Question 1

Question 1Explain with the aid of a simple diagram, the transmission of electric power from the generating station to your house.

Accepted Answer

The transmission of electric power from the generating station to our house is explained in the below diagram:The alternating voltage generated is first stepped up from 11 kV to 132 kV at the generating station (also called the grid sub station) using the step up transformer. It is then transmitted to the main sub station. At the main sub-station, the voltage is stepped down from 132 kV to 33 kV using a step-down transformer and is then transmitted to the heavy industries and intermediate sub stations. At the intermediate sub station, the voltage is again stepped down from 33 kV to 11 kV using the step-down transformer and from here it is transmitted to the light industries and city sub station. At the city sub station, it is further stepped down from 11 kV to 220 V using the step down transformer to supply it to the domestic consumers.

Question 2

Question 4Figure below shows three bulbs A, B and C each of rating 100 W, 220 V connected to the mains of 220 V. Answer the following:(a) How is the bulb A connected with the mains? At what voltage does it glow?(b) How are the bulbs B and C connected with the mains? At what voltage does the bulb B glow?(c) How is the glow of bulbs A and C affected if bulb B gets fused?(d) How is the glow of bulbs B and C affected if bulb A gets fused?

Accepted Answer

(a) Bulb A is connected with the mains in parallel connection. It glows at a voltage of 220 V.(b) Bulbs B and C are connected in series with the mains.As we know, the voltage of the source gets divided in all the appliances connected in series in ratio of their resistances.Hence, the voltage across the two bulbs B and C will be divided equally as both have same rating.Therefore, we get,VB=2202⇒VB=110VV_B = \dfrac{220}{2} \[0.5em] \Rightarrow V_B = 110 VVB​=2220​⇒VB​=110VHence, the bulb B will glow at a voltage of 110 V.(c) If bulb B gets fused then there will be no effect on bulb A, as the two are connected in parallel. However, bulb C will not glow because it is connected in series with the bulb B.(d) There is no effect on the glow of bulbs B and C if the bulb A gets fused as bulb A is connected in parallel with the bulbs B and C.

Question 3

Question 2In the transmission of power the voltage of power generated at the generating station is stepped up from 11 kV to 132 kV before it is transmitted. Why ?

Accepted Answer

The power generated at the generating station is not directly transmitted to the consumers at 11 kV, but before it's transmission, it's voltage is raised to 132 kV to reduce the loss of energy in the form off heat in the transmission line wires.From relation, P = VI, for a given power P, current I = PV\dfrac{P}{V}VP​ i.e., higher the voltage, lower is the current. Thus, by supplying a given electric power at a high voltage, the current becomes low and therefore the loss of energy due to heating (= I2Rt) in the line wires becomes less.

Question 4

Question 7Two sets A and B each of four bulbs are glowing in two separate rooms. When one of the bulbs in set A is fused, the other three bulbs also cease to glow. But in set B, when one bulb fuses, the other bulbs continue to glow.(i) Explain the difference in the two sets,(ii) Which set of arrangement is preferred in housing circuit and why?[Hint : In set A, the bulbs are in series; while in set B,the bulbs are in parallel ].

Accepted Answer

(i) In set A, the bulbs are in series connection. None of the bulb can be operated independently and thus when one bulb fuses the other two bulbs also ceases to glow. In set B, the bulbs are in parallel connection. Hence, when one of the bulbs fuses, the other bulbs continue to glow. Therefore, each bulb operates independently.(ii) In housing arrangement, parallel connection is preferred. Each appliance has a separate fuse. Therefore, if due to some fault, the fuse of one appliance burns, it does not affect the operation of the other appliances.

Question 5

Question 5What purpose is served by the terminals of a three way pin plug? Draw a diagram and name the pins.

Accepted Answer

In a three pin plug, the top pin is for earthing, the pin on the left is for live and the pin on the right is for neutral. In the good quality plugs, these are marked as E, L and N respectively.The earth pin is thicker and longer than the other two. The earth pin is made so long so that the earth connection is made first. This ensures the safety of the user because if the appliance is defective, then as soon as the live pin gets connected, the current passes to the earth and the fuse blows off.Further, the pin is made thicker so that even by mistake it cannot be inserted into the hole of live or neutral connection.The diagram of a three way pin plug with its pins labelled is shown below:

Question 6

Question 6Draw a labelled diagram of a three pin socket.

Accepted Answer

Labelled diagram of a three pin socket is shown below:It is a fixture in an electric circuit in which the plug is inserted. The socket has three holes. The upper bigger hole is for earth connection, while the hole on the right side is for connection to the live wire and the hole on the left side is for connection to the neutral wire of electric supply.

Question 7

Question 12Figure below shows two bulbs with switches and fuse connected to the mains through a three pin socket by means of a three wires cable.(a) Label each component — bulb, switch, fuse and socket.(b) Name and state the colour of insulation of each wire 1, 2 and 3.(c) How are the two bulbs joined : in series or in parallel.

Accepted Answer

(a) Labelled diagram is shown below:(b) The name and colour of wires are as follows:1 → neutral, blue colour.2 → earth, green colour.3 → live, brown colour.(c) The two bulbs are joined in parallel.

Question 8

Question 9Assertion (A): A fuse wire must have a high resistance and low melting point.Reason (R): A fuse is only used for a small current flow.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

assertion is true but reason is false.ExplanationAssertion (A) is true. A fuse wire must have a high resistance to generate enough heat when a high current flows through it, causing it to melt and break the circuit.Reason (R) is false. While fuses are used for protecting electrical circuits, they are not specifically used only for small current flow. Fuses are designed to handle normal operating currents but melt and break the circuit when the current exceeds a safe level, regardless of whether it is small or large.

Question 9

Question 10Assertion (A): Same current flows through the live wires and the filament of a bulb, but the heat produced in the filament is much higher.Reason (R): The filament of a bulb is made of low resistance and high melting point.Both A and R are true and R is the correct explanation of ABoth A and R are true and R is not the correct explanation of Aassertion is false but reason is trueassertion is true but reason is false.

Accepted Answer

assertion is true but reason is false.ExplanationAssertion (A) is true. Same current flows through the live wires and the filament of a bulb, but the heat produced in the filament is much higher. This is due to the higher resistance of the filament compared to the live wires, leading to greater heat generation in the filament.Reason (R) is false. The filament of a bulb is not made of a material with low resistance. In fact, the filament is specifically designed to have a relatively high resistance. This high resistance allows it to produce light efficiently by converting electrical energy into heat and then into light.

Question 10

Question 3How is a fuse put in an electric circuit? State the purpose of using a fuse in a circuit.

Accepted Answer

The fuse wire is stretched between the two metallic terminals T1 and T2 in a porcelain holder (since porcelain is an insulator of electricity). This holder fits into a porcelain socket having two metallic terminals to each of which the live wire of the circuit is connected. Thus the fuse wire gets connected in the live wire. The below diagram shows the fuse arrangement:The use of fuse is to safeguard the circuit and the appliances connected in that circuit from being damaged. It works on the heating effect of current. A material of low melting point is used so that it may easily melt due to overheating when current in excess to the prescribed limit passes through it.

Question 11

Question 8An electrical kettle is rated 3 kW, 250 V. Give reason whether this kettle can be used in a circuit which contains a fuse of current rating 13 A.

Accepted Answer

From relationI = PV\dfrac{P}{V}VP​Substituting the values we get,I=3000250I=12 AI = \dfrac{3000}{250} \[0.5em] I = 12 \text{ A}I=2503000​I=12 AHence, the safe limit of current for kettle is 12 ATherefore, yes, this kettle can be used in a circuit which contains a fuse of current rating 13 A as the safe limit of current for kettle is 12 A.

Question 12

Question 11The diagram in figure shows a three pin plug. Label the three pins.(a) Why is top pin thicker and longer than the other two?(b) Why are the pins splitted at the ends?

Accepted Answer

In a three pin plug, the top pin is for earthing, the pin on the left is for live and the pin on the right is for neutral. In the good quality plugs, these are marked as E, L and N respectively.(a) The earth pin is thicker and longer than the other two. The earth pin is made long so that the earth connection is made first. This ensures the safety of the user because if the appliance is defective, then as soon as the live pin gets connected, the current passes to the earth and the fuse blows off.Further, the pin is made thicker so that even by mistake it cannot be inserted into the hole of live or neutral connection.(b) The pins are splitted at the ends to provide a spring action so that they fit in the socket holes tightly.

Question 13

Question 13(a) The earthing of an electric appliance is useful only if the fuse is in the live wire. Given the reason.(b) Name the part of the appliance which is earthed.

Accepted Answer

(a) The earthing of an electric appliance is useful only if the fuse is in the live wire because in case the current in the circuit exceeds the rating of the fuse, it may melt and break the circuit first, before the current reaches the appliance.It is unsafe to connect the fuse in the neutral wire because if due to some defect in the appliance an excessive current flows in the circuit, but appliance still remains connected to the high potential point of supply through the live wire. Now if a person touches the faulty appliance, he gets an electric shock because the person comes in direct contact of the mains through the live wire. Thus, it is highly unsafe to use fuse in the neutral wire.(b) The metallic case of the appliance should be properly earthed.

Question 14

Question 14For earthing an electrical appliance, one has to remove the paint from the metallic body of the appliance where the electrical contact is made. Explain the reason.

Accepted Answer

The paint provides an insulating layer on the metal body of the appliance. Therefore, in order to make the earth connection, it is necessary to remove paint from the body part where connection is to be made.

Question 15

Question 15How does the colour code of wires in a cable help in house wiring?

Accepted Answer

The colour coding of wires help us to connect the switch, fuse, sockets, etc. through proper wire in the circuit of house wiring.

Question 16

Question 16Why is it necessary to have an earth wire installed in a power circuit, but not in a lighting circuit?

Accepted Answer

The appliances connected to the power circuit like air conditioner, geyser, washing machine, etc. consume heavy current. If due to some reason such as short circuiting, an excessive current flows through the power circuit, not only the appliance will get damaged but due to the excessive heat produced by such high current, the wires may get overheated causing a fire. If the earth wire is installed, it conducts the excessive current to the earth thus avoiding the fire.The current carrying capacity of lighting circuit is low (less than 5A). In case of a short circuit, the chances of fire due to overheating of wires are little. Hence, generally earth wire is not installed in a lighting circuit.

Question 17

Question 17Give two characteristics of a high tension wire.

Accepted Answer

A high tension wire has low resistance and large surface area. The resistance is kept low so as to allow a heavy current to pass through it.The surface area is made large so that it radiates the heat produced more quickly to the surrounding.

Question 18

Question 18Which of the cables, one rated 5 A and the other rated 15 A will be of thicker wire? Give a reason for your answer.[Hint :To carry heavy current, the resistance of wire should be low, so its area of cross section should be large].

Accepted Answer

The thicker wire is the one which is rated 15 A.In order to carry heavy current, the resistance of wire should be low, so it's area of cross section should be large. Therefore, as large current is passing through the second wire hence, it will be thicker.

Question 19

Question 19The diagram in figure shows three lamps and three switches 1, 2 and 3 connected with two cells.(a) Name the switch/switches to be closed so as to light all three lamps.(b) How are then the lamps connected : in series or in parallel.

Accepted Answer

(a) In order to light all the three lamps switch 2 and 3 should be closed.(b) The lamps are then connected in series.

Household Circuits

Exercise 9A Long Questions

Exercise 9A Short Questions

Exercise 9B Long Questions

Exercise 9B Multiple Choice Questions

Exercise 9B Short Questions