Determining Superfluid/Normal Fluid & Critical Velocity of 4He

In summary, the conversation discusses questions about a setup involving a container of liquid 4He and a released mass. The first question considers the technique to determine if the liquid is a superfluid or normal fluid, with a suggested method of observing the speed of the mass. The second question focuses on determining the critical velocity of a superfluid, with a suggested method of increasing the height and comparing the experimental velocity to a theoretical relation. Feedback and alternate answers are requested.
  • #1
MMS
148
4
Hello,

I have two questions regarding a setup concerning fluid 4He.

Assume there's a container of some height filled with liquid 4He and a mass that is released from rest at its surface. Two questions:

(1) Using what technique can it be decided whether the liquid is a superfluid or normal fluid?

(2) Assuming that the liquid is a superfluid, How can I determine the critical velocity?

My answers:

(1) If we assume that we know the speed of the mass, then if it is less than ~60 m/s (roton minimum), the mass will sink without any friction and so it is a superfluid. However, if its speed is higher, dissipation is created and so it is a normal fluid.

(2) Seems kind of simplistic but: Since it is a superfluid, the only force the mass feels while sinking is g. So the velocity it has once it reaches the bottom is v=sqrt(2*g*h). We increase the height each time, check this relation and compare it to the experimental velocity found. Once there is a distinct mismatch between the two, it means that there was dissipation in the sinking process (the mass now feels another force) and hence we reached v_c.

I'd be happy to hear some feedback or your own answers.

Thank you in advance.
 
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  • #2
anyone?
 

1. What is superfluidity?

Superfluidity is a state of matter in which a fluid flows with zero viscosity, meaning there is no resistance to its flow. This phenomenon is observed in certain liquids, such as helium-4, when they are cooled to extremely low temperatures.

2. How do scientists determine the superfluid and normal fluid fractions of helium-4?

To determine the superfluid and normal fluid fractions of helium-4, scientists use a technique called torsional oscillation. This involves rotating a torsion pendulum, which is partially submerged in the helium-4, and measuring the changes in its period of oscillation. The superfluid fraction can be calculated from these measurements.

3. What is the critical velocity of helium-4?

The critical velocity of helium-4 is the maximum velocity at which the superfluid component can flow without experiencing drag or resistance. This velocity is temperature-dependent and is usually measured in meters per second.

4. How does temperature affect the critical velocity of helium-4?

As the temperature of helium-4 decreases, its critical velocity also decreases. This is because at lower temperatures, more of the helium-4 atoms are in the superfluid state and can flow without resistance. At temperatures close to absolute zero, the critical velocity of helium-4 approaches zero, meaning the superfluid can flow without any restrictions.

5. What applications does the study of superfluidity have?

The study of superfluidity has many applications in fields such as quantum mechanics, astrophysics, and materials science. It has also led to the development of technologies such as superconductors and superfluid pumps, which have important industrial and medical applications. Understanding superfluidity can also provide insights into the fundamental nature of matter and its behavior at extreme conditions.

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