Class 10 Chapter 2 Magnetic Effect of Electric Current Physics Solution S.Chand. Part 6

 Part 6 Page 113

Very Short Answer Type Questions

1. What name is given to the device which automatically cuts off the electricity supply during short-circuiting

in household wiring ?

The device which automatically cuts off the electricity supply during short-circuiting in household wiring is called a circuit breaker or fuse.

2. What is the usual capacity of an electric fuse used (i) in the lighting circuit, and (ii) in the power circuit, of

a small house ?

• (i) The usual capacity of an electric fuse used in the lighting circuit of a small house is typically between 5 to 10 amperes.
• (ii) The usual capacity of an electric fuse used in the power circuit of a small house is typically between 15 to 20 amperes.

3. Give the symbol of an electric fuse used in circuit diagrams.

The symbol of an electric fuse used in circuit diagrams is represented by a rectangle with a diagonal line through it, indicating where the fuse would be placed in the circuit.

4. State whether the following statements are true or false :

(a) A wire with a green insulation is usually the live wire.

(b) A miniature circuit breaker (MCB) works on the heating effect of current.

• (a) False. A wire with a green insulation is typically used for the earth or ground wire, not the live wire.
• (b) False. A miniature circuit breaker (MCB) works on the principle of electromagnetic or thermal trip mechanisms, not solely on the heating effect of current.

5. Alongwith live wire and neutral wire, a third wire is also used in domestic electric wiring. What name is

given to this third wire ?

The third wire used in domestic electric wiring, along with the live wire and neutral wire, is called the earth wire or ground wire.

6. List the colours of the three wires in the cable connected to the plug of an electric iron.

The colours of the three wires in the cable connected to the plug of an electric iron typically are:

• Live wire: Brown
• Neutral wire: Blue
• Earth wire: Green and yellow (or just green)

7. What is the electric potential of the neutral wire in a mains supply cable ?

The electric potential of the neutral wire in a mains supply cable is approximately zero volts. It acts as a return path for current flow and is connected to the earth at the distribution panel.

8. If fuses of 250 mA, 500 mA, 1 A, 5 A and 10 A were available, which one would be the most suitable for

protecting an amplifier rated at 240 V, 180 W ?

To protect an amplifier rated at 240 V, 180 W, the most suitable fuse would be the one closest to the rated current of the appliance. Using the formula $\text{Current}=\frac{\text{Power}}{\text{Voltage}}$, we can calculate the current: $\text{Current}=\frac{180\text{W}}{240\text{V}}=0.75\text{A}$ Thus, the 1 A fuse would be the most suitable for protecting the amplifier.

9. When does an electric short circuit occur ?

An electric short circuit occurs when there is an unintended path of low resistance between the live and neutral wires, causing excessive current flow. This often results in a surge of current, overheating, and potentially causing damage to the circuit or appliances.

10. In which wire in an A.C. housing circuit is the switch introduced to operate the lights ?

In an AC housing circuit, the switch is introduced in the live wire to operate the lights.

11. In household circuits, is a fuse wire connected in series or in parallel ?

In household circuits, a fuse wire is connected in series with the circuit. It acts as a safety device to protect the circuit from excessive current flow.

12. Usually three insulated wires of different colours are used in an electrical appliance. Name the three colours.

The three colours of insulated wires typically used in an electrical appliance are:

• Live wire: Red
• Neutral wire: Blue
• Earth wire: Green and yellow (or just green)

13. What is the usual colour of the insulation of : (a) live wire, (b) neutral wire, and (c) earth wire ?

The usual colour of the insulation of:

• (a) Live wire: Brown
• (b) Neutral wire: Blue
• (c) Earth wire: Green and yellow (or just green)

14. What is the main purpose of earthing an electrical appliance ?

The main purpose of earthing an electrical appliance is to provide a safe path for the leakage or fault current to flow into the ground, thus preventing electric shock or fire hazards.

15. Give two reasons why different electrical appliances in a domestic circuit are connected in parallel.

Two reasons why different electrical appliances in a domestic circuit are connected in parallel are:

• Each appliance operates independently, so if one appliance fails, it does not affect the operation of other appliances.
• Each appliance receives the full mains voltage, ensuring consistent performance regardless of the number of appliances connected.

16. How should the electric lamps in a building be connected so that the switching on or off in a room has no

effect on other lamps in the same building ?

The electric lamps in a building should be connected in parallel so that the switching on or off in a room has no effect on other lamps in the same building. In a parallel connection, each lamp has its own separate path to the power source, ensuring independent operation.

17. Fill in the following blanks with suitable words :

(a) A fuse should always be placed in the live wire of a mains circuit. (b) The earth wire should be connected to the metal casing of an appliance.

Short Answer Type Questions

18. (a) Of what substance is the fuse wire made ? Why ? (b) Explain why, a copper wire cannot be used as a fuse wire.

(a) The fuse wire is typically made of a material with a low melting point, such as an alloy of lead and tin. This is because the purpose of a fuse is to protect the circuit from excessive current flow by melting when the current exceeds a certain limit. When the fuse wire melts, it interrupts the circuit, preventing further flow of current and protecting the circuit and appliances from damage.

(b) Copper wire cannot be used as a fuse wire because copper has a high melting point and excellent conductivity. It would not melt at the desired current level, thus failing to provide the protective function of a fuse. Additionally, copper's high conductivity would result in a very low resistance, allowing a large amount of current to flow through the circuit before the wire would melt, defeating the purpose of using a fuse.

19. What type of electric fuse is used in electrical appliances like car stereos ? Explain with the help of a labelled diagram.

Electrical appliances like car stereos typically use a type of electric fuse called a blade fuse. It is a flat, rectangular-shaped fuse designed to fit into a fuse holder. Here's an explanation with the help of a labelled diagram:

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       |   |                |       |    |

       |   |                |       |    |

       |   |   Fusible      |       |    |

       |   |   Link         |       |    |

       |   |                |       |    |

       |   |----------------|       |    |

       |   |                            |

       |   |                            |

       |   |----------------------------|

       |________________________________|

The blade fuse consists of a fusible link made of a material that melts when the current exceeds the rated value. When the current becomes too high, the fusible link melts, breaking the circuit and protecting the appliance from damage.

20. Distinguish between the terms ‘overloading’ and ‘short-circuiting ’ as used in domestic circuits.

• Overloading: Overloading occurs when the total electrical load connected to a circuit exceeds the circuit's rated capacity. It often leads to overheating of wires, appliances, or circuit components, posing a fire hazard.
• Short-circuiting: Short-circuiting occurs when there is an unintended path of low resistance between the live and neutral wires, causing excessive current flow. It often results in a surge of current, overheating, and potentially causing damage to the circuit or appliances.

21. (a) When does a fuse cut off current ? How does it do it ? (b) What is the maximum number of 60 W bulbs that can be run from the mains supply of 220 volts if you do not want to overload a 5 A fuse ?

(a) A fuse cuts off current when the current flowing through it exceeds its rated capacity. It does so by melting the fuse wire, thereby breaking the circuit and interrupting the flow of current. This action protects the circuit and appliances from damage due to overcurrent.

(b) To calculate the maximum number of 60 W bulbs that can be run from a mains supply of 220 volts without overloading a 5 A fuse, we use the formula $\text{Power}=\frac{\text{Voltage}\times \text{Current}}{\text{Number of bulbs}}$.

• $\text{Power}=\frac{220\text{V}\times 5\text{A}}{60\text{W}}\approx 18.33$
• Therefore, the maximum number of bulbs is $\frac{220\times 5}{60}\approx 18$. So, you can run a maximum of 18 bulbs without overloading the fuse.

22. Explain the importance of using in a household electric circuit (i) fuse, and (ii) earthing wire.

(i) Fuse: A fuse is important in a household electric circuit because it acts as a safety device to protect the circuit and appliances from damage due to overcurrent. It interrupts the flow of current when it exceeds the rated capacity, thereby preventing overheating and potential fire hazards.

(ii) Earthing wire: The earthing wire is important in a household electric circuit because it provides a safe path for the leakage or fault current to flow into the ground, preventing electric shock or fire hazards. It ensures that any excess current is safely dissipated into the earth, protecting both people and appliances from harm.

23. (a) An electric iron is rated at 230 V, 750 W. Calculate (i) the maximum current, and (ii) the number of units of electricity it would use in 30 minutes. (b) Which of the following fuse ratings would be suitable for this electric iron ? 1 A, 3 A, 5 A, 13 A

(a) (i) To calculate the maximum current: $\text{Current}=\frac{\text{Power}}{\text{Voltage}}$ $\text{Current}=\frac{750\text{W}}{230\text{V}}$ $\text{Current}\approx 3.26\text{A}$

(ii) To calculate the number of units of electricity used in 30 minutes: First, convert the time to hours: $30\text{minutes}=\frac{30}{60}\text{hours}=0.5\text{hours}$

Then, calculate the energy consumed: $\text{Energy}=\text{Power}\times \text{Time}$ $\text{Energy}=750\text{W}\times 0.5\text{hours}=375\text{Wh}$

Now, convert watt-hours (Wh) to units of electricity: $1\text{unit}=1\text{kWh}=1000\text{Wh}$ $\text{Number of units}=\frac{375\text{Wh}}{1000\text{Wh/unit}}=0.375\text{units}$

(b) The suitable fuse rating for this electric iron would be 5 A.

24. What is the function of an earth wire ? Why is it necessary to earth the metallic bodies of electrical appliances ?

(a) The function of an earth wire is to provide a path for fault current to flow safely into the ground in case of a fault or leakage in the electrical appliance or circuit. It helps protect against electric shock by ensuring that any excess current is safely discharged into the earth, preventing the metallic parts of appliances from becoming live and reducing the risk of electrical hazards.

(b) It is necessary to earth the metallic bodies of electrical appliances to prevent electric shock in case of a fault. If a fault occurs and the metallic body becomes live, touching it could result in a dangerous electric shock. Earthing the metallic body ensures that any leakage or fault current is safely directed to the ground, minimizing the risk of electric shock.

25. (a) What current is taken by a 3 kW electric geyser working on 240 V mains ?

(b) What size fuse should be used in the geyser circuit ?

(a) To calculate the current taken by a 3 kW electric geyser working on 240 V mains: $\text{Power}=\text{Voltage}\times \text{Current}$ $3000\text{W}=240\text{V}\times \text{Current}$ $\text{Current}=\frac{3000\text{W}}{240\text{V}}$ $\text{Current}=12.5\text{A}$

(b) The size of the fuse should be slightly higher than the maximum current drawn by the electric geyser to provide protection. Therefore, a 15 A fuse would be suitable for the geyser circuit.

26. (a) Why are fuses fitted in the fuse box of a domestic electricity supply ?

(b) What device could be used in place of the fuses ?

(a) Fuses are fitted in the fuse box of a domestic electricity supply to protect the electrical circuits and appliances from overcurrent and short circuits. They act as sacrificial devices that melt and break the circuit when the current exceeds the rated capacity, preventing overheating and potential fire hazards.

(b) Instead of fuses, miniature circuit breakers (MCBs) could be used in place. MCBs offer similar protection to fuses but have the advantage of being resettable after they trip. They automatically switch off the electrical circuit when they detect an overcurrent or short circuit, and they can be easily reset by toggling the switch back on after the fault has been cleared.

Long Answer Type Question

27. (a) Draw a labelled diagram to show the domestic electric wiring from an electric pole to a room. Give the

wiring for a bulb and a three-pin socket only.

(b) State two hazards associated with the use of electricity.

(c) State the important precautions which should be observed in the use of electricity.

(d) What will you do if you see a person coming in contact with a live wire ?

(e) Explain why, electric switches should not be operated with wet hands.

(a)

(b) Two hazards associated with the use of electricity are:

• Electric shock: Direct contact with live wires or faulty appliances can result in electric shock, which can be severe and even fatal.
• Fire hazard: Overloaded circuits, faulty wiring, or overheated appliances can lead to electrical fires, posing a significant hazard to life and property.

(c) Important precautions to observe in the use of electricity include:

• Ensuring all electrical installations are done by qualified professionals.
• Using electrical appliances and tools that are properly grounded and insulated.
• Avoiding overloading electrical circuits and using appropriate fuses or circuit breakers.
• Regularly inspecting electrical wiring and appliances for signs of damage or wear.
• Keeping electrical appliances away from water sources and ensuring hands are dry when handling them.

(d) If you see a person coming in contact with a live wire, you should:

• Immediately switch off the power supply to the affected area if possible.
• Call for emergency medical assistance.
• If it is safe to do so, use a non-conductive object such as a wooden stick to separate the person from the live wire.
• Do not attempt to touch the person directly with your hands to avoid getting an electric shock yourself.

(e) Electric switches should not be operated with wet hands because water is a good conductor of electricity. Wet hands increase the risk of electric shock when operating switches, as the water provides a path for electrical current to flow through the body. This can lead to severe injury or even death. Therefore, it is essential to ensure hands are dry before handling electrical switches to minimize the risk of electric shock.

Multiple Choice Questions (MCQs)

Questions Based on High Order Thinking Skills (HOTS)

38. An air-conditioner of 3.2 kW power rating is connected to a domestic electric circuit having a current rating

of 10 A. The voltage of power supply is 220 V. What will happen when this air-conditioner is switched on ?

Explain your answer.

When the air-conditioner with a power rating of 3.2 kW (or 3200 W) is connected to a domestic electric circuit with a current rating of 10 A and a voltage of 220 V, and it is switched on, there are a few possible outcomes:

1. If the air-conditioner operates efficiently and does not exceed its rated power, it will function normally without any issues. In this scenario, the current drawn by the air-conditioner can be calculated using the formula:

$\text{Power}=\text{Voltage}\times \text{Current}$

$3200\text{W}=220\text{V}\times \text{Current}$

$\text{Current}=\frac{3200\text{W}}{220\text{V}}$

$\text{Current}\approx 14.55\text{A}$

Since the current drawn (14.55 A) is higher than the circuit's rated current (10 A), this could lead to overloading of the circuit.

2. If the air-conditioner draws a current higher than the circuit's rated capacity (10 A), it will cause the circuit to trip. This occurs because the circuit's protective device, such as a fuse or circuit breaker, detects the excessive current and interrupts the flow of electricity to prevent overheating and potential fire hazards. As a result, the air-conditioner will not function until the circuit is reset or repaired.

In summary, when the air-conditioner is switched on, it may function normally if it operates within the rated power and current limits of the circuit. However, if it draws excessive current, it can overload the circuit, leading to a potential circuit trip and interruption of power supply to the air-conditioner.

39. Three appliances are connected in parallel to the same source which provides a voltage of 220 V. A fuse

connected to the source will blow if the current from the source exceeds 10 A. If the three appliances are

rated at 60 W, 500 W and 1200 W at 220 V, will the fuse blow ?

To determine if the fuse will blow, we need to calculate the total power consumption of the three appliances when connected in parallel and check if it exceeds the maximum current rating of the fuse.

Given:

• Voltage (V) = 220 V
• Maximum current rating of the fuse (I) = 10 A

Appliance 1:

• Power (P1) = 60 W
• Voltage (V) = 220 V

Appliance 2:

• Power (P2) = 500 W
• Voltage (V) = 220 V

Appliance 3:

• Power (P3) = 1200 W
• Voltage (V) = 220 V

Total power consumption (Pt) of the three appliances connected in parallel: $𝑃𝑡=𝑃1+𝑃2+𝑃3$ $𝑃𝑡=60\text{W}+500\text{W}+1200\text{W}$ $𝑃𝑡=1760\text{W}$

Now, to calculate the current (I) drawn by the appliances: $𝐼=\frac{𝑃}{𝑉}$

$𝐼=\frac{1760\text{W}}{220\text{V}}$

$𝐼\approx 8\text{A}$

Since the total current drawn by the appliances (8 A) is less than the maximum current rating of the fuse (10 A), the fuse will not blow.

40. A vacuum cleaner draws a current of 2 A from the mains supply.

(a) What is the appropriate value of the fuse to be fitted in its circuit ?

(b) What will happen if a 13 A fuse is fitted in its circuit ?

(a) To determine the appropriate value of the fuse to be fitted in the vacuum cleaner's circuit, we need to choose a fuse with a rating slightly higher than the maximum current drawn by the appliance. Since the vacuum cleaner draws a current of 2 A, an appropriate value for the fuse would be slightly higher than this to allow for any temporary surges in current. Typically, a 3 A fuse would be suitable for this circuit.

(b) If a 13 A fuse is fitted in the circuit of the vacuum cleaner, it would not provide adequate protection. The fuse is rated much higher than the maximum current drawn by the appliance (2 A). In this scenario, the fuse would not blow even if there was a fault or excessive current draw, potentially leading to overheating of the circuit and posing a fire hazard. Additionally, the vacuum cleaner's circuit may not be designed to handle such high currents, and using a fuse with a rating much higher than necessary could damage the appliance or its components. Therefore, fitting a 13 A fuse in the vacuum cleaner's circuit is not recommended.

41. Which of the following circuits will still be dangerous even if the fuse blows off and electric iron stops

working during a short circuit ?

42. An electric kettle rated as 1200 W at 220 V and a toaster rated at 1000 W at 220 V are both connected in

parallel to a source of 220 V. If the fuse connected to the source blows when the current exceeds 9.0 A, can

both appliances be used at the same time ? Illustrate your answer with calculations.

To determine if both appliances can be used at the same time without blowing the fuse, we need to calculate the total current drawn by both appliances when they are connected in parallel and compare it to the maximum current allowed by the fuse.

Given:

• Electric kettle power (P1) = 1200 W
• Toaster power (P2) = 1000 W
• Voltage (V) = 220 V
• Maximum allowed current by the fuse (I_max) = 9.0 A

Using Ohm's Law $𝑃=𝐼𝑉$, we can find the current drawn by each appliance:

For the electric kettle: ${𝐼}_1=\frac{{𝑃}_1}{𝑉}=\frac{1200\text{W}}{220\text{V}}\approx 5.45\text{A}$

For the toaster: ${𝐼}_2=\frac{{𝑃}_2}{𝑉}=\frac{1000\text{W}}{220\text{V}}\approx 4.55\text{A}$

Now, let's find the total current drawn by both appliances when they are connected in parallel: ${𝐼}_{\text{total}}={𝐼}_1+{𝐼}_2$ ${𝐼}_{\text{total}}=5.45\text{A}+4.55\text{A}=10\text{A}$

The total current drawn by both appliances (10 A) exceeds the maximum allowed current by the fuse (9.0 A). Therefore, both appliances cannot be used at the same time without blowing the fuse.

43. What is the main difference in the wiring of an electric bulb and a socket for using an electric iron in a

domestic electric circuit ? What is the reason for this difference ?

The main difference in the wiring of an electric bulb and a socket for using an electric iron in a domestic electric circuit lies in the current rating and the type of wiring used.

1. Electric Bulb Wiring:

   • Electric bulbs are typically connected to the circuit using a simple parallel wiring arrangement.
   • The wiring for an electric bulb usually consists of a live wire (phase wire) and a neutral wire.
   • The live wire is connected to the center terminal (bottom) of the bulb holder, while the neutral wire is connected to the outer threaded part (shell) of the holder.
   • The reason for this setup is that electric bulbs require relatively low power and draw lower currents compared to appliances like electric irons. Therefore, they do not require heavy-duty wiring or special provisions for high current handling.

2. Socket for Using an Electric Iron:

   • Sockets for using electric irons or other high-power appliances are equipped with a more robust wiring setup to handle the higher currents required by these appliances.
   • They typically consist of three wires: live wire (phase wire), neutral wire, and earth wire.
   • The live wire and neutral wire provide the standard electrical connection, while the earth wire provides an additional safety measure, especially for appliances with metallic bodies.
   • The wiring used for sockets of high-power appliances like electric irons is designed to handle higher currents and is often thicker and more robust compared to the wiring used for electric bulbs.
   • The reason for this difference is to ensure safety and to prevent overheating or electrical hazards when operating high-power appliances like electric irons. The thicker wiring and additional earth connection help to dissipate heat effectively and minimize the risk of electrical faults or fires.

44. (a) Explain why, it is more dangerous to touch the live wire of a mains supply rather than the neutral wire.

(b) Why is it safe for birds to sit on naked power lines fixed atop tall electric poles ?

(a) It is more dangerous to touch the live wire of a mains supply compared to the neutral wire due to the difference in potential and the presence of voltage. Here's why:

• Live Wire: The live wire carries the current from the power source and is at a high electrical potential (usually around 230 volts in residential circuits). It alternates between positive and negative potentials, providing the electrical energy for devices to operate.
• Neutral Wire: The neutral wire completes the circuit and provides a return path for the current to flow back to the source. It is connected to the earth and is typically at or close to earth potential (zero volts).

When a person touches the live wire:

1. Since the human body is a conductor of electricity, current flows through the body to the ground.
2. The high voltage of the live wire can cause electric shock, which can lead to severe injuries or even death, depending on the magnitude of the current and the duration of exposure.

Touching the neutral wire, on the other hand, is safer because it is at or close to earth potential. While there may still be a small risk of shock if there is a fault in the circuit, the severity is significantly lower compared to touching the live wire.

(b) It is safe for birds to sit on naked power lines fixed atop tall electric poles because birds do not provide a path to ground, and they do not come into contact with a potential difference between the power line and the ground. Here's why:

• Insulation: Power lines are typically made of materials that provide electrical insulation, such as rubber or plastic coatings. These insulating materials prevent the flow of current from the power line to the ground.
• No Potential Difference: Birds sitting on power lines do not create a complete circuit between the power line and the ground. Since they are not grounded and do not touch any other conductive objects with a different potential, there is no potential difference for current to flow through them.
• High Resistance Path: Even if a bird were to provide a path to ground, the high resistance of its body would limit the flow of current to negligible levels, preventing electric shock.

Overall, birds sitting on power lines are safe because they do not complete a circuit and are not subject to a potential difference that would cause electric current to flow through them.

45. A domestic lighting circuit has a fuse of 5 A. If the mains supply is at 230 V, calculate the maximum number

of 36 W tube-lights that can be safely used in this circuit.

To calculate the maximum number of 36 W tube lights that can be safely used in the domestic lighting circuit with a 5 A fuse and a mains supply of 230 V, we need to determine the total power consumption of the tube lights and then divide it by the maximum current allowed by the fuse.

Given:

• Power rating of each tube light (P) = 36 W
• Maximum current allowed by the fuse (I_max) = 5 A
• Mains supply voltage (V) = 230 V

First, let's calculate the total power consumption of one tube light: $𝑃=𝐼𝑉$ $𝑃=36\text{W}$

Now, let's calculate the current drawn by one tube light: $𝐼=\frac{𝑃}{𝑉}$ $𝐼=\frac{36\text{W}}{230\text{V}}$ $𝐼\approx 0.1565\text{A}$

Now, let's calculate the maximum number of tube lights that can be safely used in the circuit: $\text{Maximum number of tube lights}=\frac{{𝐼}_{\text{max}}}{{𝐼}_{\text{per tube light}}}$ $\text{Maximum number of tube lights}=\frac{5\text{A}}{0.1565\text{A}}$ $\text{Maximum number of tube lights}\approx 32$

Therefore, the maximum number of 36 W tube lights that can be safely used in this circuit is approximately 32.