A motor supplies 04 horsepower to stir 5 kg water How long w

A motor supplies 0.4 horsepower to stir 5 kg water. How long will it take to increase the temperature of the water by 6 degree C assuming that all the work goes into heating the water by friction losses. 1 horsepower = 746 Watts.

Solution

Power P = 0.4 horsepower

             = 0.4 x 746 W

mass of water m = 5 kg

Increase in temprature dt = 6 ^o C

Specific heat of water C = 4186 J / kg ^o C

Heat required Q = mC dt

                       = 5 x 4186 x 6

                       = 125580J

Required time t = Q / P

                       = 125580 / (0.4 x746)

                       = 420.84 s

                       =(420.84/60) min

                      = 7.014 min