thermodynamics....

[jon0013]

it seems like i can never really fully understand thermodynamics....E=Q+W....when a system does work, W decreases (is negative)...when work is done on a system W increases (is positive) correct?

so would system doing work mean that it is being compressed so the gas heats and work being done on a system mean that gas is expanding so you lose PE....or do i have it backwords...

also for you TPR'ers....im having trouble telling the difference/interpreting Isothermal vs Adiabatic graphs on page 450....

if anyone has any idea then lemme know...appreciate the help...

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[Sicilian]

it seems like i can never really fully understand thermodynamics....E=Q+W....when a system does work, W decreases (is negative)...when work is done on a system W increases (is positive) correct?

so would system doing work mean that it is being compressed so the gas heats and work being done on a system mean that gas is expanding so you lose PE....or do i have it backwords...

also for you TPR'ers....im having trouble telling the difference/interpreting Isothermal vs Adiabatic graphs on page 450....

if anyone has any idea then lemme know...appreciate the help...

E is a measure of the total internal energy of a system. The total internal energy is equal to the sum of the kinetic and potential energies of the isolated system. For E = Q + W, Q measures the heat gained or lost by the system, and W measures the work done by or on the system. The system doing work would refer to the gas expanding; work being done on the system would mean the gas was being compressed.

 

[jon0013]

E is a measure of the total internal energy of a system. The total internal energy is equal to the sum of the kinetic and potential energies of the isolated system. For E = Q + W, Q measures the heat gained or lost by the system, and W measures the work done by or on the system. The system doing work would refer to the gas expanding; work being done on the system would mean the gas was being compressed.

THanks for the help...so would the conventions be -W if system is doing work by expanding gas and +W if gas is being compressed when work is done on system?

 

[Sicilian]

THanks for the help...so would the conventions be -W if system is doing work by expanding gas and +W if gas is being compressed when work is done on system?

Thats right. W is negative for an expanding gas because energy is lost (an expanding gas confined to a closed cylinder with a piston would push the piston up) and positive for a gas being compressed because as work is being done on the gas, energy is transferred to it (the gas now has the potential to expand and push the piston up).

 

[ctv1116]

Has there ever been an AAMC question that addressed this? This is too theoretical to be on a real MCAT.

 

[DrChandy]

Has there ever been an AAMC question that addressed this? This is too theoretical to be on a real MCAT.

Yes. This is standard knowledge in thermodynamics required for the MCAT.

 

[grapeflavorsoda]

THanks for the help...so would the conventions be -W if system is doing work by expanding gas and +W if gas is being compressed when work is done on system?

for mcat always think in terms of system, unless it is specifically stated otherwise.

 

[jon0013]

for mcat always think in terms of system, unless it is specifically stated otherwise.

ive seen it a few times on Kaplan PS....thanks for the help guys....

 

[jon0013]

ive seen it a few times on Kaplan PS....thanks for the help guys....

Can someone tell me why the answer is D? Based on the logic above the gas is expanding so Work should be -W...therefore shouldnt the answer be C?


41. If the amount of gas that is transferred from chamber
1 to chamber 2 does net work upon expansion, which
of the following must be true?
A. T1 > T2
B. T1 < T2
C. |P1&#8710;V1| > |P2&#8710;V2|
D. |P1&#8710;V1| < |P2&#8710;V2|