Fluid Forces in a Manometer: Understanding Pressure and Kinetic Energy

In summary, the conversation discusses the forces acting on a manometer in two different cases. In case 1, the atmospheric force exerts a downward force on the fluid, causing the fluid to exert an equal but upward force on the atmosphere. In case 2, if the left side of the manometer is connected to a gas source with higher kinetic energy, the gas molecules will exert a higher pressure and potentially cause the liquid to move or form bubbles. The conversation also suggests using a homemade manometer to observe these effects and recommends learning about the basics of pressure in gases and liquids.
  • #1
DumpmeAdrenaline
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Note: I am self-studying Material and Energy balance courses and I haven't done fluid mechanics yet.

Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.

Case 2) If we hook up the left side of the manometer to a gas source containing higher kinetic energy molecules than the air molecules on the right side exposed to the atmosphere then those gas molecules are exerting a higher pressure. What would happen in this case?
Will the gas molecules push the liquid, bounce, pass through the liquid as bubbles?

I wrote case (1) mainly to link the upward force the fluid exerts and what is it a function of?
 

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  • #2
DumpmeAdrenaline said:
Note: I am self-studying Material and Energy balance courses and I haven't done fluid mechanics yet.

Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.
By Newton's law of action-reaction, the fluid exerts and equal but upward force on the atmosphere no matter what else is going on.
 
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  • #3
DumpmeAdrenaline said:
Case 1) Consider the manometer in the figure below. Levels on both sides of the manometer which are open to the atmosphere are equal.
If we analyse the forces acting on the left side of the manometer. We have the atmospheric force exerting a downward force on the fluid and since the fluid is not undergoing acceleration it is either moving at a non zero speed or is stationary. Therefore the fluid must be exerting an equal but upward force on the atmosphere.

Case 2) If we hook up the left side of the manometer to a gas source containing higher kinetic energy molecules than the air molecules on the right side exposed to the atmosphere then those gas molecules are exerting a higher pressure. What would happen in this case?
Will the gas molecules push the liquid, bounce, pass through the liquid as bubbles?

I wrote case (1) mainly to link the upward force the fluid exerts and what is it a function of?
To answer your specific questions…

In case 1, the magnitude of the force depends on the value of atmospheric pressure and the area of the liquid’s surface. The value of the atmospheric pressure depends on the average speed and mass of the air molecules and their number-density (how many molecules there are per cubic metre).

For case 2, I recommend you get a length of (clean!) clear plastic tube with some water. You can hold the tube in a U-shape and you have made your own manometer!

Increase the pressure on one side by gently and briefly blowing into one end. Then quickly slide slide your finger over the end in your mouth (yes it can be done!) to trap the extra gas. You will see what happens.

Similarly, you can reduce the pressure on one side by briefly and gently sucking on one end. Avoid sucking too hard and choking!

It sounds like you may have to take a step back and first learn about the basics about pressure in gases and liquids.

By the way, liquids and gases are often both referred to as ‘fluids’. So here it is best to say that the atmosphere ex erts a force on the liquid (and vice versa).
 
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