Short Answers<\/strong><\/span><\/h3>\nQuestion 1:<\/strong> What does the unit kilowatt hour measure? Define it.
\nAnswer:<\/strong> The unit kilowatt hour measures the electrical energy. 1 kWh is the electrical energy consumed by an electric appliance of power 1 kW when it is used for 1 hour.<\/p>\nQuestion 2:<\/strong> Define Watt hour.
\nAnswer:<\/strong> Watt hour is a smaller commercial unit of electric energy, which is spend by an electric appliance of 1 watt power in 1 hour, i.e.
\n1 watt hour = 1 watt \u00d7 hour = 1 watt \u00d7 (60 \u00d7 60)s
\n= 3600 joule.<\/p>\nQuestion 3:<\/strong> Define Kilo watt hour.
\nAnswer:<\/strong> Kilowatt hour is a commercial unit of electric energy consumed. It is also called B.O.T. (Board of Trade) unit and is used throughout the world in calculating the amount of electric energy consumed. kWh is defined as the amount of electric energy consumed by an electric appliance of power 1kW for one hour.<\/p>\nQuestion 4:<\/strong> Express watt hour and kilowatt hour in joule.
\nAnswer:<\/strong> 1 kWh = 1W \u00d7 1 hour = 1000 W \u00d7 (60 \u00d7 60) s
\n= 1W \u00d7 1s \u00d7 105<\/sup>\u00a0\u00d7 36 = 36 \u00d7 105<\/sup> J = 3.6 \u00d7 106<\/sup>\u00a0J<\/p>\nQuestion 5:<\/strong> What is the energy conversion when an electric current passes through a metallic wire?
\nAnswer:<\/strong> When an electric current passes through a metallic wire, the wire becomes hot and the whole of electric energy is converted into heat energy.<\/p>\nQuestion 6:<\/strong> State one advantage of a.c. over d.c.
\nAnswer:<\/strong> Advantage of a.c. over d.c. is that it is able to travel long distance without loosing power.<\/p>\nQuestion 7:<\/strong> Which cable, one rated 15A and the other rated 5A will be thicker? Give a reason.
\nAnswer:<\/strong> The cable rated 15A will be thicker. The reason is that thicker the wire, less is its resistance and hence higher is its current carrying capacity.<\/p>\nQuestion 8:<\/strong> What is electric power? State and define its S.I. unit.
\nAnswer: Electric power is the rate of doing work or it is the work done in 1 second.
\nThe S. I. unit of power is ‘Watt’.
\nIf a current of 1 ampere flows through a metallic conductor at a potential difference of 1 volt, the power is said to be 1 watt.
\nor \u00a0 \u00a01 Watt = 1 Volt \u00d7 1 Ampere.<\/p>\nQuestion 9:<\/strong> State three factors on which the heat produced in a metallic wire due to passage of current in it depends.
\nAnswer:<\/strong> (i) On resistance of wire.
\n(ii) On strength of current through the wire.
\n(iii) On time for which the current flows in the wire.<\/p>\nQuestion 10:<\/strong> Explain the meaning of the statement ‘The power of an appliance is 100W’.
\nAnswer:<\/strong> This means that if the appliance is used for 1s, it will consume 100J of electrical energy.<\/p>\nQuestion 11:<\/strong> An electric current is passed through a metallic wire. The wire gets heated up. Give the reason. Is it possible to melt the wire by passing a heavy current?
\nAnswer:<\/strong> The metallic wire gets heated up on passing an electric current through it due to collisions of free electrons and energy is transferred to the positive ions which begin to vibrate more violently, causing to produce heat. It is possible to melt the wire by passing a heavy current. The wire will melt when its temperature due to heating reaches its melting point.<\/p>\nQuestion 12:<\/strong> Why is heating coil wound in the form of a helix in electric stove?
\nAnswer:<\/strong> This reduces the area over which element of heater is spread. As heat is produced over smaller area, therefore temperature of its element rises to about 1000\u00b0C.<\/p>\nQuestion 13:<\/strong> Why is the heating coil placed in a circular porcelain plate in an electric stove?
\nAnswer:<\/strong> The procelain plate (i) is bad conductor of electricity (ii) does not melt till 1000\u00b0C.<\/p>\nQuestion 14:<\/strong> What are the two main systems of wiring commonly used in houses? Which of these systems mentioned by you is better and why?
\nAnswer:<\/strong> The two main systems of wiring are: (i) the tree system, and (ii) the ring system.
\nOut of these systems, the ring system is better. The reason is that this system of wiring is cheaper because a low rating wire can be used for connection of appliances.<\/p>\nQuestion 15:<\/strong> Point out two main disadvantages of ‘Tree System’.
\nAnswer:<\/strong> The three main disadvantages are:
\n(i) Longer length of wire is required for wiring different rooms in a house. Since all circuits start from the main distribution board and so it is comparatively more Expensive.
\n(ii) If fuse blows off in any part of the building, it disconnects all the appliances in the circuit.<\/p>\nQuestion 16:<\/strong> State two advantages of ‘the ring system of house wiring’.
\nAnswer:<\/strong> The three advantages of ring system are:
\n(i) Length of wiring used is much less and so less costly, since a single ring feeds the whole house.
\n(ii) In case of short-circuit, only the fuse of that particular appliance blows off, diconnecting it from the electric circuit; while the circuits of other appliances remain unaffected.<\/p>\nQuestion 17:<\/strong> Two sets A and B of four bulbs each, are glowing in two separate rooms. When one Of the bulbs in set is fused, the other three bulbs also cease to glow. But in set B, when one bulb fuses, the remaining ones continue to glow. Explain the difference.
\nAnswer:<\/strong> In set A, the bulbs are joined in series so there is only one path of current for each bulb. But in set B, the bulbs are joined in parallel so there is separate path of current for each bulb.<\/p>\nQuestion 18:<\/strong> How should the electric lamps in a house be connected so that the switching ON or OFF in a room has no effect on other lamps in the same building?
\nAnswer:<\/strong> In parallel.<\/p>\nQuestion 19:<\/strong> State the colour coding of wires in the flexible lead used for connecting the appliances to the mains in a house circuiting.
\nAnswer:<\/strong> The flexible lead has three wires with insulation of different colours. The red (or brown) is for live, the black (or light blue) is for neutral and the green (or yellow) is for earth connections.<\/p>\nQuestion 20:<\/strong> Name three main characteristics of an heating element.
\nAnswer:<\/strong> The three main characteristics of an heating element are:
\n(i) It should have a high resistance so that a major amount of electric energy is converted into heat energy.
\n(ii) It should not get oxidised easily up to a temperature of 1000\u00b0C.
\n(iii) It should have a high melting point and a low thermal Expansion so that it does not get loosened on getting heated.<\/p>\nQuestion 21:<\/strong> Mention three uses of the carbon arc.
\nAnswer:<\/strong> (i) They are used in light houses.
\n(ii) They are used in search lights and cinema projection appliances.
\n(iii) They are used in electric furnace and electric welding.<\/p>\nQuestion 22:<\/strong> Name two common material used for heating elements.
\nAnswer:<\/strong> Nichrome and manganin are the common alloys that are mostly used for making heating elements.
\nNichrome is: 60% nickel + 25% iron + 15% chromium.
\nManganin is: 83% manganese + 13% copper + 4% nickel.<\/p>\nQuestion 23:<\/strong> In an electric radiator or room heater, state with reason, why the porcelain tube wound with a nichrome wire is placed between the pole and principal focus of the parabolic concave reflector.
\nAnswer:<\/strong> In this position, when current passes through the nichrome wire, it becomes red hot and the concave reflector helps in spreading the heat radiation in the room because it forms a divergent beam on account of the heating element being placed between the pole and the principal focus of the reflector.<\/p>\nQuestion 24:<\/strong> Why is heating element wound on a long porcelain rod in a room heater.
\nAnswer:<\/strong> This reduces the area over which element of the heater is spread. As heat is produced over smaller area, therefore temperature of its element rises to about 1000\u00b0C.<\/p>\nQuestion 25:\u00a0<\/strong>Why is a concave reflector placed behind the heating element in a room heater.
\nAnswer:<\/strong> The concave reflector, reflects the heat radiations in the forward direction. Thus, large amount of heat radiations are available in one particular direction.<\/p>\nQuestion 26:<\/strong> State the position of heating element with respect to principal focus of concave reflector and give reason for choosing this position.
\nAnswer:<\/strong> The heating element is between the pole and principal focus of concave reflector. This in turn, gives rise to a divergent beam of heat radiations.<\/p>\nQuestion 27:<\/strong> Household wiring for lamp connections can either be done in parallel or in series.Which one would you prefer? Give a reason for your answer.
\nAnswer:<\/strong> We would prefer the parallel wiring. This is done for the following reasons:
\n(i) In parallel wiring, each lamp can have its independent switch and the disconnection (putting off) of one of them will not effect the other lamps.
\n(ii) All lamps have the same potential difference (equal to that of the mains) across the terminals and are thus able to work as per their specified rating.<\/p>\nQuestion 28:<\/strong> What is meant by earthing of an electrical appliance? Why is it essential?
\nAnswer:<\/strong> Earthing of an electrical appliance meant the outer metallic case of appliance is connected with earth, wire which is welded to the end of a copper rod and it is burned in the ground.
\nIt save the appliance during short circuiting by passing Excessive current to earth.<\/p>\nQuestion 29:<\/strong> What precaution do you observe while connecting a plug top into a socket?
\nAnswer:<\/strong> The plug pins should fit in the socket tightly. Any loose connection will give rise to sparking and burning of the socket or the plug.<\/p>\nQuestion 30:<\/strong> In a three-pin plug top, one of the pins is thicker and longer than the other two, Give reason. Name this pin.
\nAnswer:<\/strong> The thicker arid longer pin is the earth pin. It is longer so that the earth connection is made first of all and it is thicker so that even by mistake it cannot be inserted into the live socket.<\/p>\nQuestion 31:<\/strong> Why is that the same current flowing through the bulb heats up the filament, while the leads are not heated up?
\nAnswer:<\/strong> The leads are much thicker than the filament wire. Thus the resistance of the leads is much smaller than that of filament wire. So with the same current, the filament gets heated while the leads are not.<\/p>\nQuestion 32:<\/strong> A lady working in her kitchen received a shock from an electric hot plate. What could be the reason for the shock?
\nAnswer:<\/strong> The reason can be that the metallic case of the electric hot plate is not earthed and the live wire of hot plate have come in contact with its case.<\/p>\nQuestion 33:<\/strong> Why is the filament in an electric bulb coiled?
\nAnswer:<\/strong> To have a high resistance, the filament wire is long and thin. To fit in a small space it is coiled.<\/p>\nQuestion 34:<\/strong> Why is the heating element of an electric oven wound on a helix? State the reason.
\nAnswer:<\/strong> In an electric oven, the china clay plate is provided either with lateral grooves or grooves in the form of a spiral helix. This type of spiral winding reduces the area of spread of nichrome wire and helps to concentrate heat in a small region.<\/p>\nQuestion 35:<\/strong> The electric bulbs used these days have a coil-coiled filament. State its reason.
\nAnswer:<\/strong> On account of having a coil-coiled filament, the area of spread of tungsten is much and so the heat now produced in a very small area, raises the temperatures to a high degree i.e., 2300\u00b0C, producting more bright light.<\/p>\nQuestion 36:<\/strong> What changes in energy occur in an (i) electric bulb and (ii) electric heater?
\nAnswer:<\/strong> (i) In an electric bulb, electrical energy changes to heat and light energy.
\n(ii) In an electric heater, electrical energy changes to heat energy.<\/p>\nQuestion 37:<\/strong> Why mica is preferred to other insulators in an electric iron?
\nAnswer:<\/strong> Mica is preferred to other insulators in an electric iron because mica is a bad conductor of electricity, but it easily conducts away heat. So when current flows through nichrome wire, the heating element is fully protected against any leakage of electric current, whereas heat is easily conducted.<\/p>\nQuestion 38:<\/strong> What is meant by ‘MCB’? Which part of the domestic electric circuit does it replace?
\nAnswer: <\/strong>‘MCB’ stands for ‘miniature circuit breaker’. This is a device that automatically ‘switches off the mains supply if the current drawn Exceeds the prescribed upper safe limit. It can be ‘reset’ with the help of a flip-over switch. It protects the entire circuit and appliances against short circuiting, etc.<\/p>\nQuestion 39:<\/strong> Name the markings of a three-pin plug. Give the colour codes of the connecting leads.
\nAnswer:<\/strong> The three pins of a three-pin plug are:
\n(i) ‘Live’ pin (ii) ‘Neutral’ pin (iii) ‘Earth’ pin .
\nThe colour of the corresponding connecting leads are (i) ‘Live’ pin\u2014Brown (or Red),
\n(ii) ‘Neutral’ pin\u2014Blue (or Black), (iii) ‘Earth pin\u2014yellow (or Green).<\/p>\nQuestion 40:<\/strong> What is a switch? State its function in an electric circuit.
\nAnswer:<\/strong> A switch is an on-off device for current flowing in a circuit or in an electric appliance. The switch should always be connected in the live wire. Its main function is either to connect or disconnect an electric appliance in an electric circuit.<\/p>\nQuestion 41:<\/strong> What will happen if switch is connected in ‘neutral wire’?
\nAnswer:<\/strong> If switch is connected in the neutral wire, an electric appliance, e.g., an electric iron will remain connected to the live wire even when the switch is off and so if any one happens to touch the appliances; he of she will get a shock which is sometimes fatal.<\/p>\nQuestion 42:<\/strong> Why a switch should not be touched with wet hands?
\nAnswer:<\/strong> switch should not be touched with wet hands because the water forms a conducting layer between the live wire of the switch and the hand and so we may get a fatal shock when the current passes from the ‘live’ wire to the hand.<\/p>\nQuestion 43:<\/strong> State the purpose served by the terminals of a three pin plug.
\nAnswer:<\/strong> The earth pin, which is thicker and longer than the other two pins; first connects the appliances to the earth and ensures safety from an electric shock to the user. The live pin on the right and the neutral pin on the left connects the appliance to the live wire and the neutral wire of the circuit at the supply voltage.<\/p>\nQuestion 44:<\/strong> What are ‘dual switches’? Mention their one main use.
\nAnswer:<\/strong> Dual switches are double pole type switches. With such switches an appliance, e.g., and electric bulb can be switched off or on from two different places. They are generally used in staircases, long corridors etc.<\/p>\nQuestion 45:<\/strong> Where is a fuse placed in an electrical circuit? Name a material suitable for making a fuse wire.
\nAnswer:<\/strong> The fuse is placed in the live wire just after the ‘electricity meter’ of the consumer. Fuse wires are generally made of an alloy of tin and lead and have a relatively low melting point.<\/p>\nQuestion 46:<\/strong> An electric filament lamp is connected to a supply of voltage higher than the recommended value. Give reasons, why compared with a normal performance, the lamp emits a brighter light and its life is shortened.
\nAnswer:<\/strong> The lamp will have the filament of a particular resistance. If it is connected to a supply of higher voltage, the heat energy produced in it will be more (since H = V2<\/sup>t\/R), i.e., the filament will get more heated so it will glow more and the lamp will emit a brighter light. On the other hand, due to Excess heating, the filament may vaporize and so its life may shorten.<\/p>\nQuestion 47:<\/strong> Two fuse wires of the same lengths are rated 5A and 20A. Which of the two fuse wires is thick and why?
\nAnswer:<\/strong> 20A wire is thick as it carries large current and so its area of cross-section will be large.<\/p>\nQuestion 48:<\/strong> Why fuse wire must always be connected in ‘live’ wire? Explain.
\nAnswer:<\/strong> Under any circumstances, fuse wire always be connected in a live wire and never in neutal wire. The reason is that in the case of over-loading, if fuse wire is connected in neutral wire, it will certainly melt and break, but the electric appliance still be in contact with the live wire, so if any person happens to touch the appliance, he will get a shock which may prove fatal.<\/p>\nQuestion 49:<\/strong> Where are the cartridge type fuse used?
\nAnswer:<\/strong> Cartidge type fuse are used to provide additional safety to an electric appliance. The fuse wire used must have a lower rating than the maximum current which the appliance or the circuit can withstand. Hence, the fuse blows off just the current nearly approaches the maximum value and thus it protects the appliance from any damage done due to heating.<\/p>\nQuestion 50: <\/strong>‘The current rating of a fuse is 5A’. Comment on this statement.
\nAnswer:<\/strong> This statement means that the current in the circuit should not Exceed 5A. As the current in circuit will Exceed 5A, the fuse will melt and the circuit will break.<\/p>\nQuestion 51:<\/strong> What is the rating of fuse used in (i) light and fan circuits, (ii) power circuits, in domestic use?
\nAnswer:<\/strong> (i) The rating of fuse used in light and fan circuits is 5A
\n(ii) The rating of fuse used in power circuits is 15A.<\/p>\nQuestion 52:<\/strong> Two lamps are rated 220V, 50W and the other rated 220V, 100W are connected in series with the mains of 220 V. Explain, why the 50W lamp glows more?
\nAnswer:<\/strong> From relation R = V2<\/sup>\/P, the resistance of filament of 50W lamp will be more than that of 100W lamp. If the two lamps are connected in series with the mains, same current will flow through each lamp. From relation H = I2<\/sup>Rt, since resistance of 50W lamp is more than that of 100W lamp, so the 50 W lamp will consume more electrical energy than that of 100W lamp, in the same time. Hence the 50W lamp will glow more.<\/p>\nQuestion 53:<\/strong> Why does not the heating coil produce any visible light? in an electric stove.
\nAnswer:<\/strong> It is because, the temperature of heating element is around 1000\u00b0C, whereas visible light is produced if the temperature rises above 1500\u00b0C.<\/p>\nQuestion 54:<\/strong> Of the three connecting wires in a household circuit: Which two of the three wires are at the same potential? In which of the three wires should the switch be connected?
\nAnswer:<\/strong> Neutral wire and Earth wire are at same potential and the switch be connected to live wire.<\/p>\nQuestion 55:<\/strong> Name two precautions to be observed while using an immersion heater.
\nAnswer:<\/strong> The two main precautions to be observed are:
\n(i) Never try to find the temperature of water by dipping your fingers in water because some times when water comes in contanct with live or neutral wire, there is a possiblility of getting electric shock.
\n(ii) The heater must be surrounded by water to the desired level before it is switched on. If not,
\nso the heat produced in the nichrome wire will not dissipate easily and so the heating element will melt.<\/p>\nQuestion 56:<\/strong> State two advantages of filling an inert gas in an evacuated electric filament lamp.
\nAnswer:<\/strong> (i) The inert gas is filled at very low pressure to prevent oxidation as well as evaporation and thus increases its life.
\n(ii) While bulb is working, a convection current is set-up within it which carries the spurting particles of tungsten upward towards the cap and hence the blackening of the lower part (transparent portion) of the bulb is avoided.<\/p>\nQuestion 57:<\/strong> A device is used to transform 12 V a.c. to 200 V a.c. What is the name of this device? Name the principle on which it works.
\nAnswer:<\/strong> Step-up transformer. Step-up transformer works on the principle of electromagnetic induction.<\/p>\nQuestion 58:<\/strong>\u00a0Which material is the calorimeter commonly made of? Give one reason for using this material.
\nAnswer:<\/strong> Copper. It is good conductor of heat.<\/p>\nQuestion 59:<\/strong> Name a metal that is used as an electron emitter. Give one reason for using this metal.
\nAnswer:<\/strong> Thoriated tungsten (tungsten coated with carbon and thorium) is used as an electron emitter. Thoriated tungsten is used since it has a work function of only 2-6 eV and it needs to be heated to only 2000K to emit electrons.<\/p>\nQuestion 60:<\/strong> What is ‘Rating’ of an electric appliance?
\nAnswer:<\/strong> Rating of an electric appliance indicates energy consumed per second or watt when connected to a supply of given voltage.<\/p>\nQuestion 61:<\/strong> If the rating of an electric bulb is 100W – 230V, Explain its meaning.
\nAnswer:<\/strong> If an electric bulb is rated as 100W – 230V, it means that the bulb is lighted on a 230V supply, it consumes 100W electric power (i.e., 100J of energy per sec.).<\/p>\nQuestion 62:<\/strong> What would happen and why, to an electric bulb when it is connected across a supply of voltage (i) lower (ii) higher than its proper rating?
\nAnswer:<\/strong> We know, Current = Voltage\/Resistance
\nFor a given bulb, the filament would have a definite characteristic resistance. Hence
\n(i) When the bulb is connected to a lower voltage supply, it would draw less current than its ‘proper current’. Hence it would not glow to its full capacity.
\n(ii) When the bulb is connected to a higher voltage supply, it would draw more current than its ‘proper current’. Hence its filament would get heated to a temperature higher than its proper permitted limit and is, therefore, likely to melt or fuse.<\/p>\nQuestion 63:<\/strong> An electric bulb is marked 100 W, 230 V. What does this mean?
\nAnswer:<\/strong> This statement means that the bulb is operated at 230 volt and consumes 100 watt of power.<\/p>\nQuestion 64:<\/strong> Two electric lamps each rating 100 W, 110 V are connected in series to a 220 V power supply and two other electric lamps each of marking 100 W, 220 V are connected in parallel to the same power supply. Will any one of the two combinations give more light than the other? Give a reason for your answer.
\nAnswer:<\/strong> Both combinations will give same light.
\nReason: Each combination has same power namely 200 W.<\/p>\nQuestion 65:<\/strong> Why should switches always be connected to the live wire? Give one precaution that should be taken while handling switches.
\nAnswer:<\/strong> The switch should always be connected to the live wire, so that current is cut off to that appliance to which it is connected. A switch should not be touched with wet hands.<\/p>\nQuestion 66:<\/strong> What is the order of current drawn by appliances connected to the ‘light circuit’ of a household?
\nAnswer:<\/strong> The appliances, connected to the light circuit, are supposed to draw currents of the order of 10A to 5A.<\/p>\nQuestion 67:<\/strong> What is the order of current drawn by appliances connected to the ‘power circuit’ of a household?
\nAnswer:<\/strong> The appliances connected to the ‘power circuit’ are supposed to draw currents of the order of 0 to 15A.<\/p>\nQuestion 68:<\/strong> Give two precautious that you would take while putting off an electric switch.
\nAnswer:<\/strong> We will make sure that no naked wire is touching the button of the switch and neither the hand nor the switch is wet.<\/p>\nQuestion 69:<\/strong> What is the function of the split rings in a d.c. motor?
\nAnswer:<\/strong> The split ring atts as a commutator in a d.c. motor. With the split ring, the direction of current through the coil is reversed after every half rotation of coil and thus the direction of couple rotating the coil remains unchanged and the coil continues to rotate in the same direction.<\/p>\nQuestion 70:<\/strong> Why is the electric power from the generating station transmitted at high voltage?
\nAnswer:<\/strong> The electric power from the generating station is transmitted at high voltage to minimise the loss of energy in the live wires.<\/p>\nQuestion 71:<\/strong> How does the heat produced by die passage of current in a metallic wire depend on: (i) the current in the wire (ii) the resistance of the wire, and (iii) the time of passage of current in the wire?
\nAnswer:<\/strong> (i) The heat produced is proportional to the square of current passed in the wire.
\n(ii) The heat produced is proportional to the resistance of the wire.
\n(iii) The heat produced is proportional to the time of passage of current in the wire.<\/p>\nLong Answers<\/strong><\/span><\/h3>\nQuestion 1:<\/strong> What do you understand by ‘earthing’? What are the advantages of earthing in a household electric circuit? Explain, how it is done?
\nAnswer:<\/strong> By earthing we mean that the metallic body of an electric appliance is connected to thick copper wire, which is buried deep in the earth and at its end is a copper plate surrounded by a mixture of charcoal and common salt.
\nAdvantages:<\/strong> It is kind of safety device which saves us from an electric shock, in case when the
\nmetal casing of the appliance happens to touch the live wire or due to short circuiting or leakage of electric current.
\nWhenever an appliance which is earthed, get short circuited, the current from the metal casing of the appliance flows into the earth which acts as an ‘electric sink’, i.e., its potential always remains zero. Due to the flow of heavy current the fuse in that circuit melts and disconnects the appliance from the circuit.
\nSo the user who happens to touch the appliance is protected from receiving any electric shock. Another advantage is that due to overheating the house wiring system is saved from being damaged and same time from being burnt out.
\nProcedure:<\/strong> A three core, cord having three wires coated with insulation of red, brown and green colour, is used for connecting die appliance to the mains for drawing current from the mains. At one end of the cord, red is connected to the pin marked L (live), the brown to the pin marked N (neutral) and green to thick pin of the plug. The three wires at the other end of the cord are connected to the appliance such that live and neutral wires are connected to the element and the earth wire is connected to the metal body of the appliance so as to earth it.
\nOnce the plug is put in the socket, the current through the appliance becomes as soon as the switch is pressed. The live wire gets connected to the live wire of the mains. Neutral wire is connected to the neutral of the mains and the earth wire gets connected to the earth in the mains.<\/p>\nQuestion 2:<\/strong> State relative advantages and disadvantages of ring system of wiring over the tree system.
\nAnswer:<\/strong> Advantages:<\/strong>
\n(i) The tree system of wiring requires the plugs and sockets of different sizes for different current capacities while they are all of the same size in ring system of wiring.
\n(ii) The tree system of wiring has one fuse in one distribution line. If the fuse in one distribution line blows, it disconnects all the appliances in that distribution line. But in the ring system, a separate fuse is connected with each appliances. So if one fuse blows, it does not affect the other appliances.
\n(iii) The tree system of wiring is expensive while the ring system of wiring is cheaper.
\n(iv) It is expensive and inconvenient to install a new appliance in the tree system of wiring since it requires to put the new leads up to the distribution box, while it is easy to install a new appliance in the ring system since the appliance can be directly connected to ring mains in the room.
\nDisadvantages:<\/strong>
\nThe only disadvantage of ring system of wiring over the tree system is that while installing a new appliance, care has to be taken that the total load on the ring circuit does not exceed the main fuse rating (i.e., generally 30 amp.).<\/p>\nQuestion 3:<\/strong> An electric heater is rated 220 V, 550 W.
\n(i) What does this statement mean?
\n(ii) Calculate the electrical energy consumed in 3 hours?
\n(iii) Electrical energy is also measured in kWh. What do you understand by a kilo-watt-hour?
\nAnswer:<\/strong> (i) The statement means that the heater should be connected to 220 V Supply and that it consumes 550 W power.
\n
\n(iii) The energy consumed in a resistor when the potential difference is 1V and 1A current flows for 1000 hrs.<\/p>\nQuestion 4:<\/strong> (i) Two sets A and B, of three bulbs each, are glowing in two separate rooms. When one of the bulbs in set A is fused, the other two bulbs also cease to glow. But in set B, when one bulb fuses, the other two bulbs continue to glow. explain why this phenomenon occurs.
\n(ii) Why do we prefer arrangements of Set B for house circuiting?
\nAnswer:<\/strong> (i) In set A:<\/strong> Bulbs are in series so if one gets fused others are also affected. In series potential difference is variable, ‘I’ is constant.
\nIn set B:<\/strong> Bulbs are in parallel so if one goes off others continue to glow potential difference is constant, current varies inversely as resistance.
\n(ii) We prefer set B for household circuits as they are in parallel. Potential difference is constant (220 V) supplied by electricity department and current varies according to value of resistors (or) appliances used. If one circuit goes off, others continue to function.<\/p>\nQuestion 5:<\/strong> State the reason why, in a three pin plug, the earth pin is longer and thicker than the other two.
\nAnswer:<\/strong> The live terminal of the three pin plug helps us to connect our appliance to the live (or high voltage wire) or the ‘mains’. The neutral terminal connects the other end of the appliance to the ‘ground potential’ wire of the ‘mains’. The ‘earth’ terminal helps us to connect the body of the appliance to a wire whose other end is burned deep in moist earth. This ‘earthing’ of the body of the appliance acts as a safety measure and saves the user from any unwarranted dangerous shocks.<\/p>\n