Heating water From the previous example, we see that if one wants a hot shower, it is necessary to wait until the water in the tank can be reheated. How long would it take the temperature to reach 130 F (54 C) if no hot water were used during the heating period and the water temperature started at 20 C?
Solution: In this case, the only energy input is the electrical heat, and there is no energy leaving the tank. Therefore, the rate of increase in internal energy is equal to the rate that electrical energy is used: