Pascal’s Principle

PASCAL’S PRINCIPLE

Pressure can be defined as force per unit area so pushing on the fluid will increase the pressure as is done when the heart pushes on blood in the heart during contraction. This doesn’t work as well in an open system because fluid will flow away from the area of pressure.

When pushing on a fluid that is enclosed, the pressure is transmitted without diminishment. This is referred to as Pascal’s principle. What it means is that the pressure is not diminished in the entirety of the fluid’s volume. What it also means is that total pressures from different sources of fluid in the same container are additive.

This can be applied in hydraulic systems, such as those that operate automobile brakes. A simple hydraulic system involves a piston in a cylinder. Force is applied to a small area that gets transmitted to a larger surface area with the pressure unchanged. This is described in figure 60:

Figure 60.

There is a relationship between forces in a hydraulic system. In the figure, the two pistons are at the same height. The pressure times the area on the left can be described as force divided by surface area 1 (A1). The pressure is the same throughout so that, on the right side, the same pressure is equal to force divided by the surface area on the left or A2. If the force on the left is 1 Newton and the right side has a surface area of 5 times that, the force on the right will be 5 Newtons. What this means is that the ratio of force to surface area will be the same on the left as on the right.

With automobile brakes, a lever system is used to press brakes and exert a force on a small piston. The pressure is sent to brakes with a larger surface area, resulting in a much larger force applied to brake pads, which stop the vehicle. Because energy cannot be added or destroyed, only so much work can go into this type of system so, with power brakes, pumps add energy to the system in order to help them work better.

Gauges that measure the pressure of the blood and tire pressure are set not to an actual zero pressure but to read zero when the pressure is equal to atmospheric pressure. This brings up the issue of “absolute pressure”, which is the atmospheric pressure plus the pressure reading on the gauge. It is impossible to have a truly negative absolute pressure.

There are various ways to measure pressure. An aneroid gauge uses a flexible bellows connected to a mechanical indicator that measures the pressure. A manometer is a simple U-shaped tube, in which one end is open to the air and the other connected to a sealed container. Air pressure pushes down on each side equally so the effect cancels out. If both sides of the U-shape are open, the level will be equal on both sides. If there is one side with greater pressure when compared to the open side, there will be a difference in the height of the columns, which reflects the pressure differences between the two sides.

A barometer is a device that measures atmospheric pressure. This involves a nearly pure vacuum above a column of mercury. Remember that the atmospheric pressure will equal the height, density, and gravity force multiplied together. It measures the atmospheric pressure as well as altitude because it will decrease with a higher altitude.