Magnetism and Electricity Diodes


Many everyday appliances run on d.c. These appliances can convert a.c. from the mains into d.c. using diodes. Diodes are components that allow current to flow in one direction only.


z When the positive end of the diode is connected to the positive end of the battery, it is said to be forward biased. This allows current to flow.


z If the negative end of the diode is connected to the positive end of the battery, this is known as reverse biased and no current flows.


Because a.c. is constantly changing direction, current only flows when it is forward biased. Mobile phone chargers, for example, contain many diodes that convert the mains electricity into d.c.


A light-emitting diode (LED) is a diode that emits light when current flows through it. LEDs come in a range of colours and can be used to show if devices are on or off. LEDs require a very small current. They must be connected in series with a resistor to stop damage by too large a current.


LEDs lighting the future


Research LED technology under the following headings:


1. Advantages of LEDs over filament bulbs. 2. Advances in LED lighting. 3. LED televisions. 4. Future uses of LEDs.


Electrical Safety


The live wire in a house circuit is high in electrical energy and the neutral wire is low.


If a fault develops that exposes the live wire, it can cause a nasty electric shock.


The fuse wire is a safety measure that prevents this from happening. A fuse is connected in a standard three-pin plug as in Fig. 18.9.5.


If an appliance is faulty, this can lead to the current in the circuit becoming too large. The fuse has a thin piece of wire running through it, designed to melt if the current goes above a set value. When the fuse wire melts, it breaks the circuit and cuts off the electricity. Fuses are connected to the live wire as this is high in electrical energy.


The earth wire will draw a large current through the plug if an appliance is faulty. This helps the fuse wire to melt.


The fuse box of a house contains other circuit breakers that help to cut off the electrical supply if there is a fault in an appliance.


 Fig. 18.9.6 Circuit breakers


327


 Fig. 18.9.5 A three-pin plug


Earth wire


Neutral wire


Live wire


Fuse


 Fig. 18.9.4 LEDs


 Fig. 18.9.3 Circuit symbol for a diode


18.9


Cable grip


Outer insulation


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