10.2 - First law of Thermodynamics and PV graphs
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Transcript of 10.2 - First law of Thermodynamics and PV graphs
Pressure and Temperature are
directly proportional as long as the
temperature is measured in Kelvins
Pressure Law
Boyle’s Law
Each line represents an experiment done at a different temperature
PistonsWork Done = Force x Change of Distance
Wd = F . Δd
but
Wd = P.A.Δd
Wd = P. ΔV
Wd = P. ΔVAs long as there is an AREA under the line then you can see that work is done (and calculate it)
𝑄=∆𝑈+𝑊First Law of Thermodynamics
It just says that if you add Heat to a gas it must be equal to the Work Done by the gas + any change in Internal Energy
This is mainly difficult because:
1. Heat can be gained or lost2. Internal Energy can rise or fall3. Work can be done ‘on the gas’ or ‘by the gas’
1. Isobaric
1. 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 +ve 4. Heat must have been added
ConstantPressure
2. Isovolumetric / Isochoric
1. 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
ConstantVolume
3. Isothermic
1. 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
ConstantTemperature
4. Adiabatic
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 - W 4. ΔU = W so work done on the gas increases the Internal energy of
the gas
No Heat is being exchanged –
perfectly insulated.
Adiabatic lines are always steeper than Isotherms
A Heat CycleA-BIsochoric / Isovolumetric Temperature rise
B-CIsobaric Expansion
C-DIsochoric / IsovolumetricTemperature drop
D-AIsobaric Contraction
The Carnot Cycle
Forward Carnot Cycle
D
Reverse Carnot Cycle