11.19 Explain why :

(a) A body with large reflectivity is a poor emitter

(b) A brass tumbler feels much colder than a wooden tray on a chilly day

(c) An optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open, but gives a correct value for the temperature when the same piece is in the furnace

(d) The earth without its atmosphere would be inhospitably cold

(e) Heating systems based on circulation of steam are more efficient in warming a building than those based on circulation of hot water

<p><strong>11.19 </strong><strong>(a)</strong> A body with a large reflectivity is a bad absorber. A bad absorber will in turn be a poor emitter of radiations.</p><p><strong>(b)</strong> Brass is a good conductor of heat, when one touches a brass tumbler, heat is conducted from the body to the brass tumbler easily. Hence, the temperature of the body reduces to a colder value and one feels cold.</p><p>On the other hand, wood is a poor conductor of heat. Very little heat is conducted from the body to the wooden tray. Resulting in negligible drop in body temperature.</p><p>Thus a brass tumbler feels colder than a wooden tray on a chilly day.</p><p><strong>(c)</strong> Black body radiation equation is given by:</p><p>E =&nbsp;<span title="Click to copy mathml"><math><mi>σ</mi></math></span>&nbsp;(&nbsp;<span title="Click to copy mathml"><math><msup><mrow><mrow><mi>T</mi></mrow></mrow><mrow><mrow><mn>4</mn></mrow></mrow></msup></math></span>&nbsp;-&nbsp;<span title="Click to copy mathml"><math><msubsup><mrow><mrow><mi>T</mi></mrow></mrow><mrow><mrow><mi>o</mi></mrow></mrow><mrow><mrow><mn>4</mn></mrow></mrow></msubsup></math></span>&nbsp;), where</p><p>E = energy radiation</p><p><span title="Click to copy mathml"><math><mi>σ</mi></math></span>&nbsp;= constant</p><p><span title="Click to copy mathml"><math><mi>T</mi><mo>=</mo><mi></mi></math></span>&nbsp;temperature of the optical pyrometer</p><p><span title="Click to copy mathml"><math><msub><mrow><mrow><mi>T</mi></mrow></mrow><mrow><mrow><mi>o</mi></mrow></mrow></msub></math></span>&nbsp;= Temperature of open space</p><p><span contenteditable="false"> <math> <mi mathvariant="normal">H</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">s</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">h</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">m</mi> <mi mathvariant="normal">p</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">u</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">f</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">p</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">s</mi> <mi mathvariant="normal">p</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">d</mi> <mi mathvariant="normal">u</mi> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">s</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">h</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">d</mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal"> </mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">n</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">g</mi> <mi mathvariant="normal">y</mi> </math> </span></p><p>When the same piece of iron placed in a furnace, the radiation energy is E =<span contenteditable="false"> <math> <mi>σ</mi> <msup> <mrow> <mrow> <mi>T</mi> </mrow> </mrow> <mrow> <mrow> <mn>4</mn> </mrow> </mrow> </msup> </math> </span></p><p><strong>(d)</strong> In the absence of atmospheric gases, no extra heat will be trapped. All the heat would be radiated back from Earth’s surface.</p><p><strong>(e)</strong> A heating system based on the circulation of steam is more efficient in warming a building than that based on the circulation of hot water. This is because steam contains surplus heat in the form of latent heat ( 540 cal/g)</p>

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