Is an adiabatic process isothermal also?

1. Feb 18, 2010

Sunny Kumar

Is an adiabatic process isothermal also???

Can anyone please tell me if it is necessary for an adiabatic process to be isothermal? Please explain with example.

2. Feb 18, 2010

Fightfish

Re: Is an adiabatic process isothermal also???

No! An adiabatic process necessarily involves a change in temperature!
An adiabatic process is one in which there is no exchange of heat between the system and the surroundings (Q = 0). This implies that whatever work that is done by the system (eg a gas) or done on the system would cause a change in the internal energy and hence temperature of the system.

For instance, when a gas undergoes an adiabatic expansion, the work done by the gas has to come from the internal energy of the gas, since there is no heat transfer. Thus, the temperature of the gas falls. This is the reason why the adiabatic curve is steeper than the isotherms.

3. Feb 18, 2010

Mapes

Re: Is an adiabatic process isothermal also???

Wrong. "Adiabatic" only means that there is no heat transfer to or from the system. Adiabatic expansion into a vacuum, for example, is isothermal for ideal gases.

4. Feb 19, 2010

Fightfish

Re: Is an adiabatic process isothermal also???

Ok...except for this special case where there is no resistance to expansion and no work is done :p

5. Feb 19, 2010

Mapes

Re: Is an adiabatic process isothermal also???

Or for processes where electrical work is done and an equal amount of P-V work extracted, or for phase changes that are achieved through hydrostatic pressure, or for multiple gases mixing... There's a universe of possible processes that are adiabatic and isothermal. It's important not to generalize from cases where the temperature does change.

6. Feb 20, 2010

Fightfish

Re: Is an adiabatic process isothermal also???

Apologies. My prior exposure to thermodynamics did not cover nonequilibrium work modes, but only conventional expansion and contraction processes, thus my first instinct in thinking in that mode.

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