Note:Before going to understand the Commercial Unit first understand electrical energy.

## Electrical Energy:

Electrical Power =Work is done by electrical current / Time takenAccording to the law of conservation of energy, Work is done by electrical current=Electrical energy consumed

so E=P x t

Unit of energy = Watt x hour or joule

- The electrical energy consumed by an electrical appliance is given by the product of its power rating and the time for which it is used.
- One Watt-Hour is the amount of electrical energy consumed when an electrical appliance of 1 Watt power is used for 1 hour.
- We will use the commercial unit of electrical energy in
**kilowatt-hour.**

### Commercial Unit Of Electrical Energy(Kilowatt-Hour):

The SI unit of electrical energy is the joule. Joule represents a very small quantity of energy, so for commercial purposes, we use a bigger unit of electrical energy which is called "kilowatt-hour".1 kilowatt-hour (1 Unit)=1 kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1kilowatt is used for 1 hour.

**Example =**Suppose 10 bulbs each one of rating is 100 watts run 1 hour then they consumed 1kwh mean

**1Unit of electricity.**

### The relation between kilowatt-hour (Unit) and joule:

1 Unit=1kilowatt-hour=1kilowatt for 1 hour=1000 watt for 1hour**Note:**1kilowatt-hour=3600000 joules ()

### How commercial Unit is measured and by which device:

- Kilowatt-hour is the "Unit" of electrical energy for which we pay to the electricity supply department of your state. One "Unit" of electricity costs anything maybe 4 to 10 rupees(lower in rural and higher in an urban area).
- By electricity meter, we found how many units we consumed in a day according to what load we use.
- If we multiply the Unit consumed on that day with the Unit cost then we can find out daily money consumed by you. Example: If that particular day your consumed Unit is 10 and the cost of per Unit in your area be 8 rupees then your cost that day will be=10 unit x8 rupee=80 rupee.

Electricity meter |

### A mixed question of S.Chand and NCERT of this topic:

**Q1:**-How much energy is consumed when a current of 5 amperes flows through the filament of a heater having a resistance of 100ohms for two hours? Express it in joules?

**Ans-**(convert hour in the second 2h=7200s)

**Q2.**An electric bulb is connected to a 220 v power supply line. If the bulb draws a current of 0.5A, calculate the power of the bulb.

**Ans-**P=Vx I = 220 x0.5 =110 w

**Q3.**In which of the following cases more electrical energy is consumed per hour?

**(I)**A current of 1 ampere passed through a resistance of 300 hours?

**(ii)**A current of 2 A passed through a resistance of 100 ohms?

**Ans- (a)**

**(b)**

so in case 2, you can see more energy is consumed

**Q3.**An electric kettle rated at 220 v, 2.2kw, works for 3 hours. Find the energy consumed and the current drawn.

**Ans-**E=P x t = 2.2kw x 3h =6.6 kwh

we know P=V x I =2.2 x 1000 w = 220 x I

so I=2200/220=10A

**Q4.**In the house, two 60w bulbs are lighted for 4 hours, and three 100w bulbs for 5 hours every day. Calculate the electric energy consumed in 30 days.

**Ans-**E1=n x P x t x days =2 x 60 x 4 x 30=14400wh=14.4kwh

E2=n x P x t x days =3 x 100 x5 x 30=45000wh=45kwh

Etotal=E1+E2=14.4+45=59.4kwh

**Note-**n= number of bulb

**Q5**a bulb is rated as 250v,0.4A. Find its (I) Power (ii) Resistance.

**Ans- (1)**P=V x I= 250 x 0.4 =100 w

**(2)**

R=100/0.16 =625 ohm

**For a heater rated at 4kw and 220 v, calculate.**

Q6

Q6

**the current**

(i)

(i)

**(ii)**the resistance of the heater

**(iii)**the energy consumed in 2 hours and

**(iv)**the cost if 1kwh is priced at 4.60 rupees.

**Ans- (1)**Given p=4kw(or 4000w )P= V x I =4kw=4000w=220 xI

so I=4000/220=18.18 A

**(2)**

so R =220 x 220/4000=48400/4000=12.1 ohm

**(3)**E=P x t = 4kw x2h =8kwh

**(4)**cost= no of unit x per unit price

(8kwh=8unit)

so cost= 8 x 4.60=36.80 rupees

**Q7.**An electric motor takes 5 A current from the 220 volt supply line, calculates the power of the motor and electrical energy consumed by it in 2 hours.

**Ans.**P= V x I=220 x 5 =1100 watt =1.1kw

E= P x t = 1.1kw x 2 h = 2.2kwh

**Q8.**Which uses more energy a 250w TV set in 1 hour or a1200w toaster in 10 minutes?

**Ans-(a)**(Energy for Tv Etv=P x t= 0.25kw x 1h=0.25kwh

**(b)**(Energy for toaster) Etostar = P x t =1.2kw x 1h =1.2kwh

so toaster takes more energy then TV

**Note=**1kw=1000w and 250w=0.25kw and 1200w=1.2kw

**Q9**.Calculate the power used in the 2ohm resister in each of the following circuits:

**(i)**A 6v battery in series with 1ohm and 2ohm resistor.

**(ii)**A 4v battery in parallel with 12 ohms and 2-ohm resister.

**Q10.**Two lamps, one rated 40w at 220v and the other 60w at 220 v are connected in parallel to the electric supply at 220 v

**(a)**Draw a circuit diagram to show the connections

**(b)**Calculate the total energy consumed by the two lamps together when they operate for one hour.

**(c)**Calculate the current drawn from the electric supply.

**Q.11.**An electric kettle connected to the 230 v mains supply draws a current of 10 A. Calculate

**(a)**the power of the kettle

**(b)**the energy transferred in 1 minute

**Q12.**A 2kw heater, a 200w TV and three 100w lamps are all switches on from 6p.m to 10 pm. What is the total cost at Rs. 5.50 per kWh?

**Q13.**What is maximum power in kilowatt of the appliance that can be connected safely to a 13 A,230 v mains socket?

**Q14.**An electric fan runs from the 230 v mains. The current flowing through it is 0.4A.at what rates electrical energy transferred by the fan?