Heating Effect Of Electric Current

HEATING EFFECT OF ELECTRIC CURRENT
 

Activity 14.2

Take an electric cell, a bulb, a switch and connecting wires. Make an electric circuit as shown in Fig.14.9. This activity has to be done using only one cell. Keep the switch in the ‘OFF’position.   Does the bulb glow? Touch the bulb. Now move the electric switch to the ‘ON’ position and let the bulb glow for a minute or so. Again touch the bulb. Do you feel any difference? After moving the switch back to the ‘OFF’ position, touch the bulb again.
 

Activity 14.3

Make a circuit as shown in Fig.14.10. Take about 10 cm long piece of nichrome wire and tie it between the nails. (You can get nichrome wire from an electric repair shop or you can use a piece of discarded coil of an electric heater.) Touch the wire.

Now switch on the current in the circuit by moving the switch to the ‘ON’ position. After a few seconds touch the wire(Do not hold it for a long time.)

Switch off the current. Touch the wire again after a few minutes.You might have seen an electric room heater or an electric heater used for cooking. All these contain a coil of wire. This coil of wire is called an element. You may have noticed that when these appliances are switched on

Boojho could not see element in an electric iron. Paheli told him that electrical appliances, such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, have elements inside them. Have you ever seen the element in any appliance?

after connecting to the electric supply, their elements become red hot and give out heat.The amount of heat produced in a wire depends on its material, length and thickness. Thus, for different requirements, the wires of different materials and different lengths and thicknesses are used.

The wires used for making electric circuits do not normally become hot. On the other hand, the elements of some electric appliances become so hot that they are easily visible. The filament of Incandescent electric bulbs (Fig. 14.12) are often used for lighting but they also give heat. It means that a part of electricity consumed is used in producing heat. This is not desirable as it results in the wastage of electricity. The fluorescent tube-lights and compact fluorescent lamps (CFLs) are better electricity efficient lighting sources. Nowadays, the use of light emitting diode (LED) bulbs is increasing. For producing a given intensity of light, LED bulbs consume less electricity as compared to incandescent bulbs or fluorescent tubes or CFLs.Thus LED bulbs are much electricity efficient and therefore being preferred.

Fig. 14.13  Electric bulb, tube-light, CFL and LED

It is advised to use electrical appliances and gadgets, which are electricity efficient. Bureau of Indian Standards, New Delhi assigns a Standard Mark on products, called ISI mark which is an assurance of conformity to the specifications given on the products. It is therefore suggested to use ISI mark products.

Note: Fluorescent tubes and CFLs contain mercury vapour, toxic in nature. Therefore, damaged fluorescent tubes or CFLs need to be disposed off safely

Fig. 14.14 Fuse used in buildings

an electric bulb gets heated to such a high temperature that it starts glowing. If a large  current  passes  through  a wire, the wire may become so hot that it may even melt and break.But is it possible for a wire to melt and break?

Activity 14.4

Make the circuit we used for Activity 14.3 again. However, replace the cell with a battery of four cells. Also, in place of the nichrome wire, tie a thin strand of steel wool. (The steel wool is commonly used for cleaning utensils and is available in grocery shops.) If there are any fans in the room, switch them off. Now pass the

Fig. 14.15 Fuses used in electrical appliances

current through the circuit for sometime. Observe the strand of steel wool carefully. Note what happens. Does the strand of steel wool melt and break? Wires made from some special materials melt quickly and break when large electric currents are passed through them. These wires are used for making electric fuses (Fig.14.14). In all buildings fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires

One reason for excessive currents in electrical circuits is the direct touching of wires. This may happen if the insulation on the wires has come off due to wear and tear. This may cause a short circuit. Another reason for excessive current can be the connection of many devices to a single socket. This may cause overload in the circuit. You might have read reports in newspapers about fires caused by short circuits and overloads. may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse  is  thus  a  safety  device  which These days Miniature circuit breakers (MCBs) are increasingly being used in place of fuses. These are switches which automatically turn off when current in a circuit exceeds the safe limit. You turn them on and the circuit  is  once  again complete.

Fig. 14.16  Miniatur circuit

prevents damages to electrical circuits and possible fires.

Fuses of different kinds  are  used for different purposes. Fig. 14.14 shows fuses used in our houses. Fuses shown in Fig. 14.15 are generally used in electrical appliances.

We observed the heating effect of the electric current and learnt how we use it to our advantage. Does the electric current have other effects also?