Electric Power


 
 
Concept Explanation
 

Electric Power

Power is the rate of doing work, or the rate at which energy is produced or consumed. It is defined as the amount of electric energy consumed in a circuit per unit time.

If W be the amount of electric energy consumed in a circuit in t seconds, then the electric power is given  by            

                    P=frac{W}{t}

But electrical energy can be defined as

W = Vq = VIt                       [because q=It]

therefore ;;P=frac{VIt}{t}

Rightarrow ;;P=VI

According to Ohm s law,  

V = IR

Substituting the value in the equation we get

large therefore        P=IRtimes I

                 =I^{2}R

               =frac{V^{2}}{R}               [putting ;I=frac{V}{R}]

Units of Power:

The SI unit of electric power is watt (W).

Electric power is said to be1 watt, if 1 ampere current flows through a circuit having 1 volt potential difference.

i.e.,    large 1watt=1volttimes1ampere=1VA

Bigger units of power are as given below:

   1 kilowatt (kW) = 10^{3}W

   1 megawatt (MW) = 10^{6}W

   1 gigawatt (GW) = 10^{9}W

2. Practical unit of power is horse power.

        1 HP = 746 W

3. Commercial unit of electrical energy

       1KWh=1000Wh=1000times3600Ws

                        =3.6times10^{6}Ws

                       =3.6times10^{6}J

4. Number of units consumed by electric appliances is

                      large =frac{watttimes hours}{1000}

Are electrons consumed in an electric circuit ?

No electrons are not consumed in an electric circuit. We pay the electricity board or electric company to provide energy to move electrons through all the electric gadgets like fan, bulb, refrigerator, etc. installed in our homes. We pay for the electricity that we use.

Illustration: An electric refrigerator rated 500W operates 6 hours/day. What is the costof the energy to operate it for 30 days at Rs. 4.5 per kWh?

Solution: Energy consumed by refrigerator in 30 days

                           =500Wtimes6frac{hour}{day}times30days

                          =90000Wh=90kWh

large therefore  Cost of energy to operate the refrigerator for 30 days

                       =90kWhtimes Rs. 4.5per;kWh=Rs. 405

Sample Questions
(More Questions for each concept available in Login)
Question : 1

An electric kettle consumes 1 kW of electric power when operated at 220 V. A fuse wire of what rating must be used for it ?

Right Option : D
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Explanation
Question : 2

An  electric  bulb is rated 220 volt and 80 watt.power consumed by it when oprated on 110 volt is:-

Right Option : D
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Explanation
Question : 3

An electric bulb is rated 220 V and 40W.When it is operated on 110V,the power consumed will be-

Right Option : A
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Explanation
 
 


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