Class 11 Physics Work Energy Power Power

Power

  • Power is defined as the time rate at which work is done or energy is transferred.

  • Average Power: Ratio of work done(W) in a total time interval of t.
    Pav = W/t
  • Instantaneous Power: When the time interval ,t ,approaches zero the limiting value of average power becomes instantenous power.
    P = dW/dt
  • We can write W = F.dr,
    P = F . dr/dt
    P = F.v , where v is instantaneous velocity.
  • Power is a scalar quanitity
  • SI unit of power – Watt(W)
  • 1 hp = 746 W

Example: 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?

 Solution:  P = W/t = mgh/t
mass of water to be moved = ρV = 30000 kg
g = 9.8
h = 40 m
t = 15x60 = 900 s
P = 13066.67 W = 13.067 kW
Now η = P/PACTUAL  =>  PACTUAL = 13.067/0.3 = 43.6 kW

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