IIT JEE , AIEEE Forum - Physics , Mechanics - banging my head with surface tension please help

cricmancharlie

now it is written that the force of surface tension is perpendicular to surface of water but there is a whole plane perpendicular to it please describe its precise direction

gomurali

While defining surface tension it is defined that force is perpendicular to an imaginary line lying on the surface of liquid surface in equilibrium. This force acts on both sides of the imaginary line perpendicular to it but parallel to the surface of the liquid and produces a kind of tension to the liquid surface. This force per unit length of the imaginary line drawn on the surface of the liquid is called surface tension. Hope the concept is clear.

gomurali

While defining surface tension it is defined that force is perpendicular to an imaginary line lying on the surface of liquid surface in equilibrium. This force acts on both sides of the imaginary line perpendicular to it but parallel to the surface of the liquid and produces a kind of tension to the liquid surface. This force per unit length of the imaginary line drawn on the surface of the liquid is called surface tension. Hope the concept is clear.

biki

Force of surface tension = force per unit length on an imaginary line drawn on the free surface of liquid and is perpendicular to the imaginary line.....

krishna.gopal

Good answer gomurali. Make a small element on the surface. Now surface tension on this line acts perpendicular to it in the direction going into the water

cricmancharlie

now


gomurali

is saying perpendicular to line and paralell to surface of water

and krishna.gopal is saying force is directed into the water

gomurali

krishna, thanku for the complement but surface tension is the surface phenomenon. Its role is only to stretch the surface of liquid and produce some tension. Surface tension do not act into the liquid.

Prakriteesh

I think gomurali is right. Let us divide the surface of the liquid into two parts by an imaginary line. Let one part attract the other with a force F. If the length of the imaginary line be L then surface tension S = F/ L.

Caution: Surface tension is a confusing topic and even authors of different textbooks don't have a common opinion. For example, in Concepts of Physics, surface tension is given as tangential to the surface and lying on the surface (gomurali's view), while in OP Tandon's Physical Chemistry, it is given as going into the liquid (krishna gopal's view). However, it can be seen that the former is correct and not the latter, as, the relation of surface tension with surface energy can be derived and is valid only if the former viewpoint is accepted.

cricmancharlie

now listen up if you have done some problems on surface tension then you would know that the liquid exerts force on objects on its surface in a downward direction but theory says it is on the surface of liquid(paralell)

still confused someone help

Prakriteesh

In all the problems of surface-tension that I have done, I have never found that surface-tension acts downwards, I have always found it to be parallel to the liquid surface. Please give me a concrete example where you found surface tension to be downwards, and also the reason why you think it to be acting downwards.

cricmancharlie

hi

check out mechanics of fluids chapter in ncert textbook in the surface tension subtopic you will find an example in which weights are used to determine surface tension of a liquid

check the old edition not the latest one

Prakriteesh

In all such problems, as well as in other cases, the surface tension is parallel to the liquid surface. As an example I am giving you the first solved numerical of ABC Physics (previous NCERT course).

A U-shaped wire is dipped in soap solution and removed. The thin soap film formed between the wire and the light slider supports a weight of 1.5X10-2N (which includes the small weight of the slider). The lenght of the slider is 30 cm. What is the surface tension of the film?

Now, in this problem, a U-shaped frame is taken and a slider is fitted in it as shown in the figure

The slider can slide to and fro in the direction of the arrows.

Now the film formed on this frame comprises of:

1) Two liquid surfaces

2)The bulk of liquid inside.

The edges of the two liquid surfaces touching the upper and lower edge of the slider exerts surface tensional force F = 2SL on the slider (not towards the bulk of the liquid but parallel to the surface itself) and thus pull the slider (towards left in the present figure). Similarly, when the frame is vertical, with the open end downward, then the surface tensional force is upward. This upward force is balanced by a known weight acting downwards. From that we find the surface tension.

If you have any doubt on the working of this frame, read Concepts of Physics (HCV) thoroughly.

gomurali

mr. charlie, in that case of weights added to find surface tension, it is like this. There u try to expand the film of the liquid with the help of weights to counter the upward pulling force which will try to make the surface of the liquid film as small as possible. Still the force of film is parallel to the surface making the surface as small as possible.

cricmancharlie

dyuds

thats not the

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