Pressure

Pressure is an important idea in science. It helps us understand how forces act on objects. Pressure tells us how much force is pushing on a surface. It is not just the total force, but how that force is spread out over an area.

For example, if you press your hand flat on a table, the force is spread out over your whole hand. But if you press just with your fingertips, the same force feels stronger. This is because the force is concentrated in a smaller area.

We use special units to measure pressure. The main unit in science is the pascal (Pa). One pascal equals one newton of force spread over one square meter of area. Other units, like the pound-force per square inch (psi), are used more often in everyday life. These units are especially common in countries that use the imperial system of measurement.

Pressure can also be described using the weight of fluids. For instance, we can talk about pressure by looking at how high a column of water or mercury rises in a special tube called a manometer. These different ways of measuring pressure help scientists and engineers solve many practical problems. They use this knowledge to design safe buildings and understand how weather works.

Definition

Pressure is the amount of force pushed perpendicular to a surface, for a certain area. We use the symbols p or P to show pressure.

Pressure shows how much force is on an area. For example, pushing a thumbtack into a wall makes more pressure than using a finger. This is because the thumbtack has a smaller area to spread the force. That is why thumbtacks can go into surfaces easily, but fingers cannot.

Types

Fluid pressure

Fluid pressure is the force that a fluid, like a liquid or a gas, pushes on nearby surfaces. This happens in two main ways: in open spaces, such as the ocean or the air, and in closed spaces, such as pipes or tanks. In open spaces, the pressure stays mostly the same, even if the fluid is moving. In closed spaces, the pressure can change depending on how the fluid moves.

Fluid pressure was important to scientists like Blaise Pascal and Daniel Bernoulli. Bernoulli’s ideas help us understand how pressure behaves when fluids move.

Explosion or deflagration pressures

Explosions or deflagrations happen when gases, mists, or dust mixtures catch fire. This can happen in both open and enclosed spaces, creating very high pressures.

Negative pressures

Sometimes pressures can be lower than normal, called negative pressures. This can happen when we compare pressures to normal air pressure, or in special cases where materials can support tension. For example, some plants use negative pressure to help move water upward.

Stagnation pressure

Stagnation pressure is the pressure a fluid creates when it is forced to stop moving. Even if a fluid is moving fast and has low pressure, it can create higher pressure when it stops.

Surface pressure and surface tension

Surface pressure is like regular pressure but happens in two dimensions, like on the surface of water. Surface tension is related to surface pressure but works in the opposite way.

Gas pressure

Gas pressure depends on factors like temperature, the amount of gas, and volume. For ideal gases, which don’t interact much, there is a simple formula to describe pressure.

Vapour pressure

Vapour pressure is the pressure exerted by a gas when it is in balance with its liquid or solid form. The boiling point of a liquid is when its vapour pressure matches the air pressure around it.

Liquid pressure

Liquid pressure increases with depth because of the weight of the liquid above. It also depends on the liquid’s density. Deeper or denser liquids create higher pressure. Liquid pressure pushes equally in all directions, affecting objects submerged in it.

Kinematic pressure

Kinematic pressure is a way to describe pressure without directly using density. It helps simplify some physics equations, like those used to study fluid motion.