Pascal's Law (also called Pascal's Principle) says that "changes in
pressure at any point in an enclosed fluid at rest are transmitted
undiminished to all points in the fluid and act in all directions."
("Conceptual Physics," copyright 1993, HarperCollins College
Publishers.) This sounds rather overwhelming at first, so let's break
it down a bit.
When it says "enclosed fluid," that means that in order for
Pascal's Law to be true, you have to be looking at a liquid in a closed
Pressure is basically a fancy word for how much something pushes on
its container and on things in it. For example, air pressure is how
hard air pushes on things. When you pump more and more air into your
bike tire, you're increasing its pressure. If you increase its pressure
too much, then it will be pushing out more than the plastic is capable
of pushing in, and your tire will explode. Water pressure works the
So lets say you have a long, closed tube of water with a piston at
one end. (The piston is the piece of the tube at one end, except that
it can slide back and forth through the tube... so it can be used to
compress the water.) If you push on the piston, then you're changing
the amount of space that the water has to take up, so it will push back
on the piston even more. This means that you've changed it's pressure.
By Pascal's Law, we know that not only did the pressure change right
next to the piston, but it changed through the whole tube. So now, the
water is pushing out on the entire tube more, and it's pushing out in
Let's say, instead, that we had a long tube of water with pistons
at /both/ ends. Then, if we were to push on one piston, the pressure in
the water would increase, like before, so the water would be pushing on
the other piston, too. If there weren't anything holding the other
piston down, then the water's pushing on it would cause it to slide
through the tube away from the water. But because the pressure is
transmitted "undiminished" through the water, it would actually move
exactly as far as the first piston had moved.
Pressure is measured as the force felt by a certain amount of
surface area. For example, you could have a pressure of 1 lb/cm^2. This
means that the water is pushing on the container with a force of 1
pound on every square centimeter of the container. The reason that we
care about this is because we can use it with Pascal's Law.
For example, let's say we have a container with a big open space in
the middle, and two tubes sticking out of it. Like before, there's a
piston at the end of each tube. The difference here, though, is that
one of the tubes is a lot bigger than the other one. Let's say that the
small piston has 1 square cm (1 cm^2) of surface area, and the big one
has 100 square cm (100 cm^2) of surface area. (So the big one is 100
times as big as the small one is.) Then we push on the little one with
a force of 1 lb. Since we were pushing on a 1 cm^2 with a force of 1
lb, we've increased the pressure in the entire container (by Pascal's
Law) by 1 lb/cm^2. This means that the big piston will also be pushed
on by a pressure of 1 lb/cm^2. But since the big piston has so much
more area to be pushed on (100 cm^2), it will feel a force of 100 lb!
(This is because 1lb / 1cm^2 = 100 lb / 100cm^2.)
This is really useful because it means that we can turn a small
force into a big one without doing any extra work! This is the concept
behind all kinds of hydraulic pumps. For example, if you've ever taken
your car to the shop and had them put it up on a lift, this is probably
how they did it!
This is just one example of how Pascal's Law works and why it's so important, but there's lots of others, too.
(republished on 08/02/06)