Last-minute Question

[Postictal Raiden]

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I'm taking the test in two days, and I'm still shaky on the E = q + w formula.

I was hoping someone could explain to me the main concept behind this equation and educate me on when PV is negative and when is it positive, when the process is an engine and when it's a refrigerator, and what it means when E is negative, positive, or zero.

I know that's a lot to ask on a forum, but I really need someone to explain this to me in simple words.


Thank you,

 

[MedPR]

I'm taking the test in two days, and I'm still shaky on the E = q + w formula.

I was hoping someone could explain to me the main concept behind this equation and educate me on when PV is negative and when is it positive, when the process is an engine and when it's a refrigerator, and what it means when E is negative, positive, or zero.

I know that's a lot to ask on a forum, but I really need someone to explain this to me in simple words.


Thank you,

http://mcat-review.org/thermodynamics-thermochemistry.php#1-classic

First law (ΔE = q + w, conservation of energy)
1st law of thermodynamics is based on the principle of conservation of energy, and it basically says that the change in total internal energy of a system is equal to the contributions from heat and work.
ΔE is the same thing as ΔU, which is the change in internal energy.
Q is the contribution from heat
Q is positive when heat is absorbed into the system (ie. heating it).
Q is negative when heat leaks out of the system (ie. cooling it).
W is the contribution from work.
W is positive when work is done on the system (ie. compression).
W is negative when work is done by the system (ie. expansion).

 

[Postictal Raiden]

http://mcat-review.org/thermodynamics-thermochemistry.php#1-classic

First law (ΔE = q + w, conservation of energy)
1st law of thermodynamics is based on the principle of conservation of energy, and it basically says that the change in total internal energy of a system is equal to the contributions from heat and work.
ΔE is the same thing as ΔU, which is the change in internal energy.
Q is the contribution from heat
Q is positive when heat is absorbed into the system (ie. heating it).
Q is negative when heat leaks out of the system (ie. cooling it).
W is the contribution from work.
W is positive when work is done on the system (ie. compression).
W is negative when work is done by the system (ie. expansion).

Great! Thank you very much.

Just so I understand, w is positive when v (in the pv) decreases, and it's negative when v increases, right?

Another question:

When I substitute -PV for w in the equation, when do I include the negative sign? when w is positive or negative?

 

[Postictal Raiden]

Ok, never mind. I think I got it now. Thank you

 

[Opejay]

Goodluck on the Mcat!!

 

[Postictal Raiden]

Goodluck on the Mcat!!

Thank you.

 

[Fivo]

Thank you.

Good luck bro, kill that verbal once and for all!