Physics: Transfer Of Heat

1. TRANSFER OF HEAT

2. CONTENTS MODES OF TRANSMISSION OF HEAT THERMAL RADIATIONS CONDUCTION, CONVECTION, RADIATION STEADY STATE REFLECTANCE, ABSORPTANCE AND TRANSMITTANCE SOLAR CONSTANT SURFACE TREMPERATURE OF SUN NEWTONS LAW OF COOLING KIRCHOFF’S LAW STEFAN’S LAW WIEN’S DISPLACEMENT LAW ENERGY DISTRIBUTION OF BLACK BODY RADIATIONS

3. TRANSFER OF HEAT

4. THERMAL RADIATIONS THERMAL RADIATION ARE THOSE WHICH PROVIDE US I N THE SENSATION OF WARMTH. THEY ARE EMITTED BY A BODY ON ACCOUNT OF ITS TEMPERATURE. THE ENERGY EMITTED DEPENDS ON:- THE TEMPERATURE OF THE BODY NATURE OF RADIATING SURFACE OF THE BODY. IT WAS ESTABLISHED LATER THAT EVERY OBJECT WHOSE TEMPERATURE IS ABOVE 0 KELVIN, EMITS THERMAL RADIATIONS.

5. THERMAL RADIATIONSLET THEIR BE THERMAL RADIATIONS FALLING ON A METALLIC CONTAINER, THEN A PART OF RADIATIONS WILL BE ABSORBED BY THE CONTAINER. A PART WILL BE TRANSMITTED BY THE METALLIC BODY. A PART OF THE RADIATIONS WILL BE REFLECTED BACK TO THE SURROUNDINGS (as shown in the diagram)

6. THERMAL RADIATIONS REFLECTED RAY TRANSMITTED RAY ABSORBED RAY

8. TRAVEL IN STRAIGHT LINE.

9. DONOT HEAT THE INTERVEANING MEDIUM INFACT THEY DONOT REQUIRE ANY MEDIUM FOR PROPAGATION.

10. OBEY INVERSE SQUARE LAW.

11. OBEY THE LAWS OF REFRACTION AND REFLECTION.

13. CONVECTION IT IS THE MODE OF TRANSFERENCE OF HEAT IN WHICH THE ACTUAL MOVEMENT OF THE PARTICLES OF THE MEDIUM TAKES PLACE.

14. RADIATION 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.

15. STEADY STATE(THERMAL CODUCTIVITY) STEADY STATE IS THE STATE WHEN THERE IS NO MORE ABSORPTION OF HEAT AND THE TEMPERATURE BECOMES CONSTANT. DURING STEADY STATE, DIFFERENT PARTICLES MAY HAVE DIFFERENT TEMPERATURES BUT DIFFERENT PARTICLES HAVE CONSTANT TEMPERATURE.

16. REFLECTANCE 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. IT IS REPRESENTED BY “r” FOR EXAMPLE, THE REFLECTANCE OR REFLECTING POWER OF POLISHED SURFACES IS LARGE

17. ABSORPTANCE 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”. 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.

18. TRANSMITTANCE 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”.

19. SOLAR CONSTANT 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 THE VALUE OF SOLAR CONSTANT IS 1388 WATT sq. METERS OR 2 CAL.sq.CM PER MINUTE

20. EXPRESSION FOR SURFACE TEMPERATURE OF THE SUN SUN FIG. 1

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

22. NEWTON’S LAW OF COOLING 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. .i.eE  (T-T0) THE LAW MEANS THAT A HOTTER LIQUID WILL COOL DOWN MORE QUICKLY THAN A WARM LIQUID , IN THE SAME SURROUNDINGS.

23. KIRCHOFF’S LAW 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.

24. STEFAN’S LAW 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. ET4 E= T4 WHERE  IS A CONSTANT OF PROPORTIONALITY AND IS CALLED STEFAN’S CONSTANT.ITS VALUE IS 5.67X10-8 Wm-2 K-4

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 b=2.898 X10-3 m K.

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