A water heater of power 2000 W is used to heat water. The specific heat capacity of water is 4200 J kg$$^{-1}$$ K$$^{-1}$$. The efficiency of heater is 70%. Time required to heat 2 kg of water from 10°C to 60°C is ______ s. (Assume that the specific heat capacity of water remains constant over the temperature range of the water).

Correct Answer: 300

Power = 2000 W, efficiency = 70%, mass = 2 kg, $$\Delta T = 50°C$$, specific heat = 4200 J/(kg·K).

Heat required: $$Q = mc\Delta T = 2 \times 4200 \times 50 = 420000$$ J

Effective power: $$P_{eff} = 0.7 \times 2000 = 1400$$ W

Time: $$t = \frac{Q}{P_{eff}} = \frac{420000}{1400} = 300$$ s

The answer is $$\boxed{300}$$.

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