Pascal’s law states that when the pressure at any point of a static fluid in a closed system is changed, the pressure change will disperse equally throughout the fluid.
That is, the pressure at a point away from the region of change will change by the same amount as at a point near it. A closed system can simply be a closed container, or it can be something more complex, such as two or more interconnected containers;
The important thing is that no fluid can enter or leave the system. It’s also important to note that, in physics, a fluid can be a liquid or a gas. The law can be demonstrated by a series of simple experiments and has important applications, such as in the hydraulic press.
The principle takes its name from the French mathematician and philosopher Blaise Pascal who discovered it in the 1600s. It applies to static situations and not to dynamic conditions in which other factors could influence the pressure values. For example, it does not apply to fluids in motion or subject to temperature changes.
There are a variety of Pascal’s law experiments that can be used to demonstrate the effect. Pascal himself demonstrated that it worked by filling a barrel with water and inserting a long tube into the top. When he poured water into the top of the tube, the barrel burst. The weight of the water in the pipe caused an increase in pressure inside the pipe which pushed against the walls until it gave way.
Perhaps the most basic way to prove the law at home is simply to squeeze a balloon. In this example, the flexible walls of the container show how the pressure buildup caused by squeezing is dispersed throughout the balloon. The balloon will inflate evenly in all directions, not just on the side opposite the squeezed one.
In another common demonstration, a bottle is filled to the brim with water and a few match heads are dropped into it so that they float. The neck of an inflated balloon is stretched over the bottle and then slightly squeezed. Match heads now sink some distance into the water. This is because the pressure increase from squeezing the balloon is transmitted into the water, forcing some of it into the porous match heads and causing them to sink, due to the extra weight. When the pressure on the balloon is removed, the water pressure drops, the air pressure in the matchheads pushes the water out, and they float again.
Perhaps the best-known application of Pascal’s law is the hydraulic press, a device that converts a small force into a large one. It generally consists of two connected chambers, each with a piston – a movable barrier that can be pushed down or pulled up without allowing fluid to escape – and containing fluid that cannot be compressed. One chamber-piston combination is larger than the other – this is the “output”. The idea is that a small force applied to the smaller, or “input” piston will result in a larger output force. Pushing down the inlet increases pressure and that increase will be the same against the larger outlet piston.
Calculation of the output force
The output force is calculated by dividing the area of the output piston by the area of the input piston, then multiplying the result by the input force. If the output piston has ten times the area of the inlet, the output force will be ten times the input force. For example, if the input force is 5 units, the input area is 2 units, and the output area is 20 units, the output force will be 50 units. In this way, heavy objects can be lifted without the need to apply great force.
This doesn’t mean that extra energy is appearing out of nowhere. The amount by which the outlet piston is lifted will be less than the amount by which the input piston is pressed, which evens things out. In the example above, if the input piston is lowered 10 units, the output piston will be raised 1 unit. The principle is similar to using a lever to lift a rock. Hydraulic mechanisms of many types, such as the braking systems of aircraft and some vehicles, are based on Pascal’s law.