Name: fluid pressure in everyday life
Uploaded: 2023-04-30T11:04:12.000Z
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K12 Standards

Alabama Course of Study Science

9-12

Physics

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

9-12.P.1.4

Pressure of Fluids

Вы когда-нибудь задумывались, как жидкость течет из одной точки в другую?
Чтобы понять это, рассмотрим жидкость постоянной плотности в состоянии покоя. Если параллельно поверхности жидкости приложена сила, то для приведения этой силы жидкость приводится в движение.
Напротив, если на поверхность жидкости прикладывается нормальная сила, то в результате реакции в жидкости развивается равная и противоположная сила. Эта нормальная сила, оказываемая жидкостью на единицу площади, называется давлением. Единицей давления в системе СИ является паскаль.
Давление всегда нормально для любой поверхности, контактирующей с жидкостью, и поэтому является скалярной величиной. Для сравнения, сила имеет определенное направление и является векторной величиной.
Давление на глубине h от поверхности жидкости представляет собой сумму атмосферного давления и давления, обусловленного весом жидкости.
Когда мы подставляем эквивалентные выражения для веса, массы и объема, мы получаем давление, вызванное весом жидкости, как произведение плотности, ускорение, вызванное гравитацией, и h.

Давление жидкостей

В повседневной жизни существует множество примеров давления в жидкостях, таких как кровь (высокое или низкое артериальное давление) и погодные условия (системы высокого и низкого давления). Одна и та же сила может оказывать значительно разное воздействие в зависимости от площади, на которую она действует. Например, сила, приложенная к площади 1 мм², имеет давление, в 100 раз превышающее давление этой же силы, приложенной к площади 1 см². Поэтому острая игла способна проколоть кожу при небольшой силе, но та же сила, приложенная пальцем, не проколет кожу.

Давление играет очень важную роль в изучении жидкостей. Одной из ключевых характеристик жидкостей является то, что существует незначительное сопротивление к компоненту силы, параллельному поверхности жидкости. Молекулы жидкости просто перемещаются, чтобы приспособиться к горизонтальной силе. Сила, приложенная перпендикулярно поверхности, сжимает или расширяет жидкость. Например, если попытаться сжать жидкость, в каждой точке внутри жидкости развивается реакция в направлении наружу, уравновешивая силу, приложенную к молекулам на границе. Давление в любой точке статической жидкости зависит только от глубины в этой точке.

Давление в жидкости около Земли меняется с глубиной из-за веса жидкости над определённым уровнем. Таким образом, в бассейне плотность примерно постоянна, и воду на дне немного сжимает вес воды сверху. С другой стороны, перемещение в верхние слои атмосферы - совсем другая ситуация, поскольку плотность воздуха начинает значительно меняться всего в небольшом расстоянии от поверхности Земли. Более того, давление жидкости, будучи скалярной величиной, не имеет направления, в то время как силы, обусловленные давлением, имеют четко определенные направления - они всегда направлены перпендикулярно любой поверхности. Причина этого заключается в том, что жидкости не могут выдерживать или создавать расслаивающие силы. Таким образом, в статической жидкости, находящейся в закрытом баке, сила, оказываемая на стенки бака, действует перпендикулярно внутренней поверхности. Аналогично, давление оказывается перпендикулярно поверхностям любого объекта внутри жидкости.

Этот текст адаптирован из <a href="https://openstax.org/books/university-physics-volume-1/pages/14-1-fluids-density-and-pressure">Openstax, University Physics Volume 1, Section 14.1: Fluids, Density, and Pressure.</a>

Существует множество примеров давления в жидкостях в повседневной жизни, например, по отношению к крови (высокое или низкое кровяное давление) и по отношению к погоде...

Education

JoVE Core

Физика

Unit 1

Раздел 1

Fluid Mechanics

Механика текучих сред

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

fluid pressure in everyday life

Fluid pressure appears in many everyday situations, including blood pressure and weather systems. The same force can have a very different effect depending on the area it acts on. A force spread over 1 mm² creates 100 times more pressure than the same force spread over 1 cm². That is why a sharp needle can pierce skin with a small force, while the same force from a finger usually does not.

Pressure is especially important when studying fluids. A fluid does not offer much resistance to a force applied parallel to its surface. Instead, the fluid molecules flow to fit that horizontal force. A force applied perpendicular to the surface behaves differently. It can compress or expand the fluid.

When a fluid is compressed, a reaction force forms inside the fluid at each point. This force acts outward and balances the force applied at the boundary. In a static fluid, the pressure at a point depends only on depth. Near Earth, fluid pressure changes with depth because of the weight of the fluid above that point.

This effect is easy to see in a swimming pool, where the water density is nearly constant. The water at the bottom is compressed very little by the water above it. The atmosphere is different because air density changes noticeably just a short distance above Earth’s surface. Pressure is a scalar quantity, so it has no direction. The forces caused by pressure do have direction, and they act perpendicular to any surface. Fluids cannot withstand or exert shearing forces, so pressure on tank walls or on objects in a fluid is always perpendicular to the surface.

Fluid Pressure in Everyday Life

characteristics of fluids

Fluid Behavior Under Force

Fluid behavior under force differs from the behavior of solids. When a force acts parallel to the top surface of a solid, the solid resists it because of internal friction between its layers. This is called shearing resistance. If the force is removed, the shearing forces help the solid return to its original shape. Other deformation forces can also change the shape of a solid for a short time, as long as the force stays below a threshold magnitude. Solids usually keep their shape, which makes...

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density

Density, Floating, and Specific Gravity

Density helps explain why objects sink or float in a fluid. It is also useful for identifying a substance and for learning about its phase and substructure. The SI unit of density is kg/m3, and the cgs unit is g/cm3. Liquids and solids usually have densities that are roughly comparable. That matches the fact that their atoms are in close contact. Gases have much lower densities than liquids and solids because their atoms are separated by large amounts of empty space. Density does not always...

pressure of fluids

Fluid pressure appears in many everyday situations, including blood pressure and weather systems. The same force can have a very different effect depending on the area it acts on. A force spread over 1 mm² creates 100 times more pressure than the same force spread over 1 cm². That is why a sharp needle can pierce skin with a small force, while the same force from a finger usually does not. Pressure is especially important when studying fluids. A fluid does not offer much resistance to a force...

variation of atmospheric pressure

Atmospheric Pressure and Altitude

Atmospheric pressure and altitude are closely linked. As height increases, atmospheric pressure decreases. This happens because the gravitational force per unit area becomes smaller as we move away from Earth’s surface. To describe this change, the atmosphere is treated with a constant air temperature at a given altitude. The ideal gas law of thermodynamics is also used as a good approximation for the air. Let p(y) be the atmospheric pressure at height y. The density ρ at y, the temperature T...

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pascal's law

Pascal’s Principle in Static Fluids

Pascal’s principle explains how pressure changes move through a fluid that is not flowing. A fluid at rest is in static equilibrium. If the fluid is water, it is in hydrostatic equilibrium. In this state, the net force on any part of the fluid must be zero, or the fluid will begin to flow. Blaise Pascal described this idea in 1653 in Treatise on the Equilibrium of Liquids. He showed that when pressure changes in an enclosed fluid, that change is transmitted without loss through the fluid and...

application of pascal's law

Hydraulic Force Transfer with Pascal's Law

Hydraulic force transfer uses Pascal's law to move pressure through an enclosed fluid. Pascal showed experimentally that a change in pressure applied to a confined fluid is transmitted undiminished throughout the fluid and to the walls of its container. That idea is the basis of hydraulics, a major part of modern mechanical technology. Hydraulic systems appear in automotive brakes, hydraulic jacks, and many other machines. A simple hydraulic system uses two pistons at the same height. Because...

pressure gauges

Gauge vs Absolute Pressure

Pressure gauges often show pressure relative to atmospheric pressure. Many gauges, including those on scuba tanks, are set to read zero at atmospheric pressure. That reading is called gauge pressure. When the pressure inside the tank is higher than atmospheric pressure, the gauge shows a positive value. Some gauges can also measure negative pressure. This is useful in a vacuum chamber, which is a rigid chamber where some air has been pumped out. Because the pressure inside the chamber is below...

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archimedes' principle

Buoyant Force from Fluid Pressure

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density and archimedes' principle

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surface tension and surface energy

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excess pressure inside a drop and a bubble

excess-pressure-inside-a-drop-and-a-bubble

Surface Tension in Drops and Bubbles

Surface tension sets the pressure difference in small drops and bubbles. For a tiny liquid drop, gravity can be neglected, so the drop is treated as spherical. It can also be viewed as two equal hemispherical drops joined together by surface tension. The forces on the drop come from three sources. The liquid inside pushes outward, the air outside pushes inward, and surface tension acts along the diameter between the two hemispheres. Since surface tension is force per unit length, it creates a...

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buoyancy

Buoyant Force in Sinking and Floating

Buoyant force explains why objects float, sink, or stay suspended in a fluid. A fluid can be a liquid or a gas, and every object placed in it experiences this upward force. A metal ball sinks, while a rubber ball floats. A submarine also changes its buoyancy to move up or down. The force comes from pressure changes in the fluid. Fluid pressure increases with depth because the weight of the fluid above also increases. That means the bottom of an object feels more pressure than the top. This...

contact angle

Wetting and Surface Curvature

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rise of liquid in a capillary tube

Capillary Action in Thin Tubes

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laminar and turbulent flow

Fluid Motion Patterns in Air and Water

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bernoulli's equation

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bernoulli's principle

Fluid Pressure and Speed Changes

Bernoulli's equation links fluid pressure and fluid speed in a static, incompressible fluid. For horizontal flow, where gravitational energy density stays constant, the equation is especially useful. In that setting, the result is known as Bernoulli's principle. Bernoulli's principle says that fluid pressure drops when speed increases, and pressure rises when speed decreases. This pressure-speed relationship helps explain how moving fluids behave in simple flow situations. It also provides a...

bernoulli's principle: applications

Bernoulli's Principle in Fluid Devices

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energy conservation and bernoulli's equation

energy-conservation-and-bernoulli-s-equation

Bernoulli’s Equation in Streamlined Flow

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viscosity

Viscosity and How Fluids Flow

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poiseuille's law and reynolds number

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stokes' law

Terminal Velocity in a Viscous Fluid

Viscous forces act like friction in a liquid. They resist the relative motion of molecules and pull back on a solid body moving through the fluid. The size of this force depends on the body’s shape and size, its speed, and the liquid’s viscosity, density, and temperature. For a spherical solid object, the force in a fluid is described by Stokes’ law. This law applies only at low Reynolds number, when the flow is laminar and not turbulent. Experiments show that the force is proportional to the...

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Fluid Pressure in Everyday Life

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