Chapter 4 - Current Electricity - Electrical Power and Household Circuits Excercise 201
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.
Distinguish between electric power and electrical energy. State their units. Give the other name of kWh.
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.
'A fuse is rated 8 A'. Can it be used with an electrical appliance of rating 5 kW, 200 V?
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.
Calculate the electric current supplied by a 12 V battery of an automobile to a starter that develops a power of 2.4 kW.
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.
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.
State two characteristics of a fuse wire.
Characteristics of fuse wire:
(i) It should have low melting point.
(ii) It should have high resistance.
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
(ii) low, lead, tin, melts.
(iii) series, live
Chapter 4 - Current Electricity - Electrical Power and Household Circuits Excercise 202
A 200 v electric transfer draws an electric current of 11 A. What is its electrical resistance?
A heater is rated at 1200 W when the electric current is 5 A. Calculate the resistance and the potential difference.
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.
Is kWh the unit of electrical energy?
Yes, kWh is the unit of electrical energy.
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.
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.
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?
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.
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.
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
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.
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?
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.
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.
Draw a labeled diagram of a three pin socket.
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).
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.
An electric bulb is rated at 220 V, 100 W. What is its resistance? What maximum current can be passed through it?
What purpose is served by the terminals of a three way pin plug?
A three pin-plug has three terminals:
(i) Earth pin: It provides connection for earthing.
(ii) Neutral pin: It provides connection to the neutral wire.
(iii) live pin: It provides connection to the live wire.
Name the unit which a consumer pays as the cost of electricity.
Consumer pays the bill in kilowatt-hour.
State the commercial unit of electrical energy.
Kilowatt-hour is the commercial unit of electricity.
Write different expressions for electric power (P), using V, I and R.
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.
Name the physical quantity which equals the product of voltage and current.
The physical quantity is electric power.
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.
The resistance of 100 W (220 V) bulb is greater than that of 60 W (220 V) bulb. Is it true or false?