Household Electricity

Objective Questions

Question 1

Write true or false for each statement:

(a) A fuse wire has a high melting point.

(b) Flow of protons constitutes electric current.

(c) A fuse wire is made of silver.

(d) S.I. unit and commercial unit of electrical energy are same.

(e) Overloading of electric current in circuits can lead to an electrical short circuit.

(f) Our body can pass electricity through it.

(g) The metallic cases of all appliances are insulators of electricity.

(h) The earth wire protects us from an electric shock.

(i) A switch should not be touched with wet hands.

(j) All electrical appliances in a household circuit work at the same voltage.

(k) In a cable, the green wire is the live wire.

(l) A fuse is connected to the live wire.

(m) A switch is connected to the neutral wire.

Answer

(a) False
Correct Statement — A fuse wire has a low melting point.

(b) False
Correct Statement — Flow of electrons constitutes electric current.

(c) False
Correct Statement — A fuse wire is made of an alloy of lead and tin.

(d) False
Correct Statement — S.I. unit and commercial unit of electrical energy are different. S.I. unit of electrical energy is joule(J) and commercial unit is kilowatt hour(kWh).

(e) True

(f) True

(g) False
Correct Statement — The metallic cases of all appliances are conductors of electricity.

(h) True

(i) True

(j) True

(k) False
Correct Statement — In a cable, the green wire is the Earth wire.

(l) True

(m) False
Correct Statement — A switch is connected to the live wire.

Question 2

Fill in the blanks:

(a) The unit in which we pay the cost of electricity is ............... .

(b) The electrical energy consumed in a house is measured by ............... .

(c) In a household electrical circuit, the appliances are connected in ............... with the mains.

(d) A switch is connected to the ............... wire.

(e) The insulated wire in red color in a cable is the ............... wire.

(f) One kilowatt hour is equal to ............... joule.

(g) A fuse wire should have low ............... .

Answer

(a) kWh

(b) kWh meter

(c) parallel

(d) live

(e) live

(f) 3.6 x 10⁶

(g) melting point

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

All wires used in electric circuits should be covered with:

1. Coloring material
2. Conducting material
3. an insulating material
4. none of the above

Answer

an insulating material

Reason — All wires used in electric circuits should be covered with an insulating material to protect from short circuit and to protect people from electric shock.

Question 4b

Select the correct alternative:

Electrical work done per unit time is:

1. electrical energy
2. electric current
3. electric voltage
4. electrical power

Answer

electrical power

Reason — Electrical power is the rate of doing work.

Electrical Power = $\dfrac{\text{Work done}}{\text{time taken}}$

Question 4c

Select the correct alternative:

One kilowatt is equal to:

1. 100 watt
2. 1000 watt
3. 10 watt
4. none of these

Answer

1000 watt

Reason — 1 kilowatt = 1000 watt

Question 4d

Select the correct alternative:

Fuse wire is an alloy of:

1. tin-lead
2. copper-lead
3. tin-copper
4. lead-silver

Answer

tin-lead

Reason — Fuse wire is an alloy of lead and tin having 50% of each metal to have a low melting point.

Question 4e

Select the correct alternative:

A fuse wire should have:

1. a low melting point
2. high melting point
3. very high melting point
4. none of the above

Answer

a low melting point

Reason — A fuse wire should have low melting point so that when a large amount of current pass through it, the wire will melt and does not allow excess current to pass thus protecting the appliance or circuit from getting damaged.

Question 4f

Select the correct alternative:

When switch of an electrical appliance is put off, it disconnects:

1. the live wire
2. the neutral wire
3. the earth wire
4. the live and the neutral wires

Answer

the live wire

Reason — When switch of an electrical appliance is put off, it disconnects the live wire because switch is connected in live wire.

Question 4g

Select the correct alternative:

The purpose of an electric meter in a house is:

1. to give the cost of electricity directly
2. to give the consumption of electrical energy
3. to safeguard the circuit from electrical short circuit
4. to put on or off the mains.

Answer

to give the consumption of electrical energy

Reason — An electric meter measures the amount of electrical energy consumed in that house over a given period of time.

Question 4h

Select the correct alternative:

If out of the two lighted bulbs in a room, one bulb suddenly fuses, then:

1. the other bulb will glow more
2. the other bulb will glow less
3. the other bulb will also fuse
4. the other bulb will remain lighted and unaffected.

Answer

the other bulb will remain lighted and unaffected

Reason — The two bulbs are connected in parallel connection so that if one bulb gets fused other will glow without being affected. In household electrical circuits, all appliances are connected in parallel connection.

Short/Long Answer Questions

Question 1

From where does electricity come to our homes?

Answer

The electricity comes to our homes from city substations.

Question 2

What is an electric meter? Where is it fixed in our house?

Answer

An electric meter measures the amount of electric energy consumed by the consumer in the unit kWh for which the electricity bill is paid by him to the electricity board.
Electric meter is connected after the company fuse to the electric cable coming from the pole to the house. It is usually mounted on the front or outside wall of the house.

Question 3

State the purpose of kWh meter.

Answer

The purpose of kWh meter is to measure the amount of electric energy consumed by the consumer in kWh for which the electricity bill is paid by him to the electricity board.

Question 4

For which unit do we pay our electricity bill?

Answer

Electrical energy is generally measured in a unit called B.O.T (Board of Trade) unit or kWh (kilowatt hour) for which the electricity bill is paid.

Question 5

How can you check just by seeing the meter whether electricity is in use or not?

Answer

If electricity is in use, the electric current flows which rotates the armature of the meter. The counter fixed on the armature reads the number of rotations.

Question 6

Figure shows the reading on the dials of a meter. State what is its reading.

Answer

The reading on the meter is 2789.

Question 7

One day the meter reading was 7643 units while on the following day, it was 7657 units. What is the consumption of electricity in a day?

Answer

Initial reading = 7643 units

Final reading = 7657 units

Consumption = Final reading - Initial reading
= 7657 - 7643
= 14 units

So, consumption of electricity in one day is 14 units.

Question 8

A source of potential difference V volt sends current I ampere in a circuit for time t second. Write expressions for

(a) electrical energy supplied by the source, and

(b) electrical power spent by the source.

Answer

(a) Potential difference = V volt
Current = I ampere
time = t sec
Electrical energy supplied W = VIt

(b) Power is the rate of doing work.

P = $\dfrac{\text{W}}{\text{t}}$ = $\dfrac{\text{VIt}}{\text{t}}$ = VI

P = VI

Question 9

Name the unit in which you pay the cost of your electricity bill. How is it related to joule?

Answer

We pay the cost of our electricity bill in unit kWh (kilowatt hour).

Since,

$\text{Power} = \dfrac{\text{Energy}}{\text{Time}} \\[1em] \therefore \text{Energy} = \text{Power} \times \text{Time} \\[1em] \text{i.e. } \text{Energy (in kWh)} \\[1em] = \text{Power (in kWh)} \times \text{Time (in h)}$

Hence,

1 kWh = 1 kW x 1 h

= 1000 W x (60 x 60 s)

= 36,00,000 J

= 3.6 x 10⁶ J

So 1 kWh = 3.6 x 10⁶ J.

Question 10

If an appliance of power P watt is used for time t hour, how much electrical energy is consumed in kWh?

Answer

Power = P watt or P x 10^-3 kW

time = t hour

Power = $\dfrac{\text{Energy}}{\text{Time}}$

Energy = Power x time
= P x 10^-3 kW x t h
= P x t x 10^-3 kWh

Electrical energy consumed = Pt x 10^-3 kWh.

Question 11

What is an electric fuse? State its purpose in the household electrical circuit?

Answer

An electric fuse is a device which is used to limit the current in an electric circuit. It safeguards the circuit and appliances connected in the circuit from being damaged if the current in the circuit exceeds the specified value due to voltage fluctuations or short circuiting.

Question 12

State one property of the material of a fuse wire.

Answer

Fuse wire is a short wire with a low melting point. It melts at about 200°C.

Question 13

Name the material of a fuse wire.

Answer

Fuse wire is an alloy of lead and tin, having 50% of each metal.

Question 14

Can we use copper wire as a fuse wire? Give reason.

Answer

No, we cannot use copper wire as fuse wire as melting point of copper is very high.

Question 15

How does a fuse protect the electric wiring (or an appliance) from being damaged?

Answer

During short circuiting or voltage fluctuations maximum current passes through the wires causing burning of the wires. To prevent this damage a fuse is connected to live wire of the circuit. When there is a short circuit, the fuse wire gets heated up to the extent that it melts. As a result, a gap is produced in the live wire and the circuit breaks. No current then flows and electrical appliance or circuit is protected from being damaged.

Question 16

Which fuse wire is thick: 5 A or 15 A?

Answer

15 A fuse wire is thicker than the 5 A fuse wire because the thickness of fuse wire depends on the current rating of it. Higher the rating, thicker is the wire.

Question 17

Write the full form of M.C.B.

Answer

The full form of M.C.B. is miniature circuit breaker.

Question 18

How is M.C.B. superior to the fuse wire?

Answer

M.C.B. is superior to the fuse wire in the following ways:

1. An M.C.B. switches off the circuit in a very short time (nearly 25 ms) as compared to a fuse wire which takes relatively longer to melt and break the circuit.
2. M.C.B. can be reset (i.e., raised up) after tripping, whereas fuse wire needs to be replaced after it has been tripped.
3. M.C.B. offers greater protection, flexibility, and safety compared to fuse wire.

Question 19

With which wire: live or neutral is the fuse wire connected?

Answer

The fuse wire is connected to the live wire.

Question 20

What do you mean by short circuiting of a circuit?

Answer

If the insulation on the wire of the cable breaks, the live wire touches the neutral wire which results in a short circuit. Similarly, due to voltage fluctuations, sometimes high current flows through the wires that can cause burning of wires leading to short circuit.

Question 21

Figure shows two ways of connecting the three bulbs A, B and C to a battery. Name the two arrangements. Which of them do you prefer to use in a household circuit? Give a reason to support your answer.

Answer

Figure (a) is series arrangement and Figure (b) is parallel arrangement.

Parallel arrangement is generally preferred to use in a household circuit because of the following reasons :

1. Each appliance works independently. If one appliance in the circuit stops working, it doesn't affect the working of other appliances.
2. Each appliance has an independent path for current and works at the same voltage.
3. Each appliance can have a separate switch connected to its live wire.

Question 22

How are the electrical appliances connected in a house circuit: in series or in parallel? Give reason.

Answer

The electrical appliances are connected in parallel in a house circuit due to following reasons :

1. Each appliance works independently. If one appliance in the circuit stops working, it doesn't affect the working of other appliances.
2. Each appliance has an independent path for current and works at the same voltage.
3. Each appliance can have a separate switch connected to its live wire.

Question 23

In the household electric circuit, if one bulb is fused in a room, the other bulbs keep glowing. Explain the reason.

Answer

The electrical appliances are connected in parallel in a household circuit such that each appliance has an independent path for current to pass through. So if one bulb is fused in a room, the other bulb keeps glowing.

Question 24

State the voltage at which electricity is supplied to our houses.

Answer

Electricity is supplied to our houses at 220 volt.

Question 25

Draw a labelled diagram with the necessary switches to connect a bulb, a fan and a plug socket in a room with the mains. In what arrangement will you connect them to the mains?

Answer

All electrical appliances are connected in parallel with the mains.

The diagram is shown below.

Question 26

State the colour coding of the three wires in a cable used for wiring in a household electrical circuit.

Answer

In the cable there are three wires live, neutral and earth wire. Live wire is red or brown. Neutral wire is black or light blue. Earth wire is green or yellow.

Question 27

Why is the metal covering of an electrical appliance earthed?

Answer

The metal covering of an electrical appliance is connected to the earth wire to protect from electric shock. Sometimes due to breaking of insulation of wires, live wires come in contact with the body of the appliance which can cause a fatal shock when touched. If the appliance is earthed, the current will pass to the earth and the person remains protected from getting any electrical shock.

Numericals

Question 1

An electrical appliance is rated as 60 W - 150 V.

(a) What do you understand by this statement?

(b) How much current will flow through the appliance when in use?

Answer

(a) If an electrical appliance is rated as 60 W - 150 V, it means if the appliance is put on a 150 V supply, it will consume 60 W electrical power (i.e., it will consume 60 J of electrical energy in 1 s).

(b) P = 60 W
V = 150 V

We know P = VI

∴ I = $\dfrac{\text{P}}{\text{V}}$ = $\dfrac{\text{60}}{\text{150}}$ = 0.4 A

So, current of 0.4 A will flow through the appliance when in use.

Question 2

An electric iron of power 1.5 kW is used for 30 minutes to press clothes. Calculate the electrical energy consumed in:

(a) kilowatt hour

(b) joule

Answer

Power (P) = 1.5 kW

time (t) = 30 minutes = $\dfrac{\text{1}}{\text{2}}$ hr

(a) Electrical energy consumed in kWh= P x t
= 1.5 x $\dfrac{\text{1}}{\text{2}}$
= 0.75 kWh

So electrical energy consumed in kWh = 0.75 kWh

(b) 1 kWh = 3.6 x 10⁶ J

0.75 kWh = 0.75 x 3.6 x 10⁶

= 2.7 x 10⁶ J

So electrical energy consumed in joule = 2.7 x 10⁶ J.

Question 3

Assuming the electric consumption per day to be 12 kWh and the rate of electricity to be Rs 6.25 per unit, find how much money is to be paid in a month of 30 days?

Answer

Electric energy consumed per day = 12 kWh
Electric energy consumed for 30 days = 12 x 30 = 360 kWh

1 kWh = 1 unit
360 kWh = 360 units
1 unit cost = Rs 6.25
360 units cost = 360 x 6.25 = Rs 2250

So money to be paid for 30 days = Rs 2250.

Question 4

In a premise 5 bulbs each of 100 W, 2 fans each of 60 W, 2 A.Cs each of 1.5 kW are used for 5 h per day. Find:

(a) total power consumed per day,

(b) total power consumed in 30 days,

(c) total electrical energy consumed in 30 days,

(d) the cost of electricity at the rate of Rs 6.25 per unit.

Answer

(a) Power consumed by 5 bulbs = 5 x 100 = 500 W
Power consumed by 2 fans = 2 x 60 = 120 W
Power consumed by 2 A.Cs = 2 x 1.5 = 3 kW or 3 x 1000 = 3000 W
Total power consumed per day = 500 + 120 + 3000 = 3620 W

Total power consumed per day = 3620 W.

(b) Power consumed for 30 days = 30 x 3620 = 108600 W or 108.6 kW

Power consumed for 30 days = 108.6 kW.

(c) Power consumed in 30 days = 108.6 kW
time = 5 h

Electrical energy consumed in 30 days = Power x time
= 108.6 x 5
= 543 kWh

Electrical energy consumed in 30 days = 543 kWh.

(d) Cost of 1 unit = Rs 6.25
Cost of 543 units = 6.25 x 543 = Rs 3393.75

Cost of electricity = Rs 3393.75.