CHAPTER 20         

# 3

# 5

#11

#23

#29

#39

#45

You want to design a hot water heater for use in a house.  The heater is to operate at 115 Volts, and it is to heat 150 liters of water from an initial temperature 10 degrees C to a final temperature of 60 degrees C.  If the heater is to take no more than one hour to reach this final temperature, what electric power is required?  What must be the resistance of the electric heating coil in this heater?  Assume that no water is taken out of the heater before it reaches its final temperature, and assume that the heater is well insulated and loses no heat to the environment.

The heat energy unit of calorie is defined as the amount of energy to raise the temperature of one gram of water one degree Centigrade (or Celsius) - see page 404 of your text.  In order to determine the amount of energy that is needed to heat the 150 liters of water, we must know the mass of water to be heated.
With the mass of water in hand, we proceed to finding the amount of energy required to heat the water from 10 degrees C to 50 degrees C.


The time period that we have to heat the water is one hour.  Thus, we compute the power as
The water is to be heated under standard household voltage of 115 Volts.  To determine the resistance, we use the relations for voltage and power.