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# Transfer of heat

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Physics: Transfer Of Heat

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### Transfer of heat

1. 1. TRANSFER OF HEAT<br />
2. 2. CONTENTS<br />MODES OF TRANSMISSION OF HEAT<br />THERMAL RADIATIONS<br />CONDUCTION, CONVECTION, RADIATION<br />STEADY STATE<br />REFLECTANCE, ABSORPTANCE AND TRANSMITTANCE<br />SOLAR CONSTANT<br />SURFACE TREMPERATURE OF SUN <br />NEWTONS LAW OF COOLING<br />KIRCHOFF’S LAW<br />STEFAN’S LAW<br />WIEN’S DISPLACEMENT LAW<br />ENERGY DISTRIBUTION OF BLACK BODY RADIATIONS<br />
3. 3. TRANSFER OF HEAT<br />
4. 4. THERMAL RADIATIONS<br /> THERMAL RADIATION ARE THOSE WHICH PROVIDE US I N THE SENSATION OF WARMTH. THEY ARE EMITTED BY A BODY ON ACCOUNT OF ITS TEMPERATURE.<br />THE ENERGY EMITTED DEPENDS ON:-<br />THE TEMPERATURE OF THE BODY<br />NATURE OF RADIATING SURFACE OF THE BODY.<br />IT WAS ESTABLISHED LATER THAT EVERY OBJECT WHOSE TEMPERATURE IS ABOVE 0 KELVIN, EMITS THERMAL RADIATIONS. <br />
5. 5. THERMAL RADIATIONSLET THEIR BE THERMAL RADIATIONS FALLING ON A METALLIC CONTAINER, THEN<br />A PART OF RADIATIONS WILL BE ABSORBED BY THE CONTAINER.<br />A PART WILL BE TRANSMITTED BY THE METALLIC BODY.<br />A PART OF THE RADIATIONS WILL BE REFLECTED BACK TO THE SURROUNDINGS (as shown in the diagram)<br />
6. 6. THERMAL RADIATIONS<br />REFLECTED RAY<br />TRANSMITTED RAY<br />ABSORBED RAY<br />
7. 7. PROPERTIES(THERMAL RADIATIONS)<br /><ul><li> THEY TRAVEL WITH THE SPEED OF LIGHT.
8. 8. TRAVEL IN STRAIGHT LINE.
9. 9. DONOT HEAT THE INTERVEANING MEDIUM INFACT THEY DONOT REQUIRE ANY MEDIUM FOR PROPAGATION.
10. 10. OBEY INVERSE SQUARE LAW.
11. 11. OBEY THE LAWS OF REFRACTION AND REFLECTION.
12. 12. ALSO EXHIBIT THE PHENOMENON OF INTERFERENCE AND DIFFRACTION. </li></li></ul><li>CONDUCTION<br />IT IS THE MODE OF TRANSFERENCE OF HEAT IN WHICH THE HEAT TRANSFERS FROM PARTICLE TO PARTICLE IN THE DIRECTION OF FALL OF TEMPERATURE WITHOUT THE MOVEMENT OF THE PARTICLES OF THE MEDIUM<br />IN CONDUCTION PARTICLES DO NOT TRANSFER, JUST HEAT TRANSFERS.<br />
13. 13. CONVECTION<br />IT IS THE MODE OF TRANSFERENCE OF HEAT IN WHICH THE ACTUAL MOVEMENT OF THE PARTICLES OF THE MEDIUM TAKES PLACE.<br />
14. 14. RADIATION <br />IT IS A MODE OF HEAT TRANSFER IN WHICH HET REACHES THE RECIEVER OR OBSERVER DIRECTLY FROM THE SOURCE WITHOUT HEATING THE INTERVEANING/ MIDDLE MEDIUM.<br />
15. 15. STEADY STATE(THERMAL CODUCTIVITY)<br />STEADY STATE IS THE STATE WHEN THERE IS NO MORE ABSORPTION OF HEAT AND THE TEMPERATURE BECOMES CONSTANT.<br />DURING STEADY STATE, DIFFERENT PARTICLES MAY HAVE DIFFERENT TEMPERATURES BUT DIFFERENT PARTICLES HAVE CONSTANT TEMPERATURE. <br />
16. 16. REFLECTANCE<br />THE REFLECTANCE OR REFLECTING POWER OF A BODY IS DEFINED AS THE RATIO OF THE AMOUNT OF THERMAL RADIATIONS REFLECTED BY THE BODY IN A GIVEN TIME TO THE TOTAL AMOUNT OF THERMAL RADIATIONS INCIDENT ON THE BODY AT THAT TIME.<br />IT IS REPRESENTED BY “r”<br />FOR EXAMPLE, THE REFLECTANCE OR REFLECTING POWER OF POLISHED SURFACES IS LARGE<br />
17. 17. ABSORPTANCE<br />ABSORPTANCE OR ABSORPING POWER IS DEFINED AS THE RATIO OF THE AMOUNT OF THERMAL RADIATIONS ABSORBED BY THE BODY IN A GIVEN TIME TO THE TOTALAMOUNT OF THERMAL RADIATIONS INCIDENT ON THE BODY IN THE SAME TIME. IT IS REPRESENTED BY “a”.<br />E.g-ABSORBING POWER OF DULL COPPER IS 13%, OF INDIAN INK IS 85%. AS NO REAL BODY CAN ABSORB 100% OF THE RADIATIONS FALLING ON IT, THEREFORE THE VALUE OF a IS ALWAYS LESS THAN UNITY.<br />
18. 18. TRANSMITTANCE<br />THE TRANSMITTANCE OR THE TRANSMITTING POWER OF A BODY IS DEFINED AS THE RATIO OF THE AMOUNT OF THERMAL RADIATIONS TRANSMITTED BY THE BODY IN A GIVEN TIME TO THE TOTAL AMOUNT OF THARMAL RADIATIONS INCIDENT ON THE BODY IN THE SAME TIME.IT IS REPRESENTED BY”t”.<br />
19. 19. SOLAR CONSTANT<br />SOLAR CONSTANT IS DEFINED AS THE AMOUNT OF RADIENT ENERY RECEIVED PER SECOND BY A UNIT AREA OF TA PERFECTLY BLACK BODY SURFACE HELD AT RIGHT ANGLES TO THE DIRECTIONS OF THE SUN RAYS AT THE MEAN DISTANCE OF EARTH FROM THE SUN<br />THE VALUE OF SOLAR CONSTANT IS 1388 WATT sq. METERS OR 2 CAL.sq.CM PER MINUTE<br />
20. 20. EXPRESSION FOR SURFACE TEMPERATURE OF THE SUN<br />SUN<br />FIG. 1<br />
21. 21. EXPRESSION FOR SURFACE TEMPERATURE OF THE SUNcONSIDERTHE SUN TO BE A BLACK BODY AT TEMPERATURE “T “AND RADIUS “R” AT THE CENTRE OF THE HOLLOW SPHERE OF RADIUS r AS GIVEN IN FIG. 1 , WHEREr=1 A.U and r>RACCORDING TO STEFAN’S LAW , THE ENERGY EMITTED PER SECOND PER UNIT AREA BY THE SUN IS GIVEN BYE= T4 SURFACE AREA OF THE SUN= 4R2TOTAL ENERGY EMITTED PER SECOND BY THE SUN= 4R2 E= 4R2 T4SINCE SOLAR LUMINOSITY IS ALSO EQUAL TO THE TOTAL ENERGY RADIATED PER SECOND BY THE SUN, 4R2 T4 = 4r2 S T=[r2 S / R2 ]1/4<br />
22. 22. NEWTON’S LAW OF COOLING<br />ACCORDING TO THIS LAW, THE RATE OF LOSS OF HEART OF A LIQUID IS DIRECTLY PROPORTIONAL TO THE DIFFERENCE IN THE TEMPERATURES OF THE LIQUID AND THE SURROUNDINGS , PROVIDED THE DIFFERENCE IN TEMPERATURES IS SMALL,SAY OF THE ORDER OF 30 DEGREES.<br /> .i.eE  (T-T0)<br />THE LAW MEANS THAT A HOTTER LIQUID WILL COOL DOWN MORE QUICKLY THAN A WARM LIQUID , IN THE SAME SURROUNDINGS.<br />
23. 23. KIRCHOFF’S LAW<br />KIRCHOFF’S LAW STATES THAT THE RATIO OF EMMISIVE POWER TO THE ABSORPTIVE POWER CORRESPONDING TO A PARTICULAR WAVELENGTH AND AT ANY GIVEN TEMPERATURE BLACK BODY AT THE SAME TEMPERATURE AND CORRESPONDING TO THE SAME WAVELENGTH.<br />
24. 24. STEFAN’S LAW<br />IT STATES THAT THE AMOUNT OF HEAT ENERGY(E) RADIATED PER SECOND BY UNIT AREA OF APERFECTLYBLACK BODY IS DIRECTLY PROPORTIONAL TO THE FOURTH POWER OF ABSOLUTE TEMPERATURE (T) OF THE BODY.<br /> ET4<br /> E= T4 WHERE  IS A CONSTANT OF PROPORTIONALITY AND IS CALLED STEFAN’S CONSTANT.ITS VALUE IS 5.67X10-8 Wm-2 K-4<br />
25. 25. WIEN’S DISPLACEMENT LAWACCORDING TO THIS LAW, THE WAVELENGTH (M) OF MAXIMUM INTENSITY OF EMISSION OF THE BLACK BODY RADIATION IS INVERSELY PROPORTIONAL TO ABSOLUTE TEMPERATURE OF THE BLACK BODY,I.E m1/T or m = b/TWHERE ‘ b’ IS THE CONSTANT OF PROPORTIONALITY AND IS CALLED WIEN’S CONSTANT.FOR A BLACK BODY , THE VALUE OF <br /> b=2.898 X10-3 m K.<br />
26. 26. WIEN'S DISPLACEMENT LAW<br />CONCLUSION: WITH THE RISE IN TEMPERATURE OF THE BLACK BODY, THE WAVELENGTH OF MAXIMUM INTENSITY OF EMISSION SHIFTS TOWARDS LOWER WAVELENGTH SIDE.THIS LAW IS USED TO FIND THE TEMPERATURE OF SUN AND STARS.<br /><ul><li>THE WIEN’S DISPLACEMENT LAW ACCOUNTED FOR THE CHANGE IN COLOUR OF A BODY FROM RED TO YELLOW AND THEN TO WHITE AS ITS TEMPERATURE IS INCREASED.</li></li></ul><li>ENERGY DISTRIBUTION OF BLACK BODY RADIATIONS<br />E<br />  <br />Y<br />1644 K<br />1450 K<br />1250 K<br />1100 K<br />X<br />
27. 27. ENERGY DISTRIBUTIONS OF BLACK BODY RADIATIONS<br /><ul><li>BODY EACH CURVE IN THE GRAPH REPRESENTS THE VARIATION OF MONOCHROMATIC EMITTANCE (E) OF THE BLACK BODY WITH THE WAVELENGTH (0 OF THE RADIATION EMITTED.
28. 28. DIFFERENT CURVES HAVE BEEN SHOWN FOR DIFFERENT TEMPERATURES OF THE BLACK BODY.</li></li></ul><li>OBSERVATIONS [ ENERGY DISTRIBUTION OF BLACK BODY RADIATIONS ]<br />AT A GIVEN TEMPERATURE OF BLACK BODY1.ENERGY EMITTED IS NOT DISTRIBUTED UNIFORMLY AMONGST ALL WAVELENGTHS. <br />2.ENERGY EMITTED IS MAXIMUM CORESSPONDING TOA CERTAIN WAVELENGTH (m ) AND IT FALLS ON EITHER SIDE OF IT. <br />WITH RISE IN TEMPERATURE OF BLACK BODY<br />1.TOTAL ENERGY EMITTED INCREASES RAPIDLY FOR ANY GIVEN WAVELENGTH.m  1/T or m T= a constant.<br />
29. 29. THANK YOU<br />