# Frank Solutions for Chapter 4.2 Electrical Power and Household Circuits Class 10 Physics ICSE

## Exercises

1. Define electric power. What do you mean by 'power rating' of an electric appliance?
Electrical power is defined as the rate at which energy is changed or work is done i.e., the work done per second or energy converted per second.
Electrical appliances come with certain number of watts and volts written on them. Power rating of an electrical appliance gives the knowledge of the safe current limit of an appliance and also indicates the voltage above which the appliance should not be used.

2. Distinguish between electric power and electrical energy. State their units. Give the other name of kWh.
Electrical energy is defined as the total amount of work done to maintain current in a circuit in a given time. Its SI unit is joule.
The rate at which electrical energy is consumed per unit time is called electrical power. It is denoted by P. i.e., Its SI unit is watt.
Electrical power = Electrical energy/time
The other name of kwh is board of Trade Unit (BOTU).

3. Fig. 13 represents the circuit used for the verification of ohm's law. Label the different parts from A and F. State the function of each.

Functions:
(A) Cell - It provides the potential difference in the circuit.
(B) Key - It serves as a switch in the circuit. It supplies or cuts off current as required.
(C) Ammeter- It measures the current in the circuit.
(D) Rheostat - It helps to change the resistance of the circuit without changing its voltage.
(E) Resistor - It provides a constant resistance in the circuit.
(F) Voltmeter - It measure the potential drop across the resistor.

4. Explain the meaning of the statement 'the current rating of a fuse is 5 A'.
'The current rating of a fuse is 5 A' means that the maximum safe current permitted to flow through it before it breaks down is 5A.

5. 'A fuse is rated 8 A'. Can it be used with an electrical appliance of rating 5 kW, 200 V?
We know that,
P = VI
∴ I = P/V = (5×1000)/200 = 25 A
Thus, safe limit of current through the appliance is 25 A and a fuse of rating BA cannot be used with an appliance of rating 5kw, 200 V.

6. An electric kettle is rated 3 kW, 250 V. Give reason whether this kettle can be used in a circuit which contains a 13 A fuse.
We know that,
P = VI
∴ I = P/V = (3×1000)/250 = 12 A
Thus, it can be used in a circuit which contains a 13 A fuse.

7. Calculate the electric current supplied by a 12 V battery of an automobile to a starter that develops a power of 2.4 kW.
We know that.
P = VI
∴ I = P/V = (2.4×1000)/12 = 200  A
The current supplied by the battery is 200A.

8. A 720 W heater operates from a 220 V line voltage. Calculate:
(a) The electrical energy consumed by the heater in 3 h.
(b) The cost of electrical energy consumption at the rate of 60 paise per kWh.
Given, power P = 720 W = 0.72 kW
Voltage, V = 220 volt
(a) Electrical energy consumed in 3 H = 0.72×3 = 2.16 kWh
(b) Cost of energy consumption @ 60 paise per kWh = 2.16× Rs 0.6 = Rs 1.3

9. What is fuse? Name the material of fuse.
An electric fuse is a safety device which is used to limit the current in an electric circuit. The use of a fuse thus safeguards the circuit and appliances connected in that circuit from being damaged.
A fuse is generally made of an alloy of lead and tin.

10. State two characteristics of a fuse wire.
Characteristics of fuse wire:
1. It should have low melting point.
2. It should have high resistance.

11. Complete the following sentences:
(i) A fuse is a short piece of wire of high ......... and low
(ii) Fuse wire has a . Melting point and it is made of an alloy of ......... and if the current
in a circuit rises too high, the fuse wire
(iii) A fuse is connected in (parallel/series) with the ......... (earth/neutral/live) wire.
(i) resistance, melting point
(iii) series, live

12. Calculate the amount of energy transferred by a 10 A electric current flowing through a resistor of 5 â„¦ for 15 minutes.
As per Joule's law,
H = I2Rt
Here, I = 10 A, R = 5â„¦ and t = 15 mins = 900s
∴ H = (10)2×5×900 = 4.5× 10

13. A 200 v electric transfer draws an electric current of 11 A. What is its electrical resistance?
A/C to Ohm's law,
V = IR
Here, V = 200 volt and I = 11 A
∴ R = V/I = 200/11 = 18.2 â„¦

14. A heater is rated at 1200 W when the electric current is 5 A. Calculate the resistance and the potential difference.
We know that,
P = VI = I2R
Given, P = 1200 watt, I = 5A

15. Where is the main fuse in a house circuit connected?
The main fuse is connected between the kWh meter and distribution board of the house circuit. The main fuse is connected in the live wire.

16. Is kWh the unit of electrical energy?
Yes, kWh is the unit of electrical energy.

17. What is the purpose of a switch in a circuit? Why is the switch put in the live wire?
A switch is an on-off device for current in a circuit (or in an appliance). Its main function is either to connect or to disconnect an electrical appliance in an electric circuit.
Switches are always connected in the live wire of circuits so that when a switch is in 'off' position, no current flows through the appliance and its live and neutral wires are at same potential and it is safe to touch the live wire leading to the appliance, even when the fuse is blown.

18. A switch is not touched with wet hands while putting it on or off. Give a reason for your answer.
A switch should not be touched with wet hands. This is because water forms a conducting layer between the hand and the live wire of the switch through which the current passes to the hand and we may get a fatal shock.

19. An electric bulb is rated 250 W, 230 V. What information does this convey? Calculate the current through a 60 W lamp rated for 250 V. If the line voltage falls to 200 V, how is the power consumed by the bulb affected?
'An electric bulb is rated 250 W - 230 V' ; means that if the bulb is lighted on a 230 V supply it consumes 250 W electrical power or 250 J of electrical energy converts into heat and light in 1 second.
The safe limit of current through the bulb is :

20. The metal case of an electric appliance is earthed. Explain the reason.
The metal case of an electrical appliance is earthed so that in any case of accidental contact of live wire with the metallic body of the appliance, the earthing would provide a safe and easy path for the electric charges to flow down to the earth which acts as very large sink. Thus, user is thereby protected from any fatal electric shock.

21. A bulb marked 12 V, 24 W operates on a 12 volt car battery for 20 minutes. Calculate:
(i) The current flowing through it, and
(ii) The energy liberated.

22. What is international convention of colour coding in a cable?
International convention of colour coding:

1. Live wire - Brown
2. Neutral wire - Light blue
3. Earth wire - Green or yellow

23. A current of 0.2 A flows through a wire whose ends are at a potential difference of 15 V. Calculate:
(i) The resistance of the wire, and
(ii) The heat energy produced in 1 minute.
Given, current, I = 0.2 A; p.d = 15 V
(i) A/C to Ohm's law
V = IR
∴ R = V/I = 15/0.2 = 75 â„¦
(ii) energy produced in one minutes is :

24. What should be the current rating of a fuse to be used with an electric appliance of rating 1.5 kW-220 V? How is the fuse connected with the appliance?
Given, P = 1.5 k W = 1500 W, p.d. = 220 V
We know that,
P = VI
∴ I = P/V = 1500/220 = 6.8 A
Thus a fuse of rating 7 A may be used with the appliance and it should be connected in the live wire before connecting it to the appliance.

25. Explain briefly how a fuse protects an electric circuit.
The fuse helps to control the maximum current in an electrical circuit. The fuse protects an electrical circuit by melting and breaking the circuit, whenever the current exceeds the pre-determined limit in the circuit. Due to low melting point, the fuse wire melts and then breaks the circuit.

26. Why is earthing done?
Earthing is done to save an electrical circuit or an appliance from damage. If due to some reason such as short circuiting, an excessive current flows through the line wires, it will pass to the earth if there is local earthing, otherwise it may cause a fire due to overheating of line wires.

27. Draw a labeled diagram of a three pin socket.

28. Under what circumstances does one get an electric shock from an electrical gadget?
One may get an electric shock from an electrical appliance if accidentally the live wire comes in contact with the metallic case of the body of the appliance due to break of insulation after constant use (or otherwise).

29. State the S.I. units of electrical power and electrical energy.
(i) Define the household unit of electricity.
(ii) What is the voltage of the electricity that is generally supplied to a house?
The SI unit of electrical energy is joule.
The SI unit of electrical power is watt.
(i) Kilowatt-hour is the household unit of electrical energy. It is defined as the quantity of electrical energy consumed in 1 hour when the rate of consumption is 1000 watts, i.e. 1000 J/s.
(ii) A voltage of 220 volt is generally supplied to a house.

30. An electric bulb is rated at 220 V, 100 W. What is its resistance? What maximum current can be passed through it?
Given, P = 100 W. p.d. = 220 V
We know that,

Thus a fuse of rating 7A may be used with the appliance and  it should be connected in the live wire before connecting it to the appliance.

31. Two resistors of 4 â„¦ and 6 â„¦ are connected in parallel. The combination is connected across a 6 volt battery of negligible resistance. Calculate :
(i) The power supplied by the battery,
(ii) The power  dissipated in each resistor.

32. What purpose is served by the terminals of a three way pin plug?
A three pin-plug has three terminals:
1. Earth pin: It provides connection for earthing.
2. Neutral pin: It provides connection to the neutral wire.
3. live pin: It provides connection to the live wire.

33. Name the unit which a consumer pays as the cost of electricity.
Consumer pays the bill in kilowatt-hour.

34. A battery of e.m.f. 15V and internal resistance 2 â„¦ is connected to two resistors of 4 ohm and 6 ohm joined.
(i) In series,
(ii) In parallel. Find in each case the electrical energy spent per minute in 6 ohm resistor.

35. State the commercial unit of electrical energy.
Kilowatt-hour is the commercial unit of electricity.

36. Write different expressions for electric power (P), using V, I and R.

37. What do you mean by 'power-voltage' rating of electrical appliances?
Power-voltage rating of an electrical appliance indicates the value of voltage above which the appliance should not be used. It serves as a precaution to the customer and also helps to calculate the maximum value of current that can be safely passed through the appliance.

38. Name the physical quantity which equals the product of voltage and current.
The physical quantity is electric power.

39. The marking on an electric bulb reads; 200 V, 60 W. What does it mean?
It means that if the bulb is lighted on a 200V supply it consumes 60W electrical power or 200J of electrical energy converts into heat and light in 1 second.

40. The resistance of 100 W (220 V) bulb is greater than that of 60 W (220 V) bulb. Is it true or false?
We know that, R = V2/P
Thus, voltage remaining constant, resistance is inversely proportional to the power  of the appliance; which means resistance of 60 W (220V) bulb is greater.
Hence, the statement is false.

41. What will be the current drawn by an electric bulb of 40 W when it is connected to a source of 220 V?
We know that, P = VI
∴ I = P/V = 40/220 = 0.11 A

42. What do you mean by watt-hour?
Watt-hour is the commercial unit of electrical energy.
One watt-hour is the electrical energy consumed by an electrical appliance of power 1 watt when it is used for 1 hour.
1Wh = 3600 J

43. Name the commercial unit of electrical energy.
Kilowatt-hour is the commercial unit of electricity.

44. A radio set of 60 W runs for 50 hours. How much electrical energy is consumed? Give your answer in kWh.