So TBR chem says that during the carnot cycle, overall more heat is released than absorbed and more work is done on the system than by the system... I was confused about this, can someone explain how this is true if after the carnot cycle, the material returns to its orginal state anyway?
Carnot Cycle
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My feeling is that TBR is representing a Carnot cycle in reverse, or a heat pump, in which more work is done on the system than the system performs on the surroundings and more heat would be released at Th than taken in at Tc. A Carnot cycle in reverse is still a Carnot cycle. Is there a PV graph? Is the pathway going counter-clockwise on the graph? In that case it is a heat pump. If the stages proceed isothermal, adiabatic, isothermal, adiabatic, it is still a Carnot cycle but in reverse.
The Carnot cycle is the the ideal thermodynamic cycle in that every step within each stage. Going forward there is isothermal expansion at Th, adiabatic expansion Th -> Tc, isothermal compression at Tc, adiabatic compression Tc -> Th. Because nothing happens in which changes to entropy in the surroundings and system aren't balanced against one another, every stage is microscopically reversible. In other words, if a steam engine could operate with Carnot efficiency, there would be no increase or decrease in entropy in the universe, so it would be just as likely to run in the reverse as forward.
The entropy change due to heat flow in isothermal conditions equals Q/T. Because the only heat exchanged between the system and the surroundings occurs during the isothermal stages, the entropy lost or gained by the surroundings is equal to the entropy gained or lost by the system. In the forward moving Carnot cycle this means that Qh > Qc.
More heat, Qh, is taken in during the isothermal expansion at Th than released, Qc, at Tc. The difference between Qh and Qc by the first law of thermodynamics must equal the net work done by the cycle. So in a forward moving Carnot cycle, the system absorbs more heat at Th than it releases at Tc and more work is done by the system on the surroundings than the surroundings perform on the system.
TBR must be acting tricksy here with a Carnot cycle operating in reverse. That's my guess without seeing the question.
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basically carnot cycle is a closed loop on a PV diagram. you can always get back to your starting point somehow. if you know what isochoric, isobaric, isothermic, and adiabatic mean you should be able to analyze a PV diagram easily.
the second major point i would understand is the relationship between work and energy. you can use energy to do work, or convert work into energy.
in the example of the AC/refrigerator that they give. they use energy (in the form of electricity) to drive all the components in the fridge, but the main point is that the energy is used to do work (pump heat out of the fridge an dump it to the environment). that's why your AC unit (whether its in a window or outside) is outside. if you're trying to cool your room you don't want hot air from the AC mixing with cold air from the AC. so the hot air is sent out of the room.
alternatively, you can use energy in the form of chemical energy (gasoline/hydrocarbons) to do work. this is your car. fuel burns, releases heat, pistons expand (drive a crankshaft, think about torque or lever arm attached to a wheel), pistons collapse, heat gets transferred to some cold body, and the thing happens all over again.
all of these cycles just convert between work and energy.
Thanks I figured it out, TBR was showing the heat pump/refridgerator perspecitve of the carnot cycle to introduce the idea which got me a little confused because its the heat engine that is introduced first in most textbooks.. but they're both essentially the same just in reverse so it makes sense.
ALSO, I was a bit confused about which of the processes (Adiabatic, isothermal, isochoric, isobaric) are reversible vs which are irreversible... I know that the adiabatic and isothermal processes are reversible (which is why they're used in the carnot cycle to maximize engine efficiency), but does anyone have a more concrete explanation to determine which process is reversible or not? Since we're allowing heat flow to occur during an isothermal process, Im not quite seeing how it would be reversible?
Another question, why is entropy constant for a reversible cyclic process? Does this have to do with the definition of reversibility itself?
Please help.. thanks in advance!
ALSO, I was a bit confused about which of the processes (Adiabatic, isothermal, isochoric, isobaric) are reversible vs which are irreversible... I know that the adiabatic and isothermal processes are reversible (which is why they're used in the carnot cycle to maximize engine efficiency), but does anyone have a more concrete explanation to determine which process is reversible or not? Since we're allowing heat flow to occur during an isothermal process, Im not quite seeing how it would be reversible?
Another question, why is entropy constant for a reversible cyclic process? Does this have to do with the definition of reversibility itself?
Please help.. thanks in advance!
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entropy for a reversible process is usually dQ/T where dQ is change in heat over temperature.
but honestly, you don't really need to know that for the mcat. that's more of a pchem approach. in fact, i wouldn't worry about which is reversible and which isn't. i would however know which processes are state functions and which are path dependent (process functions).
things like changes in pressure, volume, potential, and internal energy are all state functions. they only depend on point A and point Z, not anything in between. meanwhile, process functions, like work and heat depend on everything in between A and Z, and beyond that you can't get more work out than you put in heat. alternatively, you can't get more heat out of a process with only so much work.
but seriously, most of this stuff might be overkill for the MCAT because the topics list in thermodynamics only covers the following:
First law:
E=Q+W
PV diagram (work done=area under or enclosed by a curve)
