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**8. Newton's law of cooling**

According to this law, if the temperature T of the body is not very different from that of the surroundings T_{0}, then rate of cooling - is proportional to the temperature difference between them. To prove it let us assume that

T = T_{0} + Dt

if the temperature difference is small.

Thus, rate of cooling

or

as dT = dq or DT = Dq

**8.1 Variation of temperature of a body according to Newton's law **

Suppose a body has a temperature q_{i} at time t = 0. It is placed in an atmosphere whose temperature is q_{0}. We are interested in finding the temperature of the body at time t, assuming Newton's law of cooling to hold good or by assuming that the temperature difference is small. As per this law,

rate of cooling µ temperature difference

or or

Here is a constant

From this expression we see that q = q_{i} at t = 0 and q = q_{0 }at t = ¥, i.e., temperature of the body varies exponentially with time from q_{i} to q_{0} (< q_{i}). The temperature versus time graph is a shown in figure.

**Note : **If the body cools by radiation from q_{1} to q_{2} in time t, then taking the approximation

and

The equation becomes

This form of the law helps in solving numerical problems related to **Newton's law of cooling. **

**8.2 Limitations of Newton's Law of Cooling : **

(a) The difference in temperature between the body and surroundings must be small

(b) The loss of heat from the body should be radiation only.

(c) The temperature of surroundings must remain constant during the cooling of the body.

**Ex.20 A body at temperature 40°C is kept in a surrounding of constant temperature 20°C. It is observed that its temperature falls to 35°C in 10 minutes. Find how much more time will it take for the body to attain a temperature of 30°C. **

**Sol.**

for the interval in which temperature falls from 40 to 35°C

for the next interval

(30 - 20) = (35 - 20) e^{-kt}

**Alter:** (by approximate method)

for the interval in which temperature falls from 40 to 35°C

from equation

for the interval in which temperature falls from 35°C to 30°C

from equation (14.4)

⇒ required time,

**9. NATURE OF THERMAL RADIATIONS : (WIEN'S DISPLACEMENT LAW) **

From the energy distribution curve of black body radiation, the following conclusions can be drawn :

**(a) **The higher the temperature of a body, the higher is the area under the curve i.e. more amount of energy is emitted by the body at higher temperature.

**(b) **The energy emitted by the body at different temperatures is not uniform. For both long and short wavelengths, the energy emitted is very small.

**(c) **For a given temperature, there is a particular wavelength (l_{m}) for which the energy emitted (E_{l}) is maximum

**(d) ** With an increase in the temperature of the black body, the maxima of the curves shift towards shorter wavelengths.

From the study of energy distribution of black body radiation discussed as above, it was established experimentally that the wavelength (l_{m}) corresponding to maximum intensity of emission decreases inversely with increase in the temperature of the black body. i.e.

This is called Wien's displacement law.

Here b = 0.282 cm-K, is the Wien's constant.

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