Eelctricity_Concept_5
Taking control of electricity
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Subject facts
Why you need to know these facts.
Vocabulary Amazing facts
Common Misconceptions
Questions
Teaching Ideas
Subject Facts
Controlling circuits
Most circuits are simply 'on' or 'off': if the circuit is complete, the bulb (or other component) will work; if the circuit is broken, it won't. A switch is a means of simply completing or breaking a circuit without having to take the circuit apart and put it back together each time.The circuit shown in Figure 3 is in need of a switch, since the only way to turn it off without unscrewing the bulb holder or undoing pieces of tape would be to unscrew the bulb itself (which might be hot) or to cut the wire.
Simple switch
To make a very simple switch, cut the wire in a circuit and strip the ends. When you touch the bare wires together, the bulb lights up. All switches work on this basic principle: the rest is technology! You can buy switches or make them
Variable switch
A simple switch is either on or off. A variable switch, such as the volume control of a radio or TV, allows you to decide how much electricity you will allow to flow through the circuit. In classroom circuits, a variable switch takes the form of a length of high resistance material that will allow electricity through, but not very well. When placed in a circuit with a battery and bulb, it resists the flow of electricity in a variable way: the greater the length of resistor material the current has to flow through, the dimmer the bulb will be see Figure 3, Unless you actually break the circuit, electricity will continue to flow, even if the bulb is so dim that it does not seem lit at all.
Bulb
If you study a small bulb carefully, you will see a very thin piece of wire (the filament) held between two upright prongs.This filament heats up and then glows brightly when electricity passes through it (due to its resistance). The greater the voltage, the brighter the bulb glows, until it burns through and breaks the circuit. This form of electrical light source, the filament lamp, is not the only one that children may be familiar with. In many light sources, electricity is passed through a gas-filled tube to make the gas glow. Light sources of this kind need less electricity to produce the same brightness, but they cost more to make in the first place. Examples are fluorescent tubes (often found in shops and classrooms), street lamps and halogen lights.
Electric motor
When electricity passes along a wire, a magnetic field is generated around the wire.This field can be made stronger either by increasing the amount of electricity passing along the wire or by coiling the wire up. A coiled wire must be coated with an insulator to stop the electricity taking a short cut by jumping from strand to strand where they touch. When two magnets are brought next to each other, they will either attract or repel depending on whether the poles are similar or opposite. If a coil with electricity flowing through it is brought close to a magnet, it will either be repelled or attracted depending on its orientation relative to the magnet.
These forces of attraction and repulsion make a motor work (see Figure 4). Electricity flows through the coil, causing a magnetic field to form. When a ceramic magnet is brought near, the coil will either push away or pull itself nearer. As the other side of the coil comes around, that will do the opposite — so the coil will pull up, push away, pull again... causing it to spin.This is the principle of the electric motors in such devices as food mixers, vacuum cleaners and milk floats.
The process can be reversed: if, instead of using electricity to make a coil move in a magnetic field, you set up a coil in a magnetic field and then turn it, an electrical current will be induced in the coil. This system is used in all electricity-generating devices, from bicycle dynamos (if you turn the pedals, the lights come on) to power stations.
If you connect a small electric motor into a circuit, it will spin in one direction. If you connect the wires on the motor terminals the other way round, the motor will spin in the other direction. Videos providing further instructions for homopolar motors can be found on websites such as Youtube@.
Buzzer
Most buzzers contain a device called a diode which acts as a 'one-way valve' for the electricity. If you connect it to the battery one way round, it will work; the other way round, it won't.
Light emitting diode (LED)
All of the little coloured lights on a computer, hi-fi or TV are LEDs. They are cheap, have no moving parts and use very little electricity. However, they can get hot, so be careful not to touch them when they are lit.
Circuit symbols
Figure 5 shows some conventional symbols used for components in circuit diagrams.
Why you need to know these facts
Once an electrical circuit is operating and the current is flowing, the next step is to control the flow using switches. There are a wide range of electrical components which, when connected into a circuit, can do different things. when building circuits, it is important to know the purpose of each component.
Vocabulary
Bulb (lamp) — a device which, when connected into a circuit, will resist the flow of electricity and heat up, producing light. Motor (electrical) — a device for changing electricity into movement through electromagnetic induction.
Switch — a device for controlling the flow of electricity in a circuit.
Amazing Facts
Thomas Edison in the USA and Joseph Swan in the UK, working independently of each other, both produced the first electric filament light bulb (that worked for any length of time) in 1 879. Modern 'energy-saving' fluorescent lights are based on work by George Stokes (1852).
Teaching Ideas
Circuit progression (testing and observing)
Take the children through the making of a simple circuit, starting with just a battery and a bulb between each pair.They should draw pictures of the connections as they go along. By the end of a halfday session, they should be using battery and bulb holders with secure knowledge of how to complete a circuit by connecting the terminals of batteries and bulbs. Introduce manufactured switches for inclusion in circuits. Only let the children start to use buzzers just before the end of the session — I think you know why!
Make it (design and technology)
Once the children can make simple circuits with switches, challenge them to apply their circuits in practical contexts. For example: light a room in a doll's house; make a door buzzer; make a lighthouse.
Control it I (design and technology, problem solving)
Construct a range of different switches (see Figure 6) and ask the children what control situation each would be useful for. Test their ideas together.
Building to plan (circuit diagrams)
Introduce circuit symbols (see Figure 5). Present the children with circuit diagrams to copy, then ask them to make the circuits. Ask the children to construct circuits and then draw diagrams of them — can other children reproduce the circuits from the diagrams? Give them incorrect circuit diagrams: can they spot the errors? Do they have to build the circuits first?
Control it 2 (design and technology, problem solving)
The children can use a variable resistor in series with a bulb or motor to control brightness or speed. Ask them to suggest how this could be used in a model.
Electromagnet making (construction, investigation)
The children can take a length of wire and coil it around a large nail (starting at one end), then connect the ends of the wire to a battery and see what happens when they take the nail close to a pile of paper clips. Ask: How could you make the electromagnet stronger? (More coils of wire, a higher-voltage battery.) How could you get more coils of wire around the nail? (Use thinner, coated wire.)
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