Hello @JohnRennie sir

@PrateekMourya hi :-)

Sir are you free i am mean are you answering another person?

Yes, I'm free.

Ok wait

This one has various contradictory answers

I can't remember the equation for the capillary rise offhand, but it's easy to derive.

Its asking for true pressure difference

Wont it be hrhog

I'm not sure what true pressure difference means ...

Difference between what 2 pressures?

OK, I guess that seems reaonable.

Is there a problem with it?

Wont the air pressure be same

Po

I think it must mean two two tubes are connected to different vessels with different pressures inside them.

The solution says P₁ and P₂ are the pressures in the air just above the top of the water, and this only makes sense if the tops of the tubes are connected to different pressures.

The diagram does not make this clear ...

@JohnRennie or it might be same vessel with tops attached to different pressure

It's just a terribly written question.

What should it actually be?

The question should make clear what the tops of the tubes are connected to.

From the diagram it looks as if they're both just open to the atmosphere so P₁ = P₂ = 1 atm

In that case the question doesn't make sense since there is no difference between P₁ and P₂

Ok

I have one more doubt

When the concept of surface tension is introduced

The old concept such as manometeric equation pascal's law still hold ?

Oops, wrong law.

Yes Pascal's law still holds.

The point about Pascal's law is that a fluid can flow, so if there is a pressure difference between two points at the same level that will create a force that makes the fluid flow.

It only stops flowing when the pressure difference has disappeared.

Ok sir

So the presence of a surface is irrelevant.

I feared that i might did a wrong decision studying surface tension on my own

Thanks

With a manometer the curvature of the liquid surface does affect the pressure, but in most manometers the tube is the same diameter on each side so the pressure due to the curved surface cancels out.

Can you drwa a diagram to illustrate this with same and different diameters

Of which pressure we are writing manometeric law

Do you mean this calculation?

Yes

That gauge pressure also holds for points inside tube

These diagrams always assume the contact angle of the liquid with the walls of the tube is 90° i.e. that the liquid gas interface is flat.

The equation ΔP = ρgh is only exactly true in this case.

Since the excess pressure is zero in this case

Ok so to proceed in a general problem i should proceed as the question i asked

Yes. If the contact angle is not 90° then the surface is curved and that means there is a pressure difference caused by the curvature of the surface.

Ok sir thanks

But if the tube has the same diameter on each side then the curvature is the same on both sides so it cancels out and does not affect the reading.

Have a nice day

Bye :-)

@JohnRennie ok

Ok sir