TPR Practice Test 2 Fluids Q#18

[663697]

Which one of the following expressions is equal to the pressure difference between Point c and Point d?

A. ρgas*g*v

B. ρgas*g*y

C. ρliquid*g*v

D. ρliquid*g*y Correct Answer

From dimensional analysis you know that A and C are incorrect. But how do you find the answer to be D? I was trying to do this question using the pressure of the atmosphere pushing down on point D and the gas from the right end pushing down on point C but I couldn't come up with the right expression.

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[studyhowbowdah]

I'm thinking that you wouldn't use the density of the gas, because then that would include the weight of that entire empty column on the right - points b to c, whereas it's just asking for the pressure difference between C and D - which is the just the weight of that fluid times the height difference

 

[akimhaneul]

Which one of the following expressions is equal to the pressure difference between Point c and Point d?

A. ρgas*g*v

B. ρgas*g*y

C. ρliquid*g*v

D. ρliquid*g*y Correct Answer

From dimensional analysis you know that A and C are incorrect. But how do you find the answer to be D? I was trying to do this question using the pressure of the atmosphere pushing down on point D and the gas from the right end pushing down on point C but I couldn't come up with the right expression.

I may be wrong but I'm guessing that because both of them feel the same pressure from gas and the only difference between them is the height of liquid, all you have to do to find the difference in pressure is use the liquid height difference.


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[663697]

I may be wrong but I'm guessing that because both of them feel the same pressure from gas and the only difference between them is the height of liquid, all you have to do to find the difference in pressure is use the liquid height difference.


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If they both felt the same pressure, wouldn't y = 0?

 

[aldol16]

If they both felt the same pressure, wouldn't y = 0?

Yes, if they felt the same pressure, there should be no height difference between the two sides of the U-tube. I think we need the whole question/passage to make sense of this system here. By what's drawn, it looks like there's a small hole in front of "B" and if that's real, then the entire system is under the same pressure because it's all part of the same open system. Which means the height difference doesn't make sense.

 

[663697]

Yes, if they felt the same pressure, there should be no height difference between the two sides of the U-tube. I think we need the whole question/passage to make sense of this system here. By what's drawn, it looks like there's a small hole in front of "B" and if that's real, then the entire system is under the same pressure because it's all part of the same open system. Which means the height difference doesn't make sense.

Here's the info they gave:

A Pitot tube is a device used to measure the flow speed of a gas. It is composed of a U-shaped tube partially filled with liquid, which is inserted into a cylinder through which the gas is flowing, as shown in Figure 1.

Figure 1 Pitot tube

Gas flowing through the horizontal cylinder in Figure 1 will obey Bernoulli’s Law for fluids at constant elevation; that is, P + (1/2) ρv2 will be constant at all points, where P is pressure, ρ is density, and v is flow speed. The pressure at the surface of the liquid in the left arm of the U-tube will be equal to the pressure at Point a, where the gas is flowing freely. The pressure at the surface of the liquid in the right arm of the U-tube will be equal to the pressure at Point b, where the gas has been brought to a stop in the cylinder. The difference in pressure between the two points will cause the liquid to shift in the U-tube, as shown in Figure 1.

In terms of the difference in height y of the liquid in the arms of the Pitot tube, the speed v of the gas can be calculated from the equation:

An investigator uses the apparatus shown in Figure 1 to measure the speed of air (ρ = 1.2 kg/m3) moving through a cylinder whose cross-sectional area is 0.005 m2. The Pitot tube contains alcohol (ρ = 800 kg/m3).

Pitot tubes can be used in medical applications to measure air flow through the lungs, which can be important in studying asthma and other breathing disorders. In this design, a breathing tube is placed in a patient's closed mouth, so that air can flow past one end of the Pitot tube during breathing. The other end of the Pitot tube is exposed to the outside air.

 

[akimhaneul]

Here's the info they gave:

A Pitot tube is a device used to measure the flow speed of a gas. It is composed of a U-shaped tube partially filled with liquid, which is inserted into a cylinder through which the gas is flowing, as shown in Figure 1.

Figure 1 Pitot tube

Gas flowing through the horizontal cylinder in Figure 1 will obey Bernoulli’s Law for fluids at constant elevation; that is, P + (1/2) ρv2 will be constant at all points, where P is pressure, ρ is density, and v is flow speed. The pressure at the surface of the liquid in the left arm of the U-tube will be equal to the pressure at Point a, where the gas is flowing freely. The pressure at the surface of the liquid in the right arm of the U-tube will be equal to the pressure at Point b, where the gas has been brought to a stop in the cylinder. The difference in pressure between the two points will cause the liquid to shift in the U-tube, as shown in Figure 1.

In terms of the difference in height y of the liquid in the arms of the Pitot tube, the speed v of the gas can be calculated from the equation:

An investigator uses the apparatus shown in Figure 1 to measure the speed of air (ρ = 1.2 kg/m3) moving through a cylinder whose cross-sectional area is 0.005 m2. The Pitot tube contains alcohol (ρ = 800 kg/m3).

Pitot tubes can be used in medical applications to measure air flow through the lungs, which can be important in studying asthma and other breathing disorders. In this design, a breathing tube is placed in a patient's closed mouth, so that air can flow past one end of the Pitot tube during breathing. The other end of the Pitot tube is exposed to the outside air.

The last paragraph talks about how one end is exposed to outside air and the other end is exposed to patient's mouth so maybe that creates the height difference? That's my thinking.


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