Household Electricity

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

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

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

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

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

(f) One kilowatt hour is equal to 3.6 x 10⁶ joule.

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

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.

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.

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.

All electrical appliances are connected in parallel with the mains.

The diagram is shown below.

(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.

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.

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.

(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.

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.

electrical power

Reason — Electrical power is the rate of doing work.

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

1000 watt

Reason — 1 kilowatt = 1000 watt

tin-lead

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

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.

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.

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.

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.

The electricity comes to our homes from city substations.

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.

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.

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.

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.

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.

(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

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.

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.

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

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

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.

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

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.

The fuse wire is connected to the live wire.

Electricity is supplied to our houses at 220 volt.

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.

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.

The reading on the meter is 2789.

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

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.

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.

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.

(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.