how long does it take a 750watt heater operating at full rating to rais the temperature of 1kg of water From 40°C to 70°C {S. H. C=4200j/kg/k and neglecte heat loss}

168 seconds (2 min 48 s)

Explanation:

Find the heat absorbed by the water.

q = mCΔT

q = (1 kg) (4200 J/kg/K) (70°C - 40°C)

q = 126,000 J

Power is energy per time.

P = q / t

750 W = 126,000 J / t

t = 168 s

It takes 168 seconds (2 min 48 s).