- 1 What is meant by Heating Effect of Electric Current? and Explain How An Electric Fuse works on the Effect of Current.
- 1.1 Heating Effect of Electric Current
- 1.2 Applications of the Heating Effect of Current
- 1.3 1. Electric Heating Appliances
- 1.4 2. Electric Bulb
- 1.5 3. Electric Fuse (or Fuse)
- 1.6 The Cause of Large Current Flow : Short-Circuit and Overloading
- 1.7 Fuses Used in Electrical Appliances
- 1.8 Use of MCB in Place of Fuse
Studying Physics Topics can lead to exciting new discoveries and technological advancements.
What is meant by Heating Effect of Electric Current? and Explain How An Electric Fuse works on the Effect of Current.
An electric current can produce three effects :
- Heating effect,
- Magnetic effect, and
- Chemical effect.
In this class, we will study only the heating effect and magnetic effect produced by the flow of electric current. The chemical effect of electric current will be studied in higher classes. Before we discuss the heating effect of electric current, we should know the meaning of the term ‘electrical resistance’ or just ‘resistance’.
All the materials oppose the flow of current through them : some oppose less and others oppose more. The degree to which a material opposes the passage of current through itself is called its resistance. Copper and aluminium metals have very low electrical resistance whereas alloys such as nichrome have very high electrical resistance (Nichrome is an alloy of nickel, chromium, iron and manganese).
A wire made of material having high electrical resistance is called a resistance wire. Thus, nichrome wire is a resistance wire. Keeping these points in mind, we will now describe the heating effect of current.
Heating Effect of Electric Current
When an electric current is passed through a high resistance wire (like nichrome wire), the resistance wire becomes hot and produces heat. This is called the heating effect of current. Actually, when an electric current passes through a high resistance wire, the electric energy gets converted into heat energy.
This heat energy heats up the resistance wire. We can demonstrate the heating effect of current by performing an activity as follows.
Take two iron nails and fix ‘them some distance apart on a thermocol sheet (ora wooden block). Take about 10 centimetre long piece of nichrome wire and tie it between the two nails (as shown in Figure) (Nichrome wire can be obtained from an electric repair shop or a piece of discarded coil of an electric heater made of nichrome can be used). Make an electric circuit by connecting the two nails to the two terminals of a cell through a switch by using copper wires (see Figure).
Now pass the electric current in the circuit by moving the switch to ‘on’ position (by closing the gap). After a few seconds, touch the nichrome wire just for a moment (Do not hold it for long). We will feel that the nichrome wire has become hot.
The nichrome wire has become hot because the electric current passing through it has produced heat in it. This is the heating effect of current. In this activity, we should not keep the switch in the ‘on’ position for a long time otherwise the electric cell will get damaged and become weak very quickly.
The amount of heat produced in a wire due to heating effect of current depends on two factors :
- Resistance of wire (This depends on material of wire, length of wire and thickness of wire).
- Magnitude of current passed through the wire.
Greater the resistance of a wire, greater will be the heat produced in it by a given current.
For example, copper has a very low resistance, so if electric current is passed through a copper wire, then the heat produced in it will be extremely small (due to which the copper wire will become negligibly hot). The copper wires used for making electric circuits in science activities (or those used in household electric wiring) normally do not become hot on passing electric current because they have very low resistance (due to which the heat produced in them is negligible).
On the other hand, nichrome has a very high resistance, so if the same electric current is passed through a nichrome wire, then the heat produced in it will be too much (due to which it will become very hot). The ‘elements’ of electric heating appliances (such as electric room heater, electric iron, etc.) become very hot or red-hot on passing electric current because they have very high resistance (due to which the heat produced in them is very large).
Please note that a short wire has less resistance whereas a long wire has more resistance. Also, a thick wire has less resistance whereas a thin wire has more resistance.
Greater the magnitude of current passed through a given wire, greater will be the heat produced in it.
This means that a small current passing in a wire produces less heat but a large current passing through the same wire produces much more heat.
For example, when normal amount of current flows through the copper wires of household electric wiring, the wires do not become much hot but when a large current flows in the wires of household wiring accidently, then the wires become extremely hot (and a fire may be started).
Applications of the Heating Effect of Current
The heating effect of electric current has many applications (or uses) in our everyday life. Some of the important applications of the heating effect of current are given below :
- The heating effect of electric current is utilised in the working of electrical heating appliances such as electric room heater, electric iron, electric kettle, electric toaster, electric oven, water heater (geyser), immersion rod, and soldering iron, etc.
- The heating effect of electric current is utilised in electric bulbs (or electric lamps) for producing light.
- The heating effect of electric current is utilised in a safety device called ‘electric fuse’ (for protecting household wiring and electrical appliances from accidental ‘large flow’ of current).
We will now discuss all these applications of the heating effect of electric current in somewhat detail.
1. Electric Heating Appliances
The electric heating appliances (such as electric room heater, electric iron, etc.) work on the heating effect of current. All the electric heating appliances contain coils of high resistance wire made of nichrome alloy. The coil of high resistance wire (or coil of nichrome wire) present in an electric heating appliance is called the ‘heating ‘element’ (or just ‘element’) of the appliance.
In simple words, the part of electric heating appliance which ‘heats up’ on passing current is called ‘heating element’ (or just ‘element’) of the appliance. When electric current is passed through an electric heating appliance by connecting it to a power socket through insulated copper wires (called ‘cord’ or ‘connecting cable’), then a large amount of heat is produced in the heating element (or coil) due to which the heating element becomes red-hot.
A lot of heat is produced in the heating element because it is made of high resistance nichrome wire. A negligible heat is produced in the connecting wires of copper because copper has very low resistance. Since a negligible heat is produced in copper connecting wires, they do not become red hot (like the heating element). Let us now take the examples of electric room heater and electric iron.
The electric room heater (called ‘convector’) has an element made of a coil of nichrome wire (see Figure). When current is switched on in an electric room heater after connecting to a socket of power supply line, the element (or coil) of heater becomes red-hot and gives out heat (see Figure).
The element becomes red-hot because a lot of heat is produced in it on passing current (because of the high resistance of nichrome coil). The cord (or connecting cable) of heater made of copper does not become red-hot or glow because a negligible heat is produced in it on passing current (due to its extremely low resistance).
Similarly, when electric current is passed into an electric iron, the element of electric iron (made of nichrome coil) becomes red-hot and produces heat (see Figure). This heat makes the bottom of electric iron hot. The hot electric iron is then used for ironing clothes.
The heating elements of an electric room heater (or convector) and the electric heater used for cooking food can be seen by us from outside. The heating elements of most of the electric heating appliances (such as electric iron, electric kettle, electric geyser, electric oven, hotplates and hair dryers, etc.) cannot be seen by us because they are concealed inside the body of these appliances.
2. Electric Bulb
The electric bulb (or electric lamp) which we use for producing light works on the heating effect of electric current. The electric bulb has a very thin filament of tungsten metal [see Figure (a)], The filament of an electric bulb has a high resistance.
When electric current passes through the very thin, high resistance filament of an electric bulb, heat is produced in it. Due to this the filament of electric bulb gets heated to a very high temperature and starts glowing [see Figure (b)], Actually, the filament of bulb becomes white-hot and emits light.
A lot of electricity consumed by the filament of an electric bulb appears as heat due to which the glowing electric bulb becomes quite hot. We should never touch a glowing electric bulb connected to the mains electricity. It may be very hot and our hand may get burnt badly.
Please note that the same current flowing through the tungsten filament of an electric bulb produces enormous heat but almost negligible heat is produced in the connecting wires of copper. This is because of the fact that the fine tungsten filament has very high resistance whereas copper connecting wires have very low resistance.
An electric bulb is basically used for producing light but it also gives out heat. In fact, a major part of the electricity consumed by the filament of an electric bulb is converted into heat (due to which the bulb becomes hot), only a small part of electricity is converted into light. The production of heat by an electric bulb is not desirable because it leads to the wastage of electricity. So, the filament-type electric bulbs are not power efficient (because they waste a lot of electricity as heat).
The wastage of electricity can be reduced by using fluorescent tube lights and compact fluorescent lamps (CFLs) for producing light (in place of filament-type electric bulbs (see Figure). Fluorescent tube lights and compact fluorescent lamps (CFLs) do not work on the heating effect of current.
The fluorescent tube lights and compact fluorescent lamps do not have filaments. So, they do not waste electricity by producing heat and hence they are very power efficient. Another advantage of compact fluorescent lamps is that they can be fixed in ordinary bulb holders (which are used for traditional, filament-type electric bulbs).
The fluorescent tube lights and compact fluorescent lamps (CFLs) work on the same principle, only their sizes are different. A fluorescent tube light (or lamp) contains mercury vapour (which is a kind of gas). The inside of glass tube is coated with an opaque, white material called ‘phosphor’.
When a fluorescent tube light (or lamp) is switched on and current passes through it, the mercury vapour emits invisible ultraviolet radiations. The phosphor absorbs these ultraviolet radiations and glows (or fluoresces) to emit white light. Though fluorescent tube lights and CFLs are initially more expensive to buy (than filament-type bulbs) but their running cost is much less (because they consume much less electricity), and they also last much longer.
Before buying any electrical appliance, we should look for the ISI mark of the Bureau of Indian Standards on the appliance (ISI stands for Indian Standards Institution). The ISI mark ensures that the appliance is safe and energy efficient (the wastage of energy being the minimum). So, before buying electrical heating appliances, bulbs, tube lights or CFLs, we should always look for ISI mark on them.
3. Electric Fuse (or Fuse)
A fuse is a safety device which prevents electric fires and damage to electrical appliances due to excessive flow of current. The fuse for protecting our household electric wiring is fitted just near the main switch on the switch board. A fuse works on the heating effect of current. We will now describe the working of an electric fuse.
A fuse consists of a short length of a thin, tin-plated copper wire having low melting point. The thin fuse wire has a much greater resistance than the rest of the electric wiring in the house. So, when the current in a household circuit suddenly increases too much, then the thin fuse wire gets heated too much, melts and breaks the circuit (due to which current stops flowing in the circuit).
This prevents fire in the house (which could be caused due to overheating of wires), and also prevents damage to various electrical appliances (like TV and refrigerator) due to a large current flowing through them.
Electric fuses are inserted in the wiring (or circuits) of all the houses and other buildings. The traditional fuse used in the household electric circuits in shown in Figure. This fuse consists of a porcelain fuse holder and a removable porcelain fuse grip. The fuse holder is connected in the household circuit. The fuse grip has a fuse wire fixed in it (see Figure).
When the fuse grip carrying fuse wire is inserted in the fuse holder, then the circuit of household wiring is completed. So, under normal circumstances when the current flowing in the wiring is within a safe limit, the fuse wire is intact and electricity is available in the household circuit. There is, however, a maximum limit on the current which can flow safely through the fuse wire inserted in the household circuit.
If due to some reason, the current in household wiring suddenly becomes too large (and exceeds the safe limit), then the fuse will blow off (its wire will get heated too much, melt and break the circuit). When the fuse wire breaks, electricity supply in the household wiring is automatically cut off before any damage can be done to the rest of wiring or to the electrical appliances being used.
If, however, there were no fuse in the household wiring, then the large current flowing through the wiring could heat up the wires to very high temperatures and cause electric fires. The large current flowing through electrical appliances such as TV and refrigerator could also damage them.
When a fuse gets blown (or breaks), a new fuse has to be fitted in its place to restore the electricity supply in the household circuit. A blown up fuse should be replaced by a new fuse only after the cause of excessive current in the wiring has been detected and set right. Never try to fix the blown up electric fuse connected to the mains circuit in the house yourself. An electrician should be called in for this purpose.
Please note that we use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires. Due to its high resistance, the heating effect of current will be much more in the fuse wire than anywhere else in the circuit. This will melt the fuse wire whereas other wiring will remain safe.
The thickness of the fuse wire should be such that it is able to withstand only a little more current than drawn by the household circuit. We should not use a thick wire as a fuse wire because it will have a low resistance and hence it will not get heated to its melting point easily.
The fuse wire is usually made from tin-plated copper wire having low melting point so that it may melt easily. A copper metal wire (or any other metal wire or metal strip) should not be used as a fuse wire because it has a high melting point due to which it will not melt easily when a large current passes through it.
To Demonstrate the Working of a Fuse
Fix two nails about 5 to 10 cm apart on a sheet of thermocol or a wooden block. Tie a thin strand of steel wool between the two nails [as shown in Figure (a)], This thin strand of steel wool will act as a fuse wire. Connect the nails to the two terminals of a four cell battery through a switch and a 6 volt bulb by using connecting wires of copper [see Figure (a)].
A battery of four cells is being used here to pass a large current through the circuit. If there are any fans working in the room, switch them off. Now, pass the electric current through the circuit (including the strand of steel wool) by moving the switch to ‘on’ position. We will find that the electric circuit is complete due to which the bulb lights up immediately [see Figure (a)],
After a few seconds, the thin strand of steel wool breaks and the bulb goes off [see Figure (b)]. Actually, the thin strand of steel wool is like a fuse wire here. The thin strand of steel wool has much greater resistance than the copper connecting wires.
So, when a large current passes through the thin strand of steel wool, a lot of heat is produced in it due to the heating effect of current. This heat melts the thin strand of steel wool and breaks it cutting off the flow of current in the circuit. This is how a fuse works in the household electric wiring.
The Cause of Large Current Flow : Short-Circuit and Overloading
An extremely large current can flow in the household electric wiring circuits under two circumstances: short- circuit and overloading. These are discussed below.
(i) Short Circuit. In household electric wiring, electric current is supplied through two insulated wires which run together and reach each and every electrical appliance. One insulated wire is called live wire and the other insulated wire is called neutral wire. Both these wires are necessary for the working of an electrical appliance (say, an electric iron).
If the plastic insulation of the live wire and neutral wire gets torn due to wear and tear, then the two naked wires touch each other. The touching of live wire and neutral wire directly is known as short circuit. When the live wire and neutral wire touch each other directly, then a large current flows through the household wiring. This large current may heat the wires to a dangerously high temperature and a fire may be started.
Thus, one reason for the flow of excessive current in household circuits is the short circuit (touching of live wire and neutral wire). The outer insulation of connecting cable of an electric iron shown in Figure is already torn. And if the insulation of inner wires also gets torn, then short circuit will take place.
We should never use such an electrical appliance having connecting cable with torn insulation. Such connecting cables should be replaced immediately.
When too many electrical appliances are connected to a single socket, they draw an extremely large current from the household circuit. This is known as overloading (the circuit) (see Figure). The flow of large current due to overloading may heat the copper wires of household wiring to a very high temperature and a fire may be started. In fact, many times we read in newspapers about the electrical fires caused by short circuits and overloading.
To avoid this danger of electric fires, we use an electric fuse in the wiring. So, when a house or some other building is wired, the wiring is protected by fuses. When a short circuit or overloading occurs, the fuse wire blows out cutting off the electricity supply. This prevents the electric fires.
Fuses Used in Electrical Appliances
So far we have discussed the fuses which are put on the main switch-board in our house to protect the whole wiring of the house. Fuses are also used to protect the individual domestic electrical appliances from damage which may be caused due to excessive current flow through them.
Costly electrical appliances like TV sets and refrigerators have their own fuses which protect them against damage by too much current. The fuse used in an electrical appliance is shown in Figure (a). It consists of a glass tube having a thin fuse wire sealed inside it. The glass tube has two metal caps at its two ends.
The two ends of the fuse wire are connected to these metal caps. The metal caps are for connecting the fuse in the circuit in a suitably made bracket. If due to some reason, a large current tends to flow through the electrical appliance, its fuse will ‘blow up’ (melt and break), and cut off the electricity supply to the appliance [see Figure(b)].
This will protect the appliance from damage which could have been caused if the large current had passed through it. The fuses used in individual electrical appliances [like the one shown in Figure (a)] are called cartridge fuses. We should always use proper fuses (carrying ISI mark) which have been specified for particular applications.
Use of MCB in Place of Fuse
MCB stands for Miniature Circuit Breaker (see Figure). These days MCBs (miniature circuit breakers) are being used increasingly in household electric wiring in place of traditional fuses. MCB does not work on the heating effect of current. MCB works on the magnetic effect of current. MCB is a special kind of electrical switch which automatically turns off when the current in the circuit becomes too large, and cuts off the electricity supply.
MCB can be re-set in the on position when the fault in the wiring (which caused a large current to flow in the circuit) has been set right. Thus, unlike a fuse which can be used ‘only once’ and has to be replaced (when its wire melts and breaks), the same MCB can be re-set manually to restore electricity supply. When the MCB is re-set (or turned on), the circuit becomes complete and electric current starts flowing in the household wiring again.
These days more and more houses and other buildings are using miniature circuit breakers (MCBs) to protect the electric wiring from the excessive flow of current. To make sure that we buy good quality MCBs, we should look for ISI mark on them.