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# 10.2 - First law of Thermodynamics and PV graphs

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### 10.2 - First law of Thermodynamics and PV graphs

1. 1. Pressure Law Pressure and Temperature aredirectly proportional as long as the temperature ismeasured in Kelvins
2. 2. Boyle’s LawEach line represents an experiment done at adifferent temperature
3. 3. Pistons Wd = P.A.Δd Wd = P. ΔV
4. 4. Wd = P. ΔVAs long as there is an AREA under the line thenyou can see that work is done (and calculate it)
5. 5. First Law of ThermodynamicsIt just says that if you add Heat to a gas it must beequal to the Work Done by the gas + any change inInternal Energy This is mainly difficult because: 1. Heat can be gained or lost 2. Internal Energy can rise or fall 3. Work can be done ‘on the gas’ or ‘by the gas’
6. 6. 1. Isobaric Constant Pressure1. The gas is expanding – doing Work (Work is +ve)2. The temperature is increasing so ΔU is increasing (ΔU is +ve)3. ΔQ = ΔU + W so Q must be +ve4. Heat must have been added
7. 7. 2. Isovolumetric / Isochoric Constant Volume1. The gas isn’t expanding or contracting. No Work is done. ΔW=02. Because the Pressure is increasing the temperature must be rising so ΔU is +ve3. Q = ΔU + W so Q = ΔU so heat is +ve4. Heat must have been added
8. 8. 3. Isothermic Constant Temperature1. The gas is expanding – doing Work (Work is +ve)2. The temperature is constant so ΔU is 03. Q = ΔU + W so Q =W4. Heat must have been added to enable the gas to do work
9. 9. 4. AdiabaticNo Heat is being exchanged – perfectly insulated.1. No heat is exchanged so Q = 02. Volume is reducing so Work is being done ON THE GAS (W= -ve)3. Q = ΔU + W so 0 = ΔU - W4. ΔU = W so work done on the gas increases the Internal energy of the gas Adiabatic lines are always steeper than Isotherms
10. 10. A Heat Cycle A-B Isochoric / Isovolumetric Temperature rise B-C Isobaric Expansion C-D Isochoric / Isovolumetric Temperature drop D-A Isobaric Contraction
11. 11. The Carnot Cycle
12. 12. Forward Carnot Cycle
13. 13. Reverse Carnot Cycle D