to light up the bulb without using any connecting wires. If you try using a 1.5V battery with a 3.5V bulb, you will find that the bulb lights very dimly. Using two 1.5V batteries in a battery box' with a 1.5V bulb will make the bulb very bright for
a very short period of time, until the filament gets too hot and burns out.You can think of voltage as 'electrical push': the higher the voltage, the greater the push. A I .5V bulb needs a I .5V 'push' from a battery to make it work properly. If you push it too hard, it will break. If you don't push it hard enough, it won't have enough energy to light up well.
Batteries and bulbs
If you start with just a battery (1.5V) and a bulb (1.5V) and a single piece of connecting wire, you can see all of the important connections that need to be made for a larger, more complex circuit to work. Identify the two terminals at either end of the battery. On a screw-fitting bulb, the terrninals are the pimple on the bottom and the outer screw casing itself.To make the bulb light, each of the battery terminals must make contact with a bulb terminal (see Figure l).
This is not the most convenient way to hold a circuit together, so the connection can be maintained with wires. Cut each wire to length, remove the outer covering at the ends using wire strippers, and twist the strands together for safe and efficient use. Now use the wires to connect the terminals of the battery and the bulb (see Figure 2a).This is still fiddly; so place the bulb in a bulb holder, connect the wires to the terminals, and use adhesive tape to fix the wires onto the battery (see Figure 2b).This is an effective circuit.
Circuit models
Explanatory models for the flow of electricity need to be clear, well-focused and within the experience of the pupils.
Two models are commonly used for teaching about electricity:
In the 'Smartie@' model, children (acting as 'electrons') are given Smarties@ by the teacher.They eat them, climb over a chair and return to the teacher.This expresses the idea that batteries provide energy (the Smarties@) for the electrons to do work within a circuit. But it leaves unanswered questions such as 'How does the electron know that there is only one chair in the circuit and it won't have to save half a Smartie@ for later on?'
The 'skipping rope' model, though more simplistic, is more versatile: four sticks (pencils are fine) are held up in a square (a metre along each side is ideal), with a rope tied around them.A child pushes the rope around the circuit, and another child grips it lightly to resist the flow (being careful to avoid friction burns). It doesn't matter which way the 'battery' is pushing: the hands of the 'bulb' will still get warmer. If two batteries' are pushing in the same direction, the 'bulb' gets warm faster. If two 'batteries' push against each other, the pushes will cancel out (unless one pushes harder than the other). If you have one 'battery' and two 'bulbs', the hands won't get as warm. If you have two batteries and two bulbs, it will be the same as having one battery and one bulb.
This model is useful for helping children to develop an understanding and visual image of what happens in a circuit. It holds up fairly well in practice; but as with all models, you should recognise its limitations and not take it too literally.
Why you need to know these facts
Trying to make electrical circuits that work can be very frustrating if your knowledge of the components and how they connect together is deficient. Once a few simple, practical points have been understood, relevant practical work can be approached with confidence.
Vocabulary
Circuit - a complete path between the two terminals of a battery, made with materials that conduct electricity.
Common Misconceptions
Different-coloured wires have different properties
Wires are covered in different-coloured plastic sheaths to make each one easier to identify in a tangle of wires. However, this is not always practical: in a circuit where bulbs are connected in parallel, trying to use a different-coloured wire for each bulb might require too many colours. In practical terms, it is the metal inside the plastic that is the important factor when making connections. This is particularly important when distinguishing between the live (brown), neutral (blue) and earth (green and yellow) wires used in household appliances.
Some devices (such as buzzers) contain diodes that only work when connected to the battery one way round. In these it is conventional to attach a red wire to the positive terminal.
A bigger battery has more electricity in it and will make a bulb shine brighter.
The battery's voltage affects teh brightness of the bulb, so a stronger battery will lead to a brighter bulb. However, 1.5V cylindrical batteries come in four different sizes! The idea that a bigger battery contains more electricity (because it contains more chemicals) is not necessarily true: it depends on which chemicals, and the quality of them.This is why each size of commercial battery is available in a confusing range of grades: 'long lasting', 'extra power' and so on.
You need a wire from each end of the battery to light a bulb, because there is not enough electricity in one end.
The first part of the statement is right, but the reason is wrong. It often pays to ask Why? in order to assess a child's understanding. If the reason stated above were correct, you should be able to connect one end of a battery and the other end of another battery to a bulb and get it to light, without connecting the other ends of the batteries. Of course, this will not work: it is a complete circuit that is needed.
Electricity comes out of both ends of the battery and runs together in the bulb.
The 'clashing currents' theory, as this is called, is quite interesting. It contains the idea that a complete circuit is needed, involving both terminals of the battery, as well as the idea that energy is coming from the battery. It shows that the child is attempting to bring together observations, Like all theories that are based on the available evidence, it needs to be tested against further evidence.
A diode, or 'one-way valve', will show that the flow of electricity has a single direction. Unfortunately, introducing either a buzzer or an LED into a circuit in series with a bulb will mean that the LED or buzzer will work, but the bulb won't: the current will be too low to make it light up. Even so, the 'clashing currents' theory suggests that the LED or buzzer should work either way round. I have known one child to argue that an LED only works one way around because different types of electricity come out of each terminal — that's why they clash! This theory was disproved by using a terminal from each of two different batteries and observing that there was no complete circuit between them,
Teaching Ideas
Live and dead bulbs (testing and sorting)
Give the children a box of 1.5V bulbs and a 1.5V battery, and ask them to check and sort the bulbs. (This always used to save me a lot of time when a class returned the 'electricity box' to the science cupboard!) Extend this by asking the children to use two 1..5V batteries (in a battery box) to test and sort a range of bulbs: I .5V, 2.5V, 3.5V and 6V. Tell them that they don't need to test each bulb for long: a quick flash is enough. With a bit of practice, they should be able to distinguish quickly between I .5V (very bright), 2.5V (bright), 3.5V (less bright) and 6V (dim). Note that a I .5V bulb will 'blow' if it is left connected to a 3V supply for any length of time.
Batteries and bulbs (observing and sorting)
Make a collection of different batteries and bulbs (avoid using 'button' batteries of the wristwatch type with younger children, as they can easily be swallowed).The children should identify the positive and negative battery terminals, then sort the batteries by voltage, by shape and by size. Next, they should identify the terminals on different bulbs and note similarities and differences in size, shape, voltage and filament.They should match batteries to bulbs where possible. NB Make sure they are aware of the dangers of handling delicate glassware.
How does your circuit flow? (modelling and explaining)
Demonstrate the skipping rope model (see page 23) to help the children understand the flow of electricity. Invite comments on any problems with the model.
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