#### coolchix321

Removed
Why must temperature increase in an adiabatic compression?
Why must temperature decrease in an adiabatic expansion?

How come when you compress a system the particles lose heat... the system loses heat?

#### RogueUnicorn

##### rawr.
7+ Year Member
gas compression/expansion is PV work, that is to say energy. in adiabatic compression/expansion, the only place to derive this energy is from the internal energy of the gas, which is directly related to temperature.

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#### RogueUnicorn

##### rawr.
7+ Year Member
first off, what does adiabatic mean?

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#### coolchix321

Removed
No gain or loss of heat

#### RogueUnicorn

##### rawr.
7+ Year Member
exactly, so, recall this thermo equation:

Change in Internal Energy = delQ + delW

as there is no change in heat (which is the TRANSFER of energy between objects, don't get confused with the colloquial temperature definition) the change in internal energy is equal only to the change in work. since in a gas

W = PV

changes in the volume will change the internal energy. the internal energy of a gas is given by

U = c nRT

and thus directly linked to temeperature. thus, changing the volume without heat transfer changes the internal energy which then changes temperature.

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#### coolchix321

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Sorry, I am still confused... could you please elaborate
thanks

#### RogueUnicorn

##### rawr.
7+ Year Member
could you elaborate on your confusion

#### G1SG2

10+ Year Member
5+ Year Member
Why must temperature increase in an adiabatic compression?
Why must temperature decrease in an adiabatic expansion?

How come when you compress a system the particles lose heat... the system loses heat?
Internal energy E is directly proportional to Temperature. Thermal energy (at the molecular level) such as rotational, vibrational, and translational energy are considered Internal Energy, and in increase in any of those increases the temperature.

Adiabatic compression-no heat flow. From E=q + w, we can see that if q is 0, then E=W. Since work is being done on the gas, it gains internal energy, and that since E is directly proportional to T (for the reasons discussed above), the temperature increases.

Adiabatic expansion-no heat flow. From E=q+w, we can see that if q is 0, then E=W. However, w=- since the gas is doing work by expanding. It is losing internal energy or USING THAT ENERGY to do the expansion (you can also think of it as trading the internal energy for the gravitational potential energy that comes about with the expansion). Since E is directly proportional to T, a decrease in E decreases the temperature of the substance.

Just remember that internal energy E is directly proportional to T (because of the molecular energies discussed earlier, an increase in such energies will increase the temperature of the substance).

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Hi Pookiez,