Pressure exerted by a liquid is different to gas?

[scilearner]

Hello everyone,

 

I have made number of topics recently I think they have all stem from my poor understanding in this matter.

 

Ok in a gas, molecules can take up a volume and exert a pressure. Inside a fixed container when you increase the temperature pressure increases because more gas molecules are hitting the walls.

 

Now in a liquid the molecules are close together and volume is determined by the shape of the container. So inside the container liquid molecules don't hit the walls and exert pressure some other way. What is this way. Is this called hydrostatic pressure. Also when you make the molecules move faster in a liquid somehow would that increase the pressure like in a gas. According to what I have read it actually decrease hydrostatic pressure. However I have a picture in mind more kinetic more molecules hitting the walls higher the pressure. Maybe I'm just not understanding how gases and liquids behave in a molecular level when it comes to pressure.

 

So please help me with this. I would greatly appreciate. Thanks

[Mr Skeptic]

Liquids cannot expand to fill a volume like gases, due to strong intermolecular forces holding them near each other. The pressure they exert is because of their weight, pressure = density * gravitational acceleration * height. Temperature is not a factor in this, other than that high temperatures tend to slightly reduce density.

[foodchain]

Liquids cannot expand to fill a volume like gases, due to strong intermolecular forces holding them near each other. The pressure they exert is because of their weight, pressure = density * gravitational acceleration * height. Temperature is not a factor in this, other than that high temperatures tend to slightly reduce density.

 

Why do gas laws have a temperature variable then? Like pv=nrt?

[Sisyphus]

Liquids cannot expand to fill a volume like gases, due to strong intermolecular forces holding them near each other. The pressure they exert is because of their weight, pressure = density * gravitational acceleration * height. Temperature is not a factor in this, other than that high temperatures tend to slightly reduce density.

 

Gases also exert pressure due to their weight, it just usually isn't significant in confined spaces. This does matter in the Earth's atmosphere, however, where you've got a miles-high air column over your head. The weight of the column is about the same as a 33-odd inch column of mercury, which is how barometers work.

 

Also, liquid pressure doesn't have to only come from weight, but just the amount of force with which it is being compressed, e.g. pushing with more or less force on the plunger on a syringe full of water will change the pressure, too.

 

Actually, a syringe is a good example, I think. Assuming the needle is sealed off, the pressure in a syringe filled with air will (more or less) depend on the tempurature, volume, and quantity of air. The pressure in a syringe filled with water will (more or less) depend on the force currently being exerted on the plunger. (I say more or less because we're ignoring gravity, pretending they are an ideal gas and incompressible fluid, respectively, etc.)

[Mr Skeptic]

Why do gas laws have a temperature variable then? Like pv=nrt?

 

The kinetic theory of gases is applicable. You can assume gases are groups of individual molecules of gas that elastically collide. If you have an ideal gas, also assume that the molecules have zero volume and no intermolecular forces. Anyhow, temperature is a measure of how fast the molecules are moving. So if you get a bunch of molecules bouncing off a wall at one speed or a faster speed, which will push the wall harder?

[scilearner]

Thanks for all the answers but I'm still confused. In a tube with water does it have 2 kind of pressure hydrostatic and what do you called the pressure created by moving molecules? Is their a name for that

 

This is some info on Bernoulli effect

 

"When air travels through an airway at a CONSTANT FLOW RATE, the total energy of the fluid (PE + KE) decreases because friction converts some of this energy into heat. Since the fluid is moving at a constant flow rate, the loss is in potential energy (decrease in hydrostatic pressure)."

 

"As we move down the airway, the tubes begin to narrow. When a tube narrows, fluid velocity increases causing an increase in KE at the expense of PE (ie. a compensatory decrease in hydrostatic pressure). This is referred to as the BERNOULLI EFFECT."

 

Ok what I like to know is that even though hydrostatic pressure is decreases is someother pressure (caused by moving molecules) increase? Thanks!!