We know that hot water or milk, when kept at room temperature, cool down gradually. But have you noticed that the rate at which hot milk cools down to a lukewarm state is lesser as compared to that required by a lukewarm milk to cool and attain the room temperature? Have you ever wondered why this happens? In this section, we will learn about Newton’s law of cooling which provides the explanation for this behaviour.
Newton’s Law of Cooling
According to Newton’s law of cooling, the rate of loss of heat from a body is directly proportional to the difference in the temperature of the body and its surroundings.
Where k is the positive constant that depends on the area and the nature of the surface of the body under consideration.
We can understand this more clearly with the help of the following example.
Consider a body of mass m and specific heat is at a temperature T2is placed in a surrounding of temperature T1. Let the temperature fall by a small amount dT2 in time dt, then the heat loss is given by
dQ = msdT2
The rate of heat loss with respect to temperature is given by
From equation a and b we can write
Integrating both the sides we get
ln(T2-T1) = -Kt + C
where K = and C is the constant of integration.
From the equation above, we can see that the nature of graph obtained for temperature with respect to time is exponential in nature, as can be seen in the plot below. Here we notice that the cooling of fluid depends on the difference of its temperature and the surrounding. The plot for the rate of change of temperature is steeper in the beginning and as the temperature decreases with time, it becomes gradual; hence the rate of cooling is higher at the beginning and decreases as the temperature of the body falls.
This plot can be derived experimentally using the setup shown in the figure below. Here we take some water in a calorimeter with the proper stirring mechanism and cover it with a lid consisting of two holes through which a thermometer is dipped in the fluid.
The water is heated till it reaches a temperature of 40°C. The heat source is then removed, the system is left for cooling and the stopwatch is then started. The reading of the thermometer and the stopwatch is noted at regular intervals. As the water reaches the room temperature, we stop the experiment. As we plot the temperature on the Y-axis and the time on the X-axis, we get a graph similar to that shown above.
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