Water in capillary tube and friction

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Discussion Overview

The discussion revolves around the behavior of water in a capillary tube, particularly focusing on the role of surface tension and the forces acting on the water column. Participants explore why water does not continuously rise in the tube after being partially drained, examining the balance of forces involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that water rises in the capillary tube until the surface tension force equals the weight of the water, questioning why it does not continue to rise after draining some water.
  • Another participant prompts consideration of all forces acting on the water mass, including weight and surface tension on both the top and bottom sides of the water column.
  • There is a discussion about the direction of surface tension forces, with some participants asserting that both surface tensions act upwards and contribute to raising the water.
  • A participant argues that the capillary force is not always equal to the weight of the water column, suggesting that the forces must balance differently when the tube is partially filled.
  • Concerns are raised about the conditions under which surface tension acts, particularly when the tube is empty and how this relates to the suction of water when the tube is inserted into the container.
  • Another participant emphasizes that adhesive and cohesive forces exist regardless of whether they are "needed," and that changes in the water surface shape occur when these forces are unbalanced by weight.
  • One participant expresses gratitude for the explanations provided, indicating that they found the discussion helpful in understanding the concepts involved.

Areas of Agreement / Disagreement

Participants express differing views on the balance of forces acting on the water in the capillary tube, particularly regarding the role of surface tension and weight. The discussion remains unresolved, with multiple competing perspectives on the mechanics involved.

Contextual Notes

Participants reference specific forces and their interactions without reaching a consensus on the overall mechanics of water behavior in the capillary tube. The discussion includes assumptions about the nature of surface tension and its effects on water movement.

Omish
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When we put an empty capillary tube in a water container, the water goes up a little bit by itself until the surface tension (ST) force is equal to its weight .This shows that unlike friction, this ST force acts individually .
So if we take out the tube out of the container, and drain a little water (in which case there's less water in it), the ST force would be more than the weight of water. So why doesn't it go up continuously in the tube?
 
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Think about it for just a little bit longer than it takes to ask the question. What are all the forces acting on the water mass?
 
Bystander said:
Think about it for just a little bit longer than it takes to ask the question. What are all the forces acting on the water mass?
Well,
1- wight
2- ST on the top side
3- ST on the bottom side
If by the question you mean that the sum of first & third force cancle the 2nd force I think it's wrong. I believe that both STs are in the same direction (upwards) since the tube can hold more water while out of the container. I encountered this point in a book and experienced it myself.
By the way the P_atm is also cancled obviously since it affects both sides.
 
Omish said:
both STs are in the same direction (upwards)
 
Yes they're in the same direction so they both help the water go up, but this actually won't happen. My question is why?!
 
Here is a picture of it.
f057311016.jpg
 
Omish said:
So why doesn't it go up continuously in the tube?
For the reason you stated:
Omish said:
the water goes up a little bit by itself until the surface tension (ST) force is equal to its weight.
But note that only the vertical component of ST is relevant for balancing weight.
 
A.T. said:
For the reason you stated:

But note that only the vertical component of ST is relevant for balancing weight.
Please read the question carefully ! Seems you didn't get the point.
First of all it doesn't act like friction forces, in other words F_capillary is not always equal to weight of column of water necessarily! (if it was equal, the water wouldn't be sucked up in the first place)
Then we have h_1 as the height of water when the tube is in container. So : (there's ST only on top side)
F_capillary_top = Gamma * A * h_1
Then we take it out and drain some of it so it would be h_2 < h_1
Then obviously (according to the picture above and my explanation) we have:
F_capillary_top + F_capillary_bottom > Gamma * A * h_2
So the water must go up!
 
Omish said:
F_capillary is not always equal to weight of column of water necessarily!
The shapes of the water surfaces adjust, until the forces balance.
 
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  • #10
A.T. said:
The shapes of the water surfaces adjust, until the forces balance.
This could be the answer except one problem. In that way when the tube is completely empty, there's no need to be any ST (or as you say change in water sarface). Nevertheless the water is sucked from container when we put the tube in it. How do you explain this?
 
  • #11
Omish said:
...there's no need to be any ST (or as you say change in water sarface).
The adhesive and cohesive forces exist, regardless if there is any "need" for them. The changes in the surface shape occur, if those forces are not balanced with other forces (like weight).
 
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  • #12
A.T. said:
The adhesive and cohesive forces exist, regardless if there is any "need" for them. The changes in the surface shape occur, if those forces are not balanced with other forces (like weight).
I got it completely thanks to your explanation and some more little experiments. Thank you so much.
A.T. said:
The shapes of the water surfaces adjust, until the forces balance.
This was also very useful.
 

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