Hello
I have been reviewing the physics concept of work last night in order to be able to understand the concept of the expansion of a gas in thermodynamics. Something odd came up though. Work - The Physics Hypertextbook This website mentions that positive work is done in raising a textbook up in the air at a constant velocity. I thought that the force applying work had to be the net force on the system?!?! Or are they taking into account the force of gravity and the force of your hand both doing work on it ? Then wouldn't the total work be zero? I'm confused by this.
This leads me to adiabatic gas expansion. An example I see quite often is the case of a cylinder filled with gas with a piston loaded with weight. The system is in equilibrium. However when calculations are done to determine the work done by the gas on the surroundings during an expansion...they use the force exerted by the surroundings on the cylinder system. For me that calculation only makes sense for a reversible reaction. The reason I bring it up is because when all of the weights are taken off at once (irreversible expansion). The work that it can do is at a minimum and it is calculated by also using the external pressure of the surroundings. This doesn't make sense to me. It's not a reversible expansion so shouldn't the pressure in the work energy formula be the internal pressure of the gas? The gas itself is doing work on the surroundings.
Thank you for your help.