A water is being heated in a glass beaker. Although the supply of energy is constant eventually the temperature becomes steady at 80 degrees Celsius. Why is that considering that phase change doesn't happen until 100 degrees Celsius?


The temperature of the water becomes constant when the rate of heat loss due to cooling by the environment is equal to the rate at which heat is being supplied by the heater.

The rate of heat loss due to cooling will be approximately described by Newton's law of cooling:

$$\frac{dQ}{dT} = -k(T - T_0) $$

where $T$ is the temperature of the water and $T_0$ is the temperature of the environment.


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