TBR Fluids #22

[akimhaneul]

For this question, wouldn't the viscosity affect the capillary action of the liquid? If it is more viscous, wouldn't the fluid molecules want to stick to each other more than with the surface of the column? Wouldn't this lower the height in column 1?



Thanks!

 

[akimhaneul]

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[bobeanie95]

Viscosity mostly refers to the resistance to flow, or how fast it flows. A more viscous fluid will flow at a lower rate. Since the fluid height is not a function of time it won't affect the height. If the question stated to "compare the fluid height at a certain time" or if they specifically told you that "cohesive forces are decreased", then you could assume that the height may be decreased.

 

[akimhaneul]

Viscosity mostly refers to the resistance to flow, or how fast it flows. A more viscous fluid will flow at a lower rate. Since the fluid height is not a function of time it won't affect the height. If the question stated to "compare the fluid height at a certain time" or if they specifically told you that "cohesive forces are decreased", then you could assume that the height may be decreased.

Ok thanks...also when thinking about this question in terms of bernoulli's equation, since the velocity would be lower because of viscosity, wouldn't the pressure also be lower, causing the fluid level to decrease?

 

[bobeanie95]

Viscosity refers to the resistance to movement. This could be due to various factors: the structure of the molecules, intermolecular forces, and the surface on which the fluid is traveling. I'm not sure if you're referring to the atmospheric pressure or the vapor pressure, but the vapor pressure a fluid exerts mainly depends on the strength of the intermolecular forces. So you could say that viscosity and pressure could technically be related in terms of intermolecular forces but you would need more information and this probably goes beyond the scope of the mcat. On a side note, the questions stem stated that the fluid is of the same mass density. If you were to replace it with a higher density such as replacing water with mercury then you'd have a lower fluid level since mercury is more compact.

 

[akimhaneul]

Viscosity refers to the resistance to movement. This could be due to various factors: the structure of the molecules, intermolecular forces, and the surface on which the fluid is traveling. I'm not sure if you're referring to the atmospheric pressure or the vapor pressure, but the vapor pressure a fluid exerts mainly depends on the strength of the intermolecular forces. So you could say that viscosity and pressure could technically be related in terms of intermolecular forces but you would need more information and this probably goes beyond the scope of the mcat. On a side note, the questions stem stated that the fluid is of the same mass density. If you were to replace it with a higher density such as replacing water with mercury then you'd have a lower fluid level since mercury is more compact.

oh oops I actually got it backwards.. I meant that since velocity decreases due to viscosity, according to bernoulli's law which is P + 1/2pv^2 + pgy = k, since velocity is lower, P (pressure exerted against the wall by the liquid) increases. The greater the pressure exerted by the moving water against the walls, the higher that pressure will push water up the column.

So why is it that this does not apply here?

 

[bobeanie95]

In Bernoulli's law pressure is dependent on the density on the fluid. Viscosity is irrelevant. Also note that Bernoulli's only applies to ideal fluids which have negligible viscosity.