Pressure

Pressure is an important idea in science that helps us understand how forces act on objects. It tells us how much force is pushing on a surface, 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 much stronger because it is concentrated in a smaller area.

We use special units to measure pressure. The main unit in science is the pascal (Pa), which 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, especially in countries that use the imperial system of measurement.

Pressure is also described using the weight of fluids. For instance, we can talk about pressure in terms of 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, from designing safe buildings to understanding how weather works.

Definition

Pressure is the amount of force applied perpendicular to the surface of an object per unit area. We use the symbols p or P to represent pressure.

Pressure tells us how much force is being applied to a certain area. For example, pressing a thumbtack into a wall applies more pressure than pressing a finger because the thumbtack has a much smaller area to spread the force over. This is why thumbtacks can pierce surfaces easily, while fingers cannot.

Types

Fluid pressure

Fluid pressure is the force exerted by a fluid, which can be either a liquid or a gas, on the surfaces around it. It occurs in two main situations: open conditions, like the ocean or atmosphere, and closed conditions, like pipes or containers. In open conditions, pressure is mostly constant, even though the fluid might be moving. In closed conditions, the fluid can be static or moving, and pressure changes depending on how the fluid moves.

Fluid pressure was important in the work of scientists like Blaise Pascal and Daniel Bernoulli. Bernoulli’s ideas help us understand how pressure behaves in moving fluids.

Explosion or deflagration pressures

Explosions or deflagrations happen when gases, mists, or dust mixtures ignite. This can occur in both open and enclosed spaces, creating 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, certain 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 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.