Name: vapor pressure and boiling behavior
Uploaded: 2025-02-13T15:43:08.000Z
Duration: 1 min 28 s
Channel: JoVE

沸騰とキャビテーションに影響を与える蒸気圧は、特定の温度で液体と平衡状態にある蒸気によって加えられる圧力であり、分子が液体表面から逃げる傾向を示しています。
密閉された真空容器の中では、液体分子は宇宙に逃げ出し、分子が逃げる速度と戻る速度が等しくなる平衡に達するまで圧力を上げます。この平衡圧力が蒸気圧です。
蒸気圧は温度に依存します。温度が上昇すると、分子は運動エネルギーを獲得し、分子間力に打ち勝ち、気相に逃げやすくなり、蒸気圧が上昇します。
例えば、100°Cの水の蒸気圧は101.3kPaで、海面の大気圧と一致しているため、沸騰します。標高が高く、気圧が低いと、水は蒸気圧が下降した大気圧とより早く一致するため、水はより低い温度で沸騰します。
蒸気圧の監視は、キャビテーションを防ぐためにポンプとタービンを設計および運用する上で重要です。
キャビテーションは、局所的な流体圧力が蒸気圧を下回ると、蒸気の泡が形成されて激しく崩壊し、機器が損傷する現象です。

流体の蒸気圧

流体の蒸気圧は、流体力学において重要な概念であり、沸騰やキャビテーションなどの現象に影響を及ぼします。蒸気圧とは、特定の温度で対応する液体相と熱力学的平衡状態にある蒸気によって加えられる圧力を指します。これは、分子が液体表面から蒸気相に逃げる傾向を表します。

 液体を小さな空気空間のある密閉容器に入れ、その空間を真空にすると、蒸気分子が液体から空間に逃げます。逃げる分子が増えると、空間の圧力が増加し、平衡に達します。平衡に達すると、液体から逃げる分子の速度と液体に戻る分子の速度が等しくなります。この平衡圧力が液体の蒸気圧です。

 蒸気圧は温度に依存します。温度が上昇すると、分子の運動エネルギーが増加し、分子間力を克服して蒸気相に逃げる分子が増えます。したがって、蒸気圧は温度とともに増加します。たとえば、100°C の水の蒸気圧は 101.3 kPa (1 atm) で、標準大気圧で沸騰します。ただし、大気圧が低い標高の高い場所では、蒸気圧が標高の高い場所での減少した大気圧と等しいため、水はより低い温度で沸騰します。

沸騰とキャビテーションに影響を与える蒸気圧は、特定の温度で液体と平衡状態にある蒸気によって加えられる圧力です...

Education

JoVE Core

Civil Engineering

土木工学

Unit 1

ユニット1

Fluid Properties

流体の特性

KEY TERMS AND CONCEPTS

主要用語と概念

Vapor Pressure of Fluid

vapor pressure and boiling behavior

Vapor pressure and boiling behavior are closely linked in fluids. Vapor pressure is the pressure a vapor exerts when it is in thermodynamic equilibrium with its liquid at a given temperature. It shows how strongly molecules tend to leave the liquid surface and move into the vapor phase.

A closed container helps show how vapor pressure forms. If a liquid sits in a sealed space with a small air gap, and that space is evacuated, vapor molecules escape from the liquid into the empty space. The pressure rises as more molecules collect there. Soon, the rate of molecules leaving the liquid equals the rate returning to it. That balance creates the vapor pressure of the liquid.

Temperature changes vapor pressure. When temperature increases, the molecules gain kinetic energy. More molecules can overcome intermolecular forces and escape into the vapor phase. For that reason, vapor pressure increases as temperature increases.

Water gives a clear example. At 100°C, water has a vapor pressure of 101.3 kPa, or 1 atm. That is why it can boil at standard atmospheric pressure. At higher elevations, atmospheric pressure is lower. Water then boils at a lower temperature because its vapor pressure reaches the reduced outside pressure sooner.

Vapor Pressure and Boiling Behavior

characteristics of fluids

Fluid Behavior in Liquids and Gases

Fluids include liquids and gases, and they behave differently from solids because of how their molecules are arranged. In solids, molecules are tightly packed and held by strong intermolecular forces. That structure helps solids keep their shape and resist deformation. Fluids have molecules that are farther apart and held by weaker forces. Because of this, they can flow and change shape easily. A fluid is any substance that deforms continuously when shearing stress is applied. Shearing stress...

density, specific weight, specific gravity and compressibility of fluid

density-specific-weight-specific-gravity-and-compressibility-of-fluid

Fluid Properties: Density, Weight and Compression

Density, specific weight, specific gravity, and compressibility are key fluid properties. Density is the mass per unit volume of a fluid. It helps describe how much mass a fluid system contains and affects buoyancy, pressure, flow behavior, viscosity, thermal conductivity, and sound propagation. In pipeline design, knowing density helps engineers make sure the pipe can support the fluid’s mass. Specific weight is the weight per unit volume. It is found by multiplying density by gravitational...

viscosity of fluid

How Temperature Changes Fluid Viscosity

Viscosity describes how much a fluid resists flow and deformation. It comes from internal friction between fluid layers that move past each other. Dynamic viscosity, shown by the Greek letter mu (μ), measures the force needed to move one layer over another. Newtonian fluids, such as water and air, have a linear relationship between shearing stress and the rate of shearing strain. Their viscosity stays constant, even when the applied stress changes. Non-Newtonian fluids, like ketchup,...

types of fluids

Newtonian vs Non-Newtonian Fluid Behavior

Fluids can be grouped by how they respond to shear stress. Shear stress is a force that acts across a material, and shear strain rate describes how quickly it changes shape or flows. In this lesson, fluids are divided into Newtonian and non-Newtonian types. Newtonian fluids follow Newton’s law of viscosity. Their shear stress changes in a linear way as the shear rate changes. That means their viscosity stays constant, so their flow is predictable and easier to analyze. Water, air, oil, and...

newtonian fluid: problem solving

Newtonian Fluid Viscosity and Shear Stress

Newtonian fluids have a constant viscosity, so their shear stress and shear strain rate stay directly proportional. This creates a linear, predictable link between force and flow. Water, air, and light oils are common examples of Newtonian fluids. A simple way to study this behavior is to place the fluid between two parallel plates. One plate stays still, and the other moves at a constant velocity. The fluid forms a velocity gradient, which is the change in speed across the distance between...

vapor pressure of fluid

Vapor pressure and boiling behavior are closely linked in fluids. Vapor pressure is the pressure a vapor exerts when it is in thermodynamic equilibrium with its liquid at a given temperature. It shows how strongly molecules tend to leave the liquid surface and move into the vapor phase. A closed container helps show how vapor pressure forms. If a liquid sits in a sealed space with a small air gap, and that space is evacuated, vapor molecules escape from the liquid into the empty space. The...

surface tension of fluid

Why Water Drops and Needles Float

Surface tension helps explain why water forms drops, supports small objects, and moves in thin tubes. It is a property of fluids at the boundary between a liquid and a gas, or between two immiscible liquids. At the surface, molecules are pulled inward by neighboring molecules. That inward pull makes the surface act a little like a stretched elastic membrane. Inside a fluid, molecules are attracted in all directions. Surface molecules do not have the same balance of forces. They feel a net...

capillarity in fluid

Capillary Action in Narrow Spaces

Capillary action is the movement of liquid through narrow spaces without any outside force. It depends on surface tension and adhesion between the liquid and nearby solid surfaces. This effect is common in narrow tubes, porous materials, and fine particles. Surface tension comes from cohesive forces between liquid molecules at the liquid-air boundary. These forces create a thin skin that resists outside pressure. When a capillary tube is placed in a liquid, the liquid level changes because of...

design example: deciding thickness of lubricating fluid in a shaft

design-example-deciding-thickness-of-lubricating-fluid-in-a-shaft

Lubricant Film Thickness for Shaft Bearings

Lubricant film thickness for shaft bearings is calculated to keep a rotating shaft from touching its bearing housing. A controlled layer of lubricant reduces friction, wear, and energy loss. It also helps the machine run smoothly by preventing metal-to-metal contact. The calculation starts with the shaft’s tangential velocity. First, the rotational speed is converted to angular velocity. Then that value is multiplied by the shaft radius. The result is the shaft’s linear speed at its surface...

key terms and concepts

Learn how vapor pressure forms, why it rises with temperature, and how it explains boiling in water at sea level and higher elevations.

Vapor Pressure and Boiling Behavior | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Vapor Pressure and Boiling Behavior

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