What Is Pascal's Law?
Pascal's law says that when pressure is placed on a fluid in a container, the pressure will disperse equally to all parts of the container. That is, the pressure of fluid furthest away from the pressure point, and the pressure of fluid closest to the pressure point will be the same — it does not diminish, for example, with distance. The principle was named after the French mathematician and philosopher Blaise Pascal who discovered it in the 1600s. It applies to static conditions such as in a closed container, and not dynamic conditions where other conditions could affect pressure values. A common application of the law is in the hydraulic press which is able to convert a small force into a larger one.
There are a variety of Pascal's law experiments. Perhaps the most basic demonstration involves stepping on a balloon filled with air. Though Pascal's law has to do with fluids in a closed container, it's important to note that in physics a fluid can be a liquid or a gas, and air is a type of gas. In this example, the flexible walls of the container shows how the pressure caused by foot disperses the air equally throughout the balloon. The balloon will bulge uniformly in all directions, not just, for example, linearly from the foot.
Another common demonstration uses a bottle, a balloon and some matchstick heads. The bottle is filled with water nearly to the top, and matchstick heads are dropped into the water. The matchstick heads will float on the water. The neck of an inflated balloon is stretched over the bottle, and that inflated balloon is subsequently, and lightly, squeezed. Forcing the air from the balloon into the bottle increases the pressure of the air in the closed container equally. Though that increased pressure of the air would otherwise not be visible to the eye, because the matchstick heads are in the bottle, one can see that the increased pressure has the effect of causing the matchstick heads to now sink.
A common application of Pascal's law is the hydraulic press. In a hydraulic press, the closed container — or in this case, perhaps more accurately, the closed system — is comprised of more than one piston, or pump-like objects. A liquid that cannot be compressed is used and when the handle of the piston is pressed, pressure is exerted on the liquid. As a result, the pressurized liquid is transferred to a larger piston, which causes that handle to lift. The pressure that moved the smaller piston down is displaced evenly throughout the sealed unit with less effort than would be required to lift the larger piston on its own, much like a lever lifting a rock. Hydraulic mechanisms of many types, such as the braking systems on aircraft and some vehicles, rely on Pascal’s law.
Written by Josie Myers
