When you apply pressure to an incompressible fluid the pressure is transferred. If you stop applying the pressure there is no motion. Volume and density are integral parts of calculating how much energy is required to achieve certain useful motion like lift a truck or press a part. Why don't certain fluids compress and others do? If you start with a large pressure on one end of a fluid tube and you have an even larger pressure on the other end of the tube will the fluid move the moment you increase the pressure to a slightly higher level? What is the reason water doesn't compress or move like air?
[Physics] How is energy transferred from one incompressible fluid to another
fluid dynamicspressure
Related Solutions
I think that you're basically right when you make the following suggestion:
Does part of the input force (or pressure) get wasted in increasing the temperature of the gas and therefore making it less effective?
Recall from the First Law of thermodynamics, \begin{align} \Delta E = Q-W \end{align} where $\Delta E$ is the change in internal energy of the fluid, $W$ is the work done by the fluid, and $Q$ is the heat transferred to the fluid during the lifting process. Let's assume that the fluid is thermally insulated so that $Q=0$, then we have \begin{align} \Delta E = -W \end{align} The $W$ term will then be a combination of $W_1$ and $W_2$ where $W_1$ is the magnitude of the work you do to compress the fluid, and $W_2$ is the magnitude of the work done by the fluid in raising the object. In fact, we have \begin{align} W = W_2-W_1 \end{align} so that \begin{align} \Delta E = W_1 - W_2. \end{align} Now, for an incompressible liquid, no work needs to be done to compress the fluid, so we have $W_1=W_2$, and therefore $\Delta E = 0$; the internal energy of the fluid remains the same. However, as suggested by John Rennie, in the case of a gas, some work goes into compressing the gas, and we have $W_2 < W_1$, so we have \begin{align} \Delta E >0 \end{align} But the internal energy of a (ideal) gas is proportional to its temperature, so this means that its temperature increases a bit during the lifting process.
How do slight changes in these properties result in a large change in pressure, microscopically?
Slight change of volume is not so easy to accomplish for solids - it takes a great force to achieve it. Considerable external force applied by different body (wall) needs to be maintained. The pressure is a measure of this force per unit area and since the force is great, also the pressure is great.
One way to explain this is the following. In solids, the atoms are so close to each other that the inter-atomic repulsion forces become close to comparable to intra-atomic forces between charged particles that constitute them.
The electric force between particles gets stronger at small distances, since the Coulomb law says it changes with distance of particles $r$ as $1/r^2$. To change the volume of a solid appreciably, the force per area associated with one atom would have to be comparable to the Coulomb electric force of the nucleus on the electron. Its value for atoms is so immense that it is unattainable by common standards. It would be also hard to find a container that would keep its shape and resist expansion under such great forces. Hence changes in volume by forces of common magnitude are commonly negligible.
Best Answer
Fluids like water do compress, though in a fluid the bulk modulus is much higher than in a gas. The same applies to solids.
In a gas the pressure is mostly due to the motion of the gas molecules, while in fluids and solids the bulk modulus is mostly due to intermolecular/interatomic forces, so the resistance to compression comes from a completely different mechanism
The diagram above shows roughly what the force between two atoms looks like as a function of distance. If you're interested I grabbed the diagram from here. In liquids and solids the interatomic distance is about at the minimum of the force, and that means if you try and push the atoms closer together you hit the "hard core" i.e. the force increases very quickly. That's why it's hard to compress liquids and solids. In gases the interatomic spacing is far to the right on the graphs so you can compress the gas a long way before you hit the hard core.
With fluids that contain large molecules, like oil, the potential is a more complicated because the molecules in the fluid can change their shape under pressure, but the general behaviour is the same.
Re your question about transmitting changes in pressure: in a fluid (or solid) pressure changes are transmitted at the speed of sound. So if you suddenly increase the pressure in your hydraulic tube the increase in pressure takes a finite (but small) time to reach the other end of the tube.