In the hydraulic domain they are pressure and flow . The analogies between the domains can be of great benefit in developing your intuition for each domain. Chemical . . Magnetic . . Effort variables are ones for which there is a sense of pushing , while flow variables are ones for which there is a sense of motion .
Hydraulic systems contain an operating fluid.
We will assume that this operating fluid is incompressible . Water is a nearly incompressible fluid. So is hydraulic oil. Systems whose operating fluid is air are called pneumatic . Since our operating fluid is essentially incompressible, the volume of it that is within the system is a constant.