Question about work when gas is compressed by a piston adiabatically

[KeepItReal7]

This question is specifically from Kaplan FL#8 PS#12.

So a sample of gas in a cylinder is compressed adiabatically by a piston-- and the answer says that the work done by the system (gas) is negative while the work done on the system by the surroundings is positive. I know that U= -W for an adiabatic situation but I always thought that work done by a gas is always positive (such as when it expands) and that work done on a gas by its surroundings (when its compressed) is always negative.

I am confused by this concept and any clarification would be much appreciated-- thanks!

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[ipmed]

This question is specifically from Kaplan FL#8 PS#12.

So a sample of gas in a cylinder is compressed adiabatically by a piston-- and the answer says that the work done by the system (gas) is negative while the work done on the system by the surroundings is positive. I know that U= -W for an adiabatic situation but I always thought that work done by a gas is always positive (such as when it expands) and that work done on a gas by its surroundings (when its compressed) is always negative.

I am confused by this concept and any clarification would be much appreciated-- thanks!

The work done by a system is negative when energy is transferred into that system, and the work done by a system is positive when energy is transferred out of the system. W(system) = Final Energy(surroundings) - Initial Energy(Surroundings).

 

[KeepItReal7]

The work done by a system is negative when energy is transferred into that system, and the work done by a system is positive when energy is transferred out of the system. W(system) = Final Energy(surroundings) - Initial Energy(Surroundings).

thank you for the explanation- makes sense now