Household Circuits: ICSE Class 10 Physics solutions, answers

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Get summaries, questions, answers, solutions, notes, extras, workbook solutions, PDF and guide of chapter 9 Household Circuits: ICSE Class 10 Physics which is part of the syllabus of students studying under the Council for the Indian School Certificate Examinations board. These solutions, however, should only be treated as references and can be modified/changed. 

Summary

Electric power is made in power stations, often far from our homes. It is usually generated at 11,000 volts. This voltage is then increased to a very high level, like 132,000 volts, for sending it over long distances. This is done because sending power at high voltage means less electricity is lost as heat in the wires. We use alternating current (AC) because its voltage can be easily changed up or down using devices called transformers.

Before the electricity reaches our homes, the high voltage is gradually lowered at different sub-stations. Finally, at a city sub-station, it is brought down to 220 volts, which is what our home appliances use. Electricity comes to our house through a cable that usually has three wires: a live wire, a neutral wire, and an earth wire. The live wire brings the current, and the neutral wire takes it back to complete the circuit. The earth wire is for safety.

Inside the house, first, the electricity passes through a company fuse and then a meter that measures how much energy we use. After the meter, there is a main switch and a main fuse or a small switch called an MCB. The main switch can turn off all power to the house, disconnecting both live and neutral wires. The main fuse protects the whole house circuit. From a distribution box, wires go to different parts of the house. Modern homes often use a ring system for wiring. In this system, wires for a part of the house start from the distribution box, go around the rooms, and come back, forming a ring.

All appliances like bulbs, fans, and sockets are connected in parallel to the mains. This means each appliance gets the full 220 volts and can be switched on or off independently without affecting others. Each appliance has its own switch and often its own fuse, connected in the live wire.

A fuse is a safety device. It’s a short, thin piece of wire made from a special metal that melts easily. If too much current flows (due to too many appliances on, or a fault called a short circuit), the fuse wire heats up, melts, and breaks the circuit. This stops the electricity and prevents damage or fire. Fuses have a current rating, telling how much current they can safely carry. Instead of fuses, many homes now use MCBs (Miniature Circuit Breakers). These are small switches that trip off automatically if too much current flows and can be easily reset.

Switches are always connected in the live wire. This is so when you switch an appliance off, it is completely disconnected from the power supply, making it safe. Earthing is another vital safety feature. The earth wire is connected to a metal plate buried deep in the ground and also to the metal bodies of appliances like fridges. If a fault makes the live wire touch the metal body of an appliance, the current flows safely to the earth through the earth wire, and the fuse blows, preventing electric shock. Three-pin plugs have a longer, thicker earth pin that connects to the earth wire first. Wires are color-coded for easy identification: Brown for live, Light Blue for neutral, and Green or Green/Yellow for earth. Always handle electricity with care.

Workbook Solutions(Concise/Selina)

Exercise (A)

MCQ

1. At the generating station, power is generated at :

(a) 33 kV
(b) 132 kV
(c) 11 kV
(d) none of the above

Answer: (c) 11 kV

2. The electric power is generated at 11 kV because :

(a) voltage higher than 11 kV causes insulation difficulties
(b) voltage lower than 11 kV involves a very high current
(c) it reduces the loss of energy in the form of heat
(d) both (a) and (b)

Answer: (d) both (a) and (b)

3. The power generated at the generating station is transmitted to the main sub station at voltage :

(a) 33 kV
(b) 11 kV
(c) 132 kV
(d) none of these

Answer: (c) 132 kV

4. The neutral and the earth wires are connected at the local sub station so that:

(a) the potential increases
(b) the neutral and earth wires are at the same potential
(c) the potential of neutral wire increases
(d) none of the above

Answer: (b) the neutral and earth wires are at the same potential

5. The main switch is a :

(a) single pole switch
(b) triple pole switch
(c) double pole switch
(d) can be any of these

Answer: (c) double pole switch

6. The main fuse is connected in:

(a) live wire
(b) neutral wire
(c) both the live and earth wires
(d) both the earth and neutral wires

Answer: (a) live wire

7. The electrical appliances in a house are connected in:

(a) series
(b) parallel
(c) either in series or parallel
(d) both in series and parallel

Answer: (b) parallel

8. The electric meter in a house records the consumption of:

(a) charge
(b) current
(c) energy
(d) power

Answer: (c) energy

9. A person connects his household appliances in series. Which of the following is true?

(a) Each appliance operates independently
(b) The current in the circuit is reduced as resistance increases
(c) Each appliance does not operate at its rated voltage
(d) Both (b) and (c)

Answer: (d) Both (b) and (c)

Very Short Answer Type Questions

1. At what voltage and frequency is the electric power generated at the power generating station?

Answer: At the power generating station, electric power is generated at 11 kV voltage and a frequency of 50 Hz.

2. At what (i) voltage and (ii) frequency is the a.c. supplied to our houses?

Answer: The a.c. supplied to our houses is at (i) 220 V voltage and (ii) 50 Hz frequency.

3. Name the device used to (a) increase the voltage at the generating station, and (b) decrease the voltage at the sub station for its supply.

Answer: The device used to (a) increase the voltage at the generating station is a step-up transformer, and (b) to decrease the voltage at the sub-station for its supply is a step-down transformer.

4. In what unit does the electric meter in a house measure the electrical energy consumed? What is its value in S.I. unit?

Answer: The electric meter in a house measures the electrical energy consumed in kWh. Its value in S.I. unit is 1 kWh = 3.6 × 10⁶ J.

5. Where is the main fuse connected in a house circuit?

Answer: The main fuse is connected in the live wire after the kWh meter and before the main circuit in a house. In the ring system, it is connected in the live wire before the ring system.

6. How should the several electric lamps be connected with the mains so that the switching on or off a lamp has no effect on the operation of other lamps?

Answer: The several electric lamps should be connected in parallel with the mains so that the switching on or off a lamp has no effect on the operation of other lamps.

Short Answer Type Questions

1. (a) At what voltage is the electric power from the generating station transmitted? Give reason to your answer.

Answer: (a) The electric power from the generating station 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 before its transmission. Its voltage is raised to reduce the loss of energy in the form of heat in the transmission line wires. From relation P = VI, for a given power P, current I = P/V 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.

(b) What is the nature of current transmitted from the power station?

Answer: (b) An alternating voltage of frequency 50 Hz is generated and transmitted from the power station.

2. In 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?

Answer: In 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 because its voltage is raised to reduce the loss of energy in the form of heat in the transmission line wires. From relation P = VI, for a given power P, current I = P/V 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.

3. (a) Name the three connecting wires used in a household circuit.

Answer: (a) The three connecting wires used in a household circuit are: (1) live (or phase) wire (L), (2) neutral wire (N), and (3) earth wire (E).

(b) Which of the two wires mentioned in part (a) are at the same potential?

Answer: (b) The neutral and the earth wires are connected together at the local sub-station so that the neutral and earth wires are at the same potential (i.e., at 0 V).

(c) In which of the wire stated in part (a), the switch is connected?

Answer: (c) The switch is connected in the live wire.

4. What is the pole fuse? Write down its current rating.

Answer: Before connecting the cable from pole to the meter in a house, first a fuse of high rating is connected in the live wire at the pole (or just before the meter). This fuse is called the company fuse (or pole fuse). The rating of the fuse depends on the load for which the connection is taken from the company (e.g. rating is 50 A for connection of load 10 kW). For the company fuse, it is 50 A.

5. State the function of each of the following in a house circuiting :
(a) kWh meter, (b) main fuse, and (c) main switch.

Answer:
(a) kWh meter: The kWh meter is connected after the company fuse and measures the electrical energy consumed in the house.
(b) Main fuse: The main fuse (or MCB) is connected in the live wire after the kWh meter. It protects the circuit by blowing off when current in the circuit exceeds the current carrying capacity of the fuse due to the circuit being either over loaded or short circuited.
(c) Main switch: The main switch (or ELCB) is connected in both the live and neutral wires after the kWh meter. It breaks the connections of the live as well as the neutral wires simultaneously from the main supply.

6. State one advantage of using the main switch in house wiring.

Answer: One advantage of using the main switch is that it breaks the connections of the live as well as the neutral wires simultaneously from the main supply.

7. Two 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?

Answer: (i) In set A, the bulbs are in series. When one bulb fuses, the circuit breaks, and all bulbs cease to glow. In set B, the bulbs are in parallel. When one bulb fuses, the other bulbs continue to glow because they remain connected to the mains independently.
(ii) The set B arrangement (parallel connection) is preferred in housing circuit. This is because each appliance gets connected to 220 V supply for its normal working, and each appliance operates independently without being affected by the presence or working of the other appliances.

Long Answer Type Questions

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

Answer: Electric power is generated at 11 kV at the power generating station. This alternating voltage is first stepped up from 11 kV to 132 kV at the generating station (grid sub-station) using a 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 a step-down transformer and from here it is transmitted to 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.

2. Draw a circuit diagram to explain the ring system of house wiring. State two advantages of it.

Answer: In the ring system, wires starting from the distribution box run around all the rooms of that portion and then come back to the distribution box, thus forming a ring. A separate connection is taken for each appliance from the live wire of the ring. One terminal of the appliance is connected to the live wire through a separate fuse and a separate switch, while the other terminal of the appliance is directly connected to the neutral wire. The earth terminal or metal covering of the appliance is connected to the earth wire.
Two advantages of the ring system are:

(1) In the ring system, the current from mains can reach to an individual appliance through two separate paths. Thus, each appliance gets connected to the mains effectively through a thick wire. Therefore, the wire required for main ring is of a lower current carrying capacity than that which would be required for a direct connection to the mains. This reduces the cost of wiring considerably.

(2) 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.

3. Draw a labelled diagram with the necessary switch, regulator, etc. to connect a bulb and a fan with the mains. In what arrangement will they be connected to the mains: series or parallel?

Answer: A bulb and a fan will be connected in parallel with the mains. Each will have a separate switch in the live wire. The fan will also have a regulator in series with its switch and the fan.

4. The figure 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?

Answer: (a) The bulb A is connected in parallel with the mains. It glows at 220 V.

(b) How are the bulbs B and C connected with the mains? At what voltage does the bulb B glow?

Answer: (b) The bulbs B and C are connected in series with each other, and this series combination is connected in parallel with the mains. If bulbs B and C are identical, the voltage across each bulb will be 110 V, so the bulb B glows at 110 V.

(c) How is the glow of bulbs A and C affected if bulb B gets fused?

Answer: (c) If bulb B gets fused, bulb A will continue to glow with no effect on its brightness as it is in parallel with the B-C combination. Bulb C will not glow because the series circuit of B and C will be broken.

(d) How is the glow of bulbs B and C affected if bulb A gets fused?

Answer: (d) If bulb A gets fused, the glow of bulbs B and C will be unaffected as they are in a separate parallel branch to bulb A. Both B and C will continue to glow as before (B at 110V, C at 110V, assuming they are identical).

Exercise (B)

MCQ

1. In a three pin plug, the longest pin is :

(a): live pin
(b) netural pin
(c) earth pin
(d) can be any

Answer: (c) earth pin

2. The purpose of earthing in a household circuit is to :

(a) provide a connection to appliance
(b) complete the circuit
(c) prevent electric shock
(d) increase the voltage

Answer: (c) prevent electric shock

3. A fuse wire permits the flow of current through it only up to a certain limit which is called :

(a) charge rating
(b) charge limit
(c) current rating
(d) none of the above

Answer: (c) current rating

4. The rise in temperature of fuse wire depends upon :

(a) current rating
(b) radius r
(c) length of wire
(d) both (a) and (b)

Answer: (d) both (a) and (b)

5. An alloy of lead and tin is used as the material of the fuse wire because its melting point is __________ and specific resistance is __________

(a) high, high
(b) low, high
(c) low, low
(d) high, low

Answer: (b) low, high

6. The full form of MCB is :

(a) miniature current breaker
(b) miniature current board
(c) miniature circuit breaker
(d) miniature circuit board

Answer: (c) miniature circuit breaker

7. The rating of a fuse connected in the lighting circuit is:

(a) 15 A
(b) 5 A
(c) 10 A
(d) zero

Answer: (b) 5 A

8. The use of MCB is more convenient than a fuse because:

(a) it avoids the inconvenience of connecting a new fuse wire
(b) it is much safer for quick response
(c) it takes a very long time for tripping
(d) both (a) and (b)

Answer: (d) both (a) and (b)

9. Assertion (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.

(a) both A and R are true and R is the correct explanation of A
(b) both A and R are True and R is not the correct explanation of A
(c) assertion is false but reason is true
(d) assertion is true but reason is false

Answer: (d) assertion is true but reason is false

10. Assertion (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.

(a) both A and R are true and R is the correct explanation of A
(b) both A and R are True and R is not the correct explanation of A
(c) assertion is false but reason is true
(d) assertion is true but reason is false

Answer: (d) assertion is true but reason is false

Very Short Answer Type Questions

1. Complete the following sentences :

(a) A fuse is a short piece of wire of high __________ and of material of low __________.

Answer: A fuse is a short piece of wire of high resistance and of material of low melting point.

(b) A fuse wire is made of an alloy of __________ and __________. If the current in a circuit exceeds the current rating of the fuse wire, it __________.

Answer: A fuse wire is made of an alloy of lead and tin. If the current in a circuit exceeds the current rating of the fuse wire, it melts.

(c) A fuse is connected in __________ with the __________ wire.

Answer: A fuse is connected in series with the live wire.

(d) Higher the current rating, __________ is the fuse wire.

Answer: Higher the current rating, thicker is the fuse wire.

(e) Live wire is also called __________ wire.

Answer: Live wire is also called phase wire.

2. Why is the fuse wire fitted in a porcelain casing ?

Answer: The fuse wire is fitted in a porcelain casing or holder because porcelain is an insulator of electricity, which makes the fuse arrangement safe to handle.

3. Two fuse wires are rated 5 A and 20 A. Which of the two is (i) thicker, (ii) longer ?

Answer:
(i) The 20 A fuse wire is thicker.
(ii) The length of the fuse wire does not depend on its current rating, so both may be of the same length for their respective holders.

4. (a) ‘A fuse is rated 8 A’. Can it be used with an electrical appliance of rating 5 kW, 200 V ?

Answer: No, it cannot be used. The appliance draws a current of I = P/V = 5000W / 200V = 25 A, which is much higher than the 8 A rating of the fuse. The fuse would blow instantly.

(b) Name two safety devices which are connected to the live wire of a household electric circuit.

Answer: Two safety devices connected to the live wire are a switch and a fuse (or MCB).

5. What is the colour code for the insulation on the (a) live, (b) neutral, and (c) earth wire ?

Answer: According to the new international convention, the colour codes are:
(a) Live wire: Brown
(b) Neutral wire: Light blue
(c) Earth wire: Green or Yellow (commonly Green)

6. Name the colour code of the wire which is connected to (i) metallic body of an appliance, (ii) switch for the appliance.

Answer:
(i) The metallic body of an appliance is connected to the earth wire, which is Green or Yellow (commonly Green).
(ii) A switch for the appliance is connected to the live wire, which is Brown (new convention) or Red (old convention).

Short Answer Type Questions

1. What is a fuse ? Name the material of fuse. State one characteristic of the material used for fuse.

Answer: An electric fuse is a safety device which is used to limit the current in an electric circuit. The use of a fuse safeguards the circuit and the appliances connected in that circuit from being damaged.

The material generally used for a fuse wire is an alloy of lead and tin.

One characteristic of this material is that it has a low melting point (≈ 250°C) or that it has high specific resistance.

2. Name the device used to protect the electric circuits from over loading and short circuit. On what effect of current does it work ?

Answer: The device used to protect electric circuits from overloading and short circuit is an electric fuse (or a Miniature Circuit Breaker, MCB).

A fuse works on the heating effect of current.

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

Answer: A fuse is connected in series with the live wire, before the appliance or circuit it is intended to protect. For example, a main fuse is connected in the live wire after the kWh meter and before the main circuit, and individual appliances have fuses connected in the live wire before them.
The purpose of using a fuse is to limit the current in an electric circuit. It safeguards the circuit and the appliances connected in that circuit from damage due to excessive current caused by overloading or short circuits.

4. A fuse is always connected in the live wire of the circuit. Explain the reason.

Answer: The fuse is always connected in the live wire before the appliance so that when an excessive current flows and the fuse melts, it breaks the circuit and disconnects the appliance from the live supply. This prevents the heavy current from reaching the appliance, ensuring no current flows in the appliance and it becomes safe. If the fuse were in the neutral wire, even if it blows, the appliance would remain connected to the high potential live wire, making it dangerous if touched or faulty.

5. How does the (i) thickness and (ii) length of a fuse wire depend on its current rating ?

Answer:
(i) The thickness of a fuse wire depends on its current rating. Higher the current rating, thicker is the fuse wire.
(ii) The rise in temperature of a fuse wire, and thus its operation, does not depend on the length of the wire. The length is determined by the design of the fuse holder.

6. Explain the meaning of the statement ‘the current rating of a fuse is 5 A’.

Answer: The statement ‘the current rating of a fuse is 5 A’ means that the fuse wire will permit the flow of current through it only up to a maximum of 5 Amperes. If the current in the circuit exceeds 5 A due to overloading or a short circuit, the fuse wire will heat up, melt, and break the circuit, thus stopping the flow of current.

7. Why is it not advisable to use a piece of copper wire as fuse wire in an electric circuit ?

Answer: It is not advisable to use a piece of copper wire as a fuse wire for two main reasons:

• Copper has a high melting point (≈ 1080°C), so it would not melt easily even when excessive current flows, failing to protect the circuit or appliance.
• Ordinary copper wire available is usually thick and has low resistance, meaning it can carry a high current without melting, thus not serving the purpose of a fuse which needs to melt at a specific lower current limit.

8. An electric 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.

Answer: Yes, the kettle can be used.

Reason: The current drawn by the kettle is I = P/V = 3000 W / 250 V = 12 A. A fuse should have a current rating slightly more than the maximum current drawn by the appliance. A 13 A fuse is suitably rated for an appliance drawing 12 A, as it will allow the normal operating current of 12 A to pass but will blow if a fault causes the current to significantly exceed this value, thereby protecting the kettle.

9. (a) A switch is not touched with wet hands while putting it on or off. Give a reason for your answer.

Answer: A switch should not be touched with wet hands because water is a conductor of electricity. If hands are wet, water can form a conducting layer between the hand and the live parts of the switch. This can lead to an electric current passing through the body to the earth, resulting in a severe or even fatal electric shock.

(b) Name the wire to which a switch is connected.

Answer: A switch is always connected to the live wire.

10. It is dangerous to connect the switch in the neutral wire. Explain your answer.

Answer: Connecting a switch in the neutral wire is dangerous because even when the switch is in the ‘off’ position, the appliance remains connected to the high potential terminal through the live wire. Although no current flows because the neutral path is broken, the appliance is still ‘live’. If someone touches the live parts of the appliance for repair, or if a fault causes the live wire to touch the appliance’s metallic body, they could receive a fatal electric shock as the current would pass through their body to earth. The appliance is not isolated from the high potential of the mains supply.

11. The diagram shows a three pin plug. Label the three pins.

(a) Why is the top pin thicker and longer than the other two?

Answer: The top pin, which is the earth pin, is made longer so that the earth connection is made before the live connection when the plug is inserted, and broken after the live connection when removed. This ensures safety if the appliance is faulty. It is made thicker so that it cannot be accidentally inserted into the live or neutral sockets of the outlet.

(b) Why are the pins splitted at the ends ?

Answer: The pins are often splitted at the ends to provide a spring action. This ensures that they fit tightly into the socket holes, making a good and firm electrical contact and preventing loose connections which can cause sparking.

For labelling the pins: The top pin is Earth (E), the pin on the left (conventionally, when looking at the pins with earth pin up) is Live (L), and the pin on the right is Neutral (N).

12. To which wire is the metallic case of an electric appliance connected ? Give the reason.

Answer: The metallic case of an electric appliance is connected to the earth wire.

Reason: This is done for safety. If, due to a fault, the live wire comes into contact with the metallic case, the case becomes live. If it is earthed, a heavy current flows from the live wire to the earth through the earth wire. This large current blows the fuse in the live wire, disconnecting the appliance from the mains supply and preventing a person who touches the appliance from receiving an electric shock.

13. (a) The earthing of an electric appliance is useful only if the fuse is in the live wire. Give the reason.

Answer: Earthing an appliance provides a path for fault current to flow to the ground if the live wire touches the metallic case. If the fuse is in the live wire, this large fault current will cause the fuse to blow, disconnecting the appliance from the live supply and making it safe. If the fuse were in the neutral wire, even if the fault current caused it to blow, the appliance would still be connected to the live wire. While earthing would prevent the case from attaining full live potential, the source of the fault (the live wire) would not be isolated, and the appliance’s internal components would remain live, posing a risk. Thus, for complete safety, both earthing and a correctly placed fuse (in the live wire) are essential.

(b) Name the part of the appliance which is earthed.

Answer: The outer metallic case (or metallic body) of the appliance is earthed.

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

Answer: Paint is generally an insulator of electricity. To ensure a good and effective electrical connection between the earth wire and the metallic body of the appliance, any paint or other insulating coating must be removed from the point of contact. This allows for a low-resistance path for the fault current to flow to the earth.

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

Answer: The colour coding of wires in a cable helps to easily identify the live, neutral, and earth wires. This is crucial for correct and safe house wiring, ensuring that switches, fuses, sockets, and appliances are connected to the proper wires in the circuit, thereby preventing electrical hazards and ensuring correct operation.

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

Answer: An earth wire is necessary in a power circuit because appliances connected to such circuits (like geysers, electric irons, refrigerators) often have metallic bodies that are handled physically and consume significant power. Earthing provides safety against electric shock if a fault occurs where the live wire touches the metallic casing.
While earthing is a general safety measure, the perceived risk might be lower for some lighting circuits if the fixtures have no exposed metallic parts or are inaccessible. However, modern safety standards increasingly recommend earthing for all circuits where possible. Earthing for appliances we handle physically, which are more common in power circuits.

17. Give two characteristics of a high tension wire.

Answer: Two characteristics of a high tension wire are:

• It has low resistance to allow heavy current to pass with minimal heating.
• It has a large surface area to radiate heat produced more quickly to the surroundings.

18. Which of the cables, one rated 5 A and the other 15 A will be of thicker wire ? Give a reason for your answer.

Answer: The cable rated 15 A will be of thicker wire.

Reason: A cable rated for a higher current (15 A) needs to carry that current with minimal resistance and heat generation. To achieve low resistance, the wire must have a larger area of cross-section. A larger area of cross-section means the wire is thicker. This allows it to safely carry the heavier current without overheating.

19. The 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.

Answer: (a) To light all three lamps, switches 2 and 3 must be closed.

(b) In this condition, the lamps are connected in series.

Long Answer Type Questions

1. Describe with the aid of a diagram some form of a fuse which is used in the electric lighting circuit of a house. Give two reasons why a fuse must not be replaced by an ordinary copper wire.

Answer: One common form of a fuse used in electric circuits is the porcelain cartridge fuse or a rewireable fuse with a porcelain holder. In a rewireable fuse, a short piece of fuse wire is stretched between two metallic terminals (T₁ and T₂) mounted in a porcelain holder. Porcelain is an insulator, making it safe. This holder fits into a porcelain socket which is connected in the live wire of the circuit. When current exceeds the safe limit, the fuse wire melts, breaking the circuit.

A cartridge type fuse consists of a fuse wire enclosed in a glass case with metal caps at both ends, which fits into a socket.

Two reasons why a fuse must not be replaced by an ordinary copper wire are:

• Ordinary copper wire has a very high melting point (around 1080°C). It will not melt easily even if a very high current flows through it, thus failing to protect the circuit and appliances from damage due to overheating or short circuit.
• Copper wire, especially if it’s thick, has low resistance. If used as a fuse, it would allow excessive current to pass without breaking the circuit, which defeats the purpose of a safety fuse. A proper fuse wire is designed to melt at a specific, relatively low temperature when its current rating is exceeded.

2. (a) What is the purpose of a switch in a circuit ?

Answer: The purpose of a switch in a circuit is to act as an on-off device. It is used to either complete or break an electrical circuit, thereby starting or stopping the flow of current to an appliance or a part of the circuit.

(b) Why is the switch put in the live wire ?

Answer: A switch is put in the live wire so that when it is in the ‘off’ position, the appliance is completely disconnected from the high potential live supply. This ensures that the appliance is at zero potential and safe to handle or repair. If the switch were in the neutral wire, turning it ‘off’ would stop the current, but the appliance would still be connected to the live wire and would remain at high potential, making it dangerous.

(c) What precaution do you take while handling a switch ?

Answer: The main precaution to take while handling a switch is to ensure that your hands are completely dry. A switch should never be touched with wet hands, as water can conduct electricity from the live parts of the switch to the hand, potentially causing an electric shock.

3. Draw the diagram of a dual control switch when the appliance is switched ‘ON’.

Answer: A dual control switch circuit for an appliance (like a bulb) in the ‘ON’ state would show a complete path for current from the live wire, through the first dual control switch (S₁), through the appliance (bulb), through the second dual control switch (S₂), and finally to the neutral wire. For example, if S₁ connects ‘b’ to ‘c’ and S₂ connects ‘c’ to ‘b’, and the bulb is between the ‘c’ of S₁ and ‘c’ of S₂, the circuit would be on.

4. Draw a circuit diagram using the dual control switches to light a staircase electric light and explain its working.

Answer: A circuit diagram for staircase wiring uses two dual control switches (say S₁ at the bottom of the stairs and S₂ at the top) to control a single light. The live wire is connected to the common terminal of S₁, and the neutral wire is connected to one terminal of the light. The other terminal of the light is connected to the common terminal of S₂. The two remaining terminals of S₁ are connected to the two corresponding terminals of S₂.
Working:

• Initially, let the light be ‘off’ (e.g., S₁ connects ‘ba’ and S₂ connects ‘cb’, breaking the circuit).
• A person going up operates S₁: S₁ changes its connection from ‘ba’ to ‘bc’. Now a path is formed: Live -> S₁ (bc) -> S₂ (cb) -> Light -> Neutral. The light turns ‘ON’.
• On reaching the top, the person operates S₂: S₂ changes its connection from ‘cb’ to ‘ca’. The path is broken. The light turns ‘OFF’.
  Similarly, if the light is ‘off’, operating either switch will turn it ‘on’, and if the light is ‘on’, operating either switch will turn it ‘off’. This allows independent control from two locations.

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

Answer: The terminals (pins) of a three-pin plug serve the following purposes:

• Earth Pin (E – top, longer and thicker): Connects the metallic body of the electrical appliance to the earth wire of the mains supply. This is a safety feature to prevent electric shock in case of a fault.
• Live Pin (L – usually on the left when viewed from the wiring side, or right hole in socket): Connects the appliance to the live wire of the mains supply, which is at a high potential (e.g., 220V).
• Neutral Pin (N – usually on the right when viewed from the wiring side, or left hole in socket): Connects the appliance to the neutral wire of the mains supply, which is at or near zero potential and completes the circuit for current flow.

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

Answer: A labelled diagram of a three-pin socket shows three holes corresponding to the pins of a three-pin plug.

• The top, larger hole is for the Earth connection (E).
• The hole on the right (when facing the socket) is for the Live connection (L).
• The hole on the left (when facing the socket) is for the Neutral connection (N).
  These holes have metallic terminals behind them connected to the earth, live, and neutral wires of the mains supply respectively.

7. The diagram shows a three-pin socket marked as 1, 2 and 3.

(a) Identify and write live (L), neutral (N) and earth (E) against the correct number.

Answer: For a standard three-pin socket :

• 1 (top hole) is Earth (E).
• 2 (left hole) is Neutral (N).
• 3 (right hole) is Live (L).

(b) To which part of the appliance is the terminal 1 connected ?

Answer: Terminal 1, which is the Earth (E) terminal of the socket, is connected (via the earth pin of the plug and the earth wire in the cable) to the metallic body or case of the appliance.

(c) To which wire joined to 2 or 3, is the fuse connected ?

Answer: The fuse is always connected in the live wire. Wire 3 corresponds to the Live (L) connection. Therefore, the fuse is connected to the wire joined to terminal 3.

8. What do you mean by the term local earthing ? Explain how it is done.

Answer: Local earthing refers to the process of creating an electrical connection from the earth terminal at the consumer’s premises (e.g., near the kWh meter) to the general mass of the Earth. This provides a low-resistance path for fault currents to flow safely to the ground.
How it is done:

• A hole, about 2-3 metres deep, is dug in the ground.
• A thick copper plate (e.g., 50 cm × 50 cm) is welded to the lower end of a copper rod (or thick copper wire) which is often covered by a hollow insulating pipe for protection.
• This copper plate is buried deep inside the hole.
• The area around the plate is filled with a mixture of charcoal and salt. This mixture helps to maintain good conductivity with the surrounding earth and absorbs moisture.
• Water is poured into the pit through the pipe from time to time to keep the ground damp, which further improves the conductivity of the earth connection.
• The upper end of the copper rod is then connected to the earth connection point in the house’s electrical system, typically near the kWh meter.

9. A power circuit uses a cable having three different wires.

(a) Name the three wires of the cable.

Answer: The three wires of the cable are the live wire (L), the neutral wire (N), and the earth wire (E).

(b) Between which of the two wires should the heating element of an electric geyser be connected ?

Answer: The heating element of an electric geyser should be connected between the live wire (L) and the neutral wire (N).

(c) To which wire should the metallic case of the geyser be connected ?

Answer: The metallic case of the geyser should be connected to the earth wire (E).

(d) To which wire should the switch and fuse be connected ?

Answer: The switch and the fuse should both be connected to the live wire (L).

10. State two circumstances when one may get an electric shock from an electric gadget. What preventive measure must be provided with the gadget to avoid it ?

Answer: Two circumstances when one may get an electric shock from an electric gadget are:

1. If the insulation of the wires inside the gadget is damaged or poor, and the live wire touches the metallic body of the gadget, making the body live.
2. Touching the appliance with wet hands, as water reduces the body’s resistance and provides a path for current.
   A key preventive measure that must be provided with a gadget, especially one with a metallic case, is proper earthing. The metallic case of the appliance must be connected to the earth wire. This ensures that if a fault occurs and the case becomes live, a heavy current flows to earth, blowing the fuse and disconnecting the supply, thus preventing an electric shock to the user.

11. The figure shows a dual control switch circuit use to light a bulb.

(a) Complete the circuit so that bulb is switched on.

Answer:

(b) Mark the supply terminals with L and N to indicate live and neutral wires.

Answer: The supply terminals feeding the dual control switch circuit should be marked as L (Live) and N (Neutral). The Live wire typically goes to one of the switches first, and the Neutral wire typically connects to one side of the appliance (bulb).

12. The figure 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.

Answer:

(b) Name and state the colour of insulation of each wire 1, 2 and 3.

Answer: Assuming standard connections for the plug connected to the socket:

• Wire 1, if connected to the neutral pin of the plug, is the Neutral wire. Its insulation colour (new convention) is Light Blue.
• Wire 2, if connected to the earth pin of the plug, is the Earth wire. Its insulation colour is Green or Yellow (commonly Green).
• Wire 3, if connected to the live pin of the plug, is the Live wire. Its insulation colour (new convention) is Brown.

(c) How are the two bulbs joined: in series or in parallel ?

Answer: If each bulb can be operated independently by its own switch, and if one bulb fusing or being switched off does not affect the other, then the two bulbs are joined in parallel with the mains supply.