Work on a System

[Bridgette Ryning 1D]

How does doing work on a system (ex: compressing a piston) change the energy of a system? (He mentioned this in class, but if someone could explain this further, it would be greatly appreciated)

[Laura Dinh 1L]

Think of work as just another form of energy—one that is defined by a force acting over a distance. So if you do work //on// a system (compressing w a piston), then you are adding energy to the system. However, this can only be done when there is a volume change (aka: constant pressure) because if there's no volume change, there's like no change in distance and therefore no work being done.

[Julian Wang]

Hi Bridgette. One way of thinking about work is that it represents mechanical energy, as opposed to heat, which represents thermal energy. It is simply another form by which energy can enter or leave a system. Take the example of a gas pushing against a piston in a chamber. In order to increase its volume, the gas must expand and overcome the mass of the piston and friction involved in the expansion. This takes energy, and the energy required exits the system (the gas) in order to accomplish this change. Even if the temperature of all parts of the system and surroundings are constant, energy exchange has occurred and we quantify this change as work done by the system on the surroundings. In the case of the piston expansion, the gas loses energy and the surroundings gain energy. Hope this helped!

[Jon Trujillo 3L]

Think of work as the transfer of energy through forces acting on an object. Using Professor Lavelle's bicycle analogy, if you are pumping up a tire with air, you are exerting energy into changing the state of the tire. This is an example of work because the change in energy in this scenario isn't due to a change in heat, but rather an act of exerting force on an object. So far this is the only kind of work which I think we've used, but it can also come in other forms other than mechanical energy changes. To simplify this, doing work on a system changes the energy of the system because it requires energy to exert force on an object; there would be no force without energy and therefore the energy is required to do work.