- #1
Himal kharel
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I am currently studying surface tension. I came across a topic named "capillarity" where it is said that mercury falls down in capillary tube. But it rises in a barometer . Why is it so?
Contradiction regarding capillarity
- Thread starter Himal kharel
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In summary, the conversation discusses the concept of capillarity and how it affects the behavior of liquids in different situations. It is explained that in a barometer, the mercury rises due to atmospheric pressure, while in a capillary tube, it falls due to surface tension. This difference is attributed to the fact that water wets the glass, while mercury does not. The conversation also delves into the details of how a barometer is implemented and how it compares atmospheric pressure to the pressure in an enclosed bulb.
- #2
xts
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In barometer?
Hmmm... what 'barometer' means?
Maybe it is forced to enter the pipe by atmospheric pressure?
For classical (Pascal/Torricelli) barometers pipes have usually diameter of 3mm or so. Surface tension is negligible for so wide 'capillares'.
Hmmm... what 'barometer' means?
Maybe it is forced to enter the pipe by atmospheric pressure?
For classical (Pascal/Torricelli) barometers pipes have usually diameter of 3mm or so. Surface tension is negligible for so wide 'capillares'.
- #3
Studiot
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This is surely a question of language.
When one end of fine (capilliary) glass tube is dipped into liquid mercury and open to atmosphere at the other the meniscus (surface) of the mercury within the tube is observed to be lower than the general surface of the liquid.
However when the same experiment is performed in water the meniscus is found to be higher than the general surface.
This is because water 'wets' the glass, whereas mercury does not.
This is the origin of "the mercury falls down"
A barometer, however, is not an open ended tube, dipped into mercury or whatever. It is essentially a manometer comparing the pressure in an enclosed bulb with atmospheric pressure. Often the enclosed bulb actually has a vacuum in it so the implementation is a straight tube, closed and evacuated at the top and dipped into mercury at the bottom open end.
As the atmospheric pressure changes it is observed and said that the mercury rises or falls in the tube.
I have tried to make this clearer in the attachment.
When one end of fine (capilliary) glass tube is dipped into liquid mercury and open to atmosphere at the other the meniscus (surface) of the mercury within the tube is observed to be lower than the general surface of the liquid.
However when the same experiment is performed in water the meniscus is found to be higher than the general surface.
This is because water 'wets' the glass, whereas mercury does not.
This is the origin of "the mercury falls down"
A barometer, however, is not an open ended tube, dipped into mercury or whatever. It is essentially a manometer comparing the pressure in an enclosed bulb with atmospheric pressure. Often the enclosed bulb actually has a vacuum in it so the implementation is a straight tube, closed and evacuated at the top and dipped into mercury at the bottom open end.
As the atmospheric pressure changes it is observed and said that the mercury rises or falls in the tube.
I have tried to make this clearer in the attachment.
Attachments
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Related to Contradiction regarding capillarity
What is capillarity?
Capillarity is the phenomenon where liquids, such as water, move through narrow spaces, such as tubes or porous materials, without the assistance of external forces, such as gravity.
What is the contradiction regarding capillarity?
The contradiction regarding capillarity is that while it is commonly observed that liquids rise in narrow tubes, it is also observed that the height of the rise is inversely proportional to the diameter of the tube. This contradicts the idea that capillarity is solely due to surface tension, as surface tension should result in a higher rise with smaller tube diameters.
What theories attempt to explain the contradiction regarding capillarity?
Two main theories attempt to explain the contradiction - the Young-Laplace equation and the Kelvin equation. The Young-Laplace equation takes into account the surface tension and curvature of the liquid at the air-liquid interface, while the Kelvin equation considers the effects of the liquid's vapor pressure and the curvature of the solid-liquid interface.
Which theory is considered to be the most accurate explanation for the contradiction regarding capillarity?
Currently, the Kelvin equation is considered to be the more accurate explanation for the contradiction regarding capillarity. This is because it takes into account additional factors, such as the vapor pressure of the liquid, which the Young-Laplace equation does not. However, both theories have their limitations and the exact explanation for capillarity remains a subject of ongoing research and debate.
What are the practical applications of understanding capillarity and its contradiction?
Understanding capillarity and its contradiction has many practical applications, such as in the fields of engineering, materials science, and biology. It can help with the design of microfluidic devices, the selection of materials for porous structures, and the understanding of transport processes in plants and animals. It also has implications in fields such as medicine, where it can affect drug delivery and the behavior of blood in the body.
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