pgh vs. pgy for pressure in a fluild?

[ankit1ag]

I know that EK makes a distinction between pgh and pgy as height above a point and distance below a surface. However, I don't see this distinction in AAMC questions or other prax questions. I remember a specific hypothetical question asking what the hydrostatic pressure was at a certain point in a fluid filled container and believe that they only had pgh as an answer choice and not pgy.

Is the conventional representation really diff, or do pgh and pgy get mixed interchageably in real problems.

thanks,

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[CATallergy]

you definitely can't mix these up - one is gravitational potential energy, and the other is the potential energy due to pressure, which is a result of the weight of fluid, air, etc. pushing down from above.

imagine that you were a water molecule in a bucket, diffusing around. at the top of the bucket, you would have more gravitational potential energy (ρgh), and at the bottom of the bucket, you would have more potential energy from pressure (ρgy)

in bernoulli's equation:
pressure + ½ρv² + ρgh = constant, the point is that energy can be converted between pressure, kinetic, and gravitational potential, but stays the same everywhere.

 

[HristosKaran]

Hey Cat, is there anywhere besides Bernoulli's principle we will need to know rho g h for the MCAT?? I think I have somewhere in my notes that rho g h by itself can be used to calculate pressure, Is this true? Thanks

 

[Jacqui3932]

pgy refers to fluid that is moving, as opposed to static (pgh)

 

[CATallergy]

Hey Cat, is there anywhere besides Bernoulli's principle we will need to know rho g h for the MCAT?? I think I have somewhere in my notes that rho g h by itself can be used to calculate pressure, Is this true? Thanks

ρgh by itself cannot be used to calculate pressure, since this is a measure of gravitational potential energy, as compared to some arbitrary reference point. For pressure, you need to know the weight of all of the matter above the point pushing down (think about this: what would the pressure be at the 1 ft dept of a swimming pool when it is filled to 10 ft vs when it is filled to 2 ft - it would be much higher, even though h at that point is the same)

remember that density = mass / volume (ρ = m / V)

ρgh is more-or-less the same thing as potential energy = mgh (that you use for problems on land), and as you should know in those problems, h could be the distance to the floor in the room, the ground outside, sea level, etc. h = 0 only because that it the height defined in the problem as being the bottom point.

 

[HristosKaran]

ρgh by itself cannot be used to calculate pressure, since this is a measure of gravitational potential energy, as compared to some arbitrary reference point. For pressure, you need to know the weight of all of the matter above the point pushing down (think about this: what would the pressure be at the 1 ft dept of a swimming pool when it is filled to 10 ft vs when it is filled to 2 ft - it would be much higher, even though h at that point is the same)

remember that density = mass / volume (ρ = m / V)

ρgh is more-or-less the same thing as potential energy = mgh (that you use for problems on land), and as you should know in those problems, h could be the distance to the floor in the room, the ground outside, sea level, etc. h = 0 only because that it the height defined in the problem as being the bottom point.

Gotcha, now its all starting to come together, Thanks Cat!