Evaporation, boiling and humidity

Содержание и основные понятия

Evaporation comes from the surface at any temperature, boiling in all volume at a saturation vapour pressure equal to the outside, and fast molecules leave the liquid and cool it.
GEF · Grade 8
PIF · Baseline
Interactive model
Evaporative
boiling
Saturated vapour
humidity
Dew point
01
The physical picture
What happens and what boundaries the model has.
Main message
Evaporation comes from the surface at any temperature, boiling in all volume at a saturation vapour pressure equal to the outside, and fast molecules leave the liquid and cool it.
Evaporative rate increases with temperature, surface area and air movement.
The boiling point depends on the external pressure.
Relative humidity compares actual water vapour content with saturation at the same temperature.
How to reason
First, name the system and the interaction, then select the values, and only then write down the law. The formula works within the model's assumptions.
02
Laws and values
Symbols should be read as a relationship of values, not as a recipe for substitution.
Q
=
L
m
Q=Lm
Q
=
L
m
Heat of steam formation
φ
=
p
p
нас
⋅
100
%
\varphi=\frac{p}{p_{\text{нас}}}\cdot100\%
φ
=
p
us.
​
p
​
⋅
100%
relative humidity
Review of the situation
In the mountains, water boils below 100 °C due to lower atmospheric pressure, so cooking takes longer.
04
Links and application
In the real world
In the mountains, water boils below 100 °C due to lower atmospheric pressure, so cooking takes longer.
Model verification
Compare the prediction with observation, evaluate the units, the order of magnitude and the influence of the factors not taken into account. Disparity does not always mean an error of law: often idealization is no longer sufficient.
C
What needs to be understood
Outcome 1
Explain the physical meaning of the topic "Vasion, boiling and humidity" through the model and not just through the formula.
Outcome 2
Clear and correctly use the terms evaporative, boiling, saturated vapour.
Outcome 3
Indicate the conditions for the applicability of the law, verify units and limit cases.
T
Terms of the topic
The definition should indicate the physical meaning, not a substitute for a domestic synonym.
Evaporative
Coarse formation from the free surface of the liquid at any temperature.
boiling
A vapour formation of the entire fluid at the same pressure as the saturated vapour to external pressure.
Saturated vapour
A vapour in a dynamic equilibrium with its liquid.
humidity
A characteristic of water vapour content in air.
Dew point
The temperature to cool the air so that the water vapour becomes saturated.
F
Origin and conclusion of the formulas
The fundamental law cannot be "proven" by algebras: for each record, it is stated whether it is a definition, an experimental law, or a model effect.
Q
=
L
m
Q=Lm
Q
=
L
m
empirical law
Heat of steam formation.
The following is an indication of the definition, experience or law from which this entry arises and where it is applicable.
Q
∼
m
Q\sim m
Q
∼
m
Experience has established the proportionality of the heat required for steam formation, the mass of the liquid under constant conditions.
L
=
Q
m
L=\frac{Q}{m}
L
=
m
Q
​
The specific heat of steam formation L is introduced as the heat of converting a unit of liquid mass into vapour.
Q
=
L
m
Q=Lm
Q
=
L
m
For a complete phase change, the specific heat shall be multiplied by the volatilised mass.
Conditions:
Liquid becomes vapour at a constant boiling point and at a specified pressure; preheating and heat loss shall be accounted for separately.
Monitoring:
The dimensions of both parts shall coincide; the result shall be checked for sign, order and limit case.
φ
=
p
p
нас
⋅
100
%
\varphi=\frac{p}{p_{\text{нас}}}\cdot100\%
φ
=
p
us.
​
p
​
⋅
100%
definition
Relative humidity.
The following is an indication of the definition, experience or law from which this entry arises and where it is applicable.
0
≤
p
≤
p
нас
(
T
)
0\le p\le p_{\text{нас}}(T)
0
≤
p
≤
p
us.
​
(
T
)
At this temperature, the water vapour pressure is compared to the saturated vapour pressure at the same temperature.
φ
=
p
p
нас
⋅
100
%
\varphi=\frac{p}{p_{\text{нас}}}\cdot100\%
φ
=
p
us.
​
p
​
⋅
100%
The relative humidity shall be applied as a disproportionate ratio expressed in percentage terms.
Conditions:
The vapour is close to ideal, both pressures are taken at the same temperature; for unsaturated vapour without water drops 0 ≤ 100 per cent.
Monitoring:
The dimensions of both parts shall coincide; the result shall be checked for sign, order and limit case.
!
Model boundaries and typical errors
Error: Forget model condition
The key condition of this subject is that the evaporative rate rises with temperature, surface area and air motion.
How to Check the Mind
The boiling point depends on the external pressure. Then check the units, vector direction, and the behaviour of the result at zero or very large parameter.
Surface tension, wetting and capillaries
Surface-layer molecules have potential excess energy, so the surface seeks to reduce the surface area. The surface tension force along the border F = l. The coefficient of adhesion of the liquid to the surface and to the inside of the fluid determines wetting. In a thin tube, the water rises if the walls are wet; the height h = 2 cos/(gr) for round capillar. This is an in-depth model requiring a clean surface and equilibrium.
Σ
Conclusion on the topic
Evaporation, boiling and humidity
Evaporation comes from the surface at any temperature, boiling in all volume at a saturation vapour pressure equal to the outside, and fast molecules leave the liquid and cool it.
Evaporative rate increases with temperature, surface area and air movement.
The boiling point depends on the external pressure.
Any numerical result shall be accompanied by a unit, a sign and an explanation of its physical meaning.
The content is consistent with the FRP on Physics of the OS/OSS 2025 and the subject matter of the current lines of the PNP: I. M. Peryškin and co-authors (7–9), G. J. Mäkishev and coauthors (10–11), and with the lines "Engineers of the Future" and V. A. Kasyanov. The original texts are not reproduced, the text of the textbooks is not reproduced.