Table of Contents

Understanding Pressure

Pressure in everyday life

Pressure and Weather

Meteorologists often refer to high-pressure and low-pressure zones. In specific conditions, the normal pressure on Earth, at sea level, is in the order of 101 kPa. In reality, this pressure varies somewhat at different points of the planet and, for a given point, is not necessarily the same from one day to the next. How is this possible?

Warm air is not as dense (it is lighter) as cool air, which explains how hot air balloons can leave solid ground. We can therefore assume that the air pressure above a desert is lower than the air pressure above the North Pole, and that is indeed the case. This type of variation in pressure from one point of the planet to another has a direct impact on the weather.

Transparency

Generally speaking, a high-pressure zone is associated with dry and sunny weather since the air descends and dries. Inversely, a low-pressure zone is a sign of bad weather, since the air has a tendency to ascend and condense itself to form rain-bearing clouds.

Inflated Balloons

Let’s now consider the example of an inflated balloon, such as those used during birthday parties. To inflate it, we must blow hard enough to apply pressure to its opening which is greater than the atmospheric pressure. Why? Under ambient pressure, the rubber tries to get back to its initial shape. If there is no knot in the balloon and we let it go, the air will quickly escape under the effect of the internal pressure by sending the balloon in the opposite direction.

Scuba Diving

Divers are subject to much greater pressure under water than at the surface. Added to the atmospheric pressure is the pressure of the water above them. At ten metres deep, the ambient pressure is already double the pressure at the surface. At twenty metres, it’s triple, and at thirty metres, it’s quadruple. This pressure can collapse a diver’s lungs, which is why they must breathe pressurized air. At 30 m, the air pressure provided by the tank must be four times greater than the pressure of the surface air. This means that inhaling at this depth is equivalent, in terms of air volume, to inhaling four times at the surface.
According to Boyle’s Law, the air breathed during ascent will expand. Holding one’s breath in this case would cause air in the lungs to dilate to the point of seriously damaging its tissues. This is why divers are taught never to hold their breath under water.

Snowshoes and Skates

We have already stated the relationship between force and the surface to which it is applied with our snowshoe example earlier on. Snowshoes distribute our weight on a relatively large surface. The resulting pressure is therefore low.

But what happens in the case of a finely edged ice skate? The surface on which weight is applied is minimal (the edge of both blades) and the resulting pressure is therefore significant. We would tend to expect it to be difficult to move. But ice has a surprising characteristic: when it is subjected to large amounts of pressure, it turns into water. This thin layer of water between the skate’s blade and the rink makes it possible to slide with ease.

Pressure Cookers

Pressure cookers make it possible to boil water at temperatures exceeding 100 °C by letting the pressure internal to the cooker rise. For the pressure to increase, it is necessary to latch the cooker’s locking mechanism before applying heat. If, by mishap, we unlocked the cooker when the pressure was at its maximum, the lid would be blown off violently.

Activities:

• The egg and the bottle – How not to "crack" under pressure
• Pascal scales – Understanding pressure measurements
• Make your own barometer