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Work, Energy and Power

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Physics Part I

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Class 10 Class 12
A pump on the ground floor of a building can pump up water to fill a tank of volume 30 m3 in 15 min. If the tank is 40 m above the ground, and the efficiency of the pump is 30%, how much electric power is consumed by the pump?


Volume of the tank, V = 30 m

Time of operation, t = 15 min

                              = 15 × 60

                              = 900 s 

Height of the tank, h = 40 m 

Efficiency of the pump, η = 30 % 

Density of water, ρ = 103 kg/m

Mass of water, m = ρV = 30 × 103 kg 

Output power can be obtained as, 

P0 = fraction numerator Work space done over denominator Time end fractionmgh over straight t

    = 30 × 103 × 9.8 × 40 over 900 

    =  13.067 × 103 W 

For input power Pi,, efficiency η, is given by the relation, 
η =straight p subscript straight o over straight P subscript straight i=  30% 

Pi = fraction numerator 13.067 space straight x space 100 space straight x space 10 cubed over denominator 30 end fraction 

   = 0.436 × 105 W 

   =  43.6 kW, is the electric power consumed by the pump. 

What are different units of energy?

The different units of energy are :

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


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 .

Define watt.

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

What is watt?

SI unit of power is Watt. 

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.