ELECTRICITY SCIENCE OR DESIGN AND TECHNOLOGY?
Any electrical activities undertaken in the classroom beg this question. Our unit on electricity will tackle electricity from a design and technology stance, but what is this? Basically the content required for electrical activities in the primary classroom is specified in the N.C. orders for science. This includes a knowledge of: Everyday electrical appliances, simple circuits, use of switches to control electrical devices, varying the current in a circuit, simple circuit diagrams. Thus in this instance science provides the knowledge base for design and technology activities. Design and technology is not so much about knowing something but knowing how to apply this knowledge to meet human needs. It proceeds through a process of recognising those needs, the generation of a design, planning, making and evaluating as specified in the N.C. orders for Technology. This unit will provide the knowledge of basic circuitry and an understanding of how to apply this knowledge in meeting human needs.
The main resources likely to be needed for basic electrical work are: 4mm plugs for power supply aluminium foil batteries battery holders battery snaps battery tester bell bulb holder clip on bulb holders bulbs flashing bulbs 2.5V bulbs 3.5V buzzers low voltage card crocodile clips crocodile clip leads drawing pins electro magnet motors 1.5-4V low torque mounting clips paper clips paper fasteners and washers pliers snipe nose power supply screwdrivers electrical sellotape switches, push to make wire wire strippers BATTERY HOLDERS Manufactured battery holders come in a variety of systems. (See the datafile card on batteries.) However home made battery holders can be quite adequate. 35mm film containers are useful for this. Electrical contact can be made via paper fasteners fixed through the lid and the base. BATTERY SNAPS Usually sold in packs of 10. Care needs to be taken when stripping the wire, as if they are continuously stripped you cannot attach them to the component. Beware of leaving these attached to the cell holder is storage as the leads may touch and short circuit. This can be prevented by removing the battery snap or physically stopping the wires from touching. This can be achieved by cutting one lead much shorter, or by using a card separator or similar.
ELECTRICAL WIRE This is usually sold on spools. A variety of colours are usually available and it is helpful to purchase different colours to enable clear identification of different circuits or positive and negative poles etc. However it is worth considering alternative supplies e.g. a parent who is an electrician, local businesses etc. The main types available are multi stranded or single stranded. Multi stranded flex tends to be more durable and can be prevented from fraying by twisting the strands together after stripping. CONNECTIONS Good connections are essential in electrical work if the children are not to become frustrated. For simple ideas to make electrical connections see the relevant datafile cards (Connecting wires and Connecting batteries and bulbs). BULBS The main ones which may be needed are - 2.5V, 3.5V and 6V. Ensure that the bulbs being used are compatible with the supply of electricity e.g. a 6V bulb used with a 1.5V battery will not appear to be working. It will help if the different rated bulbs are sorted into different containers. Flashing bulbs can now be bought which, when wired up, will flash on and off. BULB HOLDERS MES bulb holders – some difficulty may be encountered, as the screw connections can be easily lost if the children unscrew them too much. This can be prevented by attaching wires or paperclips for the children before use. Mini bulb holders – useful in the construction of models as they do not take up a lot of space. MES clip on bulb holder – These bulb holders will fasten on to the end of lolly pop sticks and are so easily attached to models. These are also very useful if there is a need for a number of lights. SWITCHES There are many types of manufactured switches available (see the datafile card on components). Toggle switches – these come apart to show how the switch system works. Knife switches – these work by moving the lever to one of two positions. Push switches come in two types: Push to make switches – the button must be depressed for the circuit to work. Push to break switches – these switches make a complete circuit unless pressed, then the circuit is broken.
Reed switches – these switches are operated with a magnet. Micro switches – very small switches, which can be wired either to switch on a circuit or to break it. Pressure pads – these are pressure sensitive switches. Reversing switches – these allow a motor to be operated in forward and reverse. Double pole double throw (DPDT) switches – these have six connection points and can be used to operate two separate components or as a reversing switch. Sensors – these are usually electronic switches, which respond to different stimuli eg.light, moisture, pressure or heat. Relay switches – these are electronically operated switches which allow a motor or similar to be switched by use of a small applied current. BUZZERS These can be magnetic or piezo electric, however they must be connected correctly in the circuit to operate. The red connecting wire must be connected to the positive (+) end of the battery. MOTORS These can be obtained fairly cheaply and will add movement to a model. The most suitable type will operate on 1.5V or 3V however it will be necessary to gear the motor down in some way to gain sufficient torque (turning effect). For further information on this see the activity sheets section on Under Control.
RESOURCE MANAGEMENT With any electrical work there are always many small components, which will need storage. This being the case a good supply of small containers such as 35mm film pots, plastic margarine tubs and yoghurts pots will be needed. It is advisable to give each working group a complete set of equipment in a plastic tray or similar. This will enable the children to gain easy access to the equipment without undue movement around the classroom. Lengths of wire will need wrapping around something or coiling separately to prevent them from becoming entangled. The numerous small components necessary can be stored long term in cut down photocopier boxes, plastic gallon containers or margarine tubs.
The equipment, which the children use, will also need checking regularly to ensure that batteries and bulbs are operable. There are various way in which this can be done: 1) By the teacher before a session 2) By an ancillary helper or parent 3) By a pupil monitor 4) By the children themselves The last two options should save work for hard pressed staff and could have positive educational benefits for all the children. If these are the preferred options it will be necessary to have a test circuit set up which the children can use to test individual components. The children will also need to be made familiar with the operation of this.
Faulty components need disposing of before they become mixed up with the good stock. Nothing is more frustrating than continually finding that things do not work.
A good stock control system will also be needed. This will help to ensure an adequate supply of electrical equipment before work starts and that depleted resources are replaced as and when necessary. Again it would help if pupils could administer this system. Rechargeable cells although expensive at the outset could save money in the long run, however the safety aspect of their use needs careful consideration (see safety section). If this type of cell is adopted there will need to be some system for recharging the cells (Overseen by an adult).
BATTERY INFORMATION BATTERY SAFETY GUIDELINES Batteries are a source of safe and portable power. If batteries are misused or abused problems may occur. ALWAYS 1) 1 Observe the + and – signs marked on the battery and the appliance. (Incorrect fitting can cause leakage, or in extreme cases fire or explosion). 2) 2 Replace the whole set of batteries at one time. Do not mix old batteries with new ones. Do not mix different types of batteries together. 3) 3 Store new batteries in their packaging. Never store batteries in a metal container or near metal objects. This can cause short circuits, leakage, fire or even explosion. 4) 4 remove dead and partly used batteries from equipment that you know you are not going to use for a long time. The batteries may leak and cause damage. 5) Supervise children if they are replacing batteries themselves. Ensure that children know the safety guidelines when using batteries. 6) Remember that the button cell batteries that are used in hearing aids, toys and other equipment can be easily swallowed. 7) Make sure that battery compartments are secure. 8) Seek medical advice if a battery is swallowed. NEVER 1. Attempt to recharge ordinary batteries either by a charger or applying heat to them. (rechargeable batteries should be marked clearly). 2. Never dispose of batteries in a fire.
Pupils may come into contact with electricity in two modes:- mains electricity and low voltage equipment. MAINS EQUIPMENT Pupils and teachers should not bring in their own mains powered electrical equipment to school. It is wise, if not essential, that all electrical equipment is checked annually. Some local education authorities arrange this automatically. If they do not, the necessary arrangements should be made with the LEA. Equipment such as power supplies, electric kettles, boiling rings, cookers, fridges, computers, lighting and heating equipment for aquaria and other uses should be safe to use if bought from a reliable supplier. In General pupils will not need to handle mains electrical equipment at school. Teach children NEVER to experiment with mains equipment and warn them of the dangers in the home. Do not use mains equipment, which has damaged plugs or frayed leads. These are dangerous and the commonest source of danger with electrical equipment. Wherever possible safety plugs having shrouded terminals should be used. Use only approved plugs. Teach children to switch off the mains at the wall socket before attempting to unplug mains electrical equipment. Earth leakage breakers should be used if children are to use electrical equipment. These earth leakage breakers are usually called power breakers. A useful address for supply of the Power Breaker: B & R Electrical PLC Templefields Harrow Essex CM20 2BG Telephone 0279 – 434561 Power Breakers are added protection against the risk of electrocution. Power breakers respond to residual current which if left undetected could create a fire. They are suitable for fuse with electrical household appliances and power tools e.g. food mixers. They are also useful with kettles and irons where there is water present. An RCD (residual current detector) is not a substitute for normal precautionary measures. However there may be instances where the use of mains equipment is useful e.g. low voltage power supplies. These are a convenient way of replacing batteries if a lot of
time is spent investigating electricity. Children should be taught and shown how to use these safely. The Irwin power supply has a stabilised output. The outputs can be from 3 – 12V. Electronic overload protection prevents the supply from damage in the event of a short circuit.
PROCEDURE WHEN USING AN IRWIN POWER SUPPLY UNIT Always ensure that the mains is switched OFF when connecting the power supply to it. Connect the circuit to the positive and negative terminal. Select the voltage required by adjusting the rotary switch. Switch on the power supply. If you need to change the voltage setting TURN OFF THE MAINS SUPPLY and disconnect the leads. This avoids brief voltage spikes being applied to the circuit during switching transition. The supply will operate continuously at any of the voltage settings, supplying a current of up to 2A. If more than 2A is drawn, the output voltage will start to fall, this will not happen immediately. If this occurs, turn off the supply and reduce the load. If the supply is too heavily loaded for an extended time (for example if there is a short circuited for a minute or two) it may be necessary to leave it disconnected for about ten minutes, before correct operation can resume. For maintenance return to the supplier. IRWIN – DESMAN LTD 294 Purley Way CROYDON 081 686 6441 Mains voltage exists inside the unit. Repair should only be carried out by suitably qualified personnel. Power supplies as used for model trains and cars are not recommended for use in electricity experiments. LOW VOLTAGE SOURCES Sources of low voltage for experiments and for powering models include dry batteries, rechargeable batteries and low-voltage power supplies. DRY BATTERIES These types of batteries are alkaline or zinc carbon or zinc chloride. Children should not cut open dry batteries since their contents are poisonous and can be corrosive. Discard old batteries as they often leak. Never discard them in a fire. These are the safest source of electricity. The running costs however can be expensive. It is not possible to receive an electric shock through the skin from a 1.5-volt battery unless
they are joined together. This type of battery CANNOT be recharged and no attempt should be made to do so. Never put them in an oven in mistaken effort to rejuvenate them. Tiny batteries such as those used in watches are not suitable. Batteries should always be inserted the correct way round noting the position of the + and – terminals. Battery cells of different types should never be mixed together in a single application. New batteries should never be mixed with old or partly discharged batteries. Store batteries in such a way that their terminals cannot touch and short circuit. RECHARGEABLE BATTERIES Great care needs to be taken with these since they can get very hot if allowed to short circuit and can melt the wires and start a fire. If they are allowed to discharge too quickly, for instance through a short circuit, the wires and the batteries can get very hot. This can damage the plastic battery box as well as giving children an unpleasant surprise. A member of staff should always supervise their recharging using a charger designed for this kind of battery. High capacity rechargeable batteries are best avoided. Car batteries and accumulators should NOT be used.
A SIMPLE GLOSSARY OF TERMS
ANODE the positive electrode. AMPERE the basic unit for measuring electric current. BATTERY usually the arrangement of two or more cells but often used to describe a single cell. CAPACITY the quantity of electricity which a battery can deliver. CATHODE the negative electrode CELL the basic unit for the conversion of chemical energy into electrical energy. CIRCUIT a complete pathway of conducting material. CONDUCTOR a material which allows the passage of an electric current. CURRENT the flow of electricity in a circuit. CURRENT DRAIN the current supplied by the battery when in use. DISCHARGE the conversion of chemical energy of the battery into electrical energy. DISCHARGE RATE the speed at which available capacity is taken from the cell or battery. INSULATOR a material which prevents the passage of an electric current. LOGIC GATES these are usually the part of an electronic circuit which processes the input signal befor operating an output. ( see the datafile card on logic gates). PARALLEL CIRCUIT an electrical circuit connected in such a way that some components lie on alternative pathways. A parallel circuit has junctions whereas a series circuit does not. PRIMARY CELL one which cannot be recharged. SECONDARY CELL one which can usually be recharged after being discharged. SERIES CIRCUIT a circuit containing several components, which lie along the same pathway. SERVICE LIFE the period of useful service of a primary battery before its on-load voltage falls to a specified end point voltage. SHORT CIRCUIT a faulty circuit which has a complete pathway of conducting material containing no other electrical components. This will very quickly deplete a
battery or cause damage to a mains supply if left connected inadvertently. FIRE RISK INVOLVED. VOLTAGE a measure of the electrical force which pushes electric current around a circuit.
USEFUL ADDRESSES FOR ELECTRICITY RESOURCES
Understanding Electricity The Electricity Council 30 Millbank London SW1P 4RD 071 834 23 MEB Headquarters Mucklow Hill Halesowen West Midlands B62 8BP 021 423 2345 Ever Ready Education Services Ever Ready Limited Ever Ready House Burleigh Garden Southgate London N14 5AQ Contact public relations department further information about resources school visits etc. Mrs Judy Camp is the Community affairs assistant and will advise you further. Catalogue available of teaching Resources (many free)
Free resources available include “Making Battery Power work for your pupils” and use ful leaflets about batteries.
Technology Teaching Systems Ltd Supplier of resources for science and
Unit 4 Holmewood Fields Business Park Park Road Holmewood Chesterfield technology. Well illustrated catalogue available.
S42 5UY 0246 850085 Duracell
Buracell House Church Road Lowfield Heath Crawley West Sussex RH11 0PQ 0293 517527 Various leaflets and catalogues on batteries available.
Lynn Lane Shenstone Lichfield STAFFS WS14 0EE 0543 480077
Suppliers of equipmejnt for science and technology. Comprehensive catalogue available.