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Work, Energy and Power

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
A family uses 8 kW of power.
(a) Direct solar energy is incident on the horizontal surface at an average rate of 200 W per square meter. If 20% of this energy can be converted to useful electrical energy, how large an area is needed to supply 8 kW?
(b) Compare this area to that of the roof of a typical house.


Power used by the family, P = 8 kW

                                          = 8 × 103 W 

Solar energy received per square metre = 200 W 

Efficiency of conversion from solar to electricity energy = 20 %

Area required to generate the desired electricity = 

We have

8 × 103 = 20% × (A × 200)

             = (20 /100) × A × 200 
∴        A = 8 × 103 / 40  =  200 m2


The area of a solar plate required to generate 8 kW of electricity is almost equivalent to the area of the roof of a building having dimensions 14 m × 14 m. (≈ √200).

What are different units of energy?

The different units of energy are :

(i) Joule            
(ii) Erg
(iii) eV            
(iv) KWh
(v) Calorie.


Is work a scalar or a vector quantity?

Work is dot product of two vectors. i.e. F with rightwards arrow on top times S with rightwards arrow on top.

And dot product is a scalar quantity. Therefore, work is a scalar quantity. 

Define the unit joule.

Work done is said to be one joule if one newton of  force displaces the body through a distance of one meter in the direction of applied  force .

What is watt?

SI unit of power is Watt. 

Define watt.

Power is said to be one watt if one joule of work is done in one second.