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| Newton's Law of Cooling |
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| The rate of loss of heat by a body is directly proportional to the temperature difference between the body and the surroundings, provided the difference is not very large. |
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| Newton was the first person to investigate the heat lost by a body in air. He found that the rate of heat loss is proportional to the temperature excess of the body over the surroundings. This result, called Newton's law of Cooling, is approximately true in still air only for a temperature excess of 20 K or 30 K. |
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| Newton's law of cooling deals mainly with cooling caused by convection. On the other hand, Stefan-Boltzmann's law deals mainly with cooling by radiation. |
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| Consider a hot body at a temperature t, placed in surroundings at temperature q0. |
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| Using Newton's law of cooling, |
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 or  |
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| where K is a constant. |
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| where m is the mass of the body and c is the specific heat. |
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| This is the equation of a straight line
having negative slope. |
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| The graph drawn between the temperature of a body and time is known as cooling curve. The slope of the tangent to the curve at any point gives the rate of fall of temperature. If rate of fall of temperature is plotted against excess temperature, we get a straight line passing through the origin. |
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| According to this, the wavelength (lm) of maximum intensity of emission of black body radiation is inversely proportional to the absolute temperature (T) of the black body i.e., |
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| where b is Wien's constant and is 2.898 x 10-3 mK |
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| Lummer and Pringsheim confirmed through experiments that for a black body with rise in temperature the wavelength of maximum intensity of emission shifts towards the lower wavelength. This law helps to find temperatures of the sun and stars. |
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