# What is a super fluid ?

1. Mar 19, 2014

### Entanglement

Some fluid act weirdly at zero kelvin like Helium

1- it loses viscosity
2- flows upwards along the wall against gravity !!!

And there is a property I'm not sure about, that it can't be held in a container because it leaks out of it.

However what's the physical explanation for all of that what really happens to the fluid at zero kelvin at the atomic level to make it lose viscosity and move against gravity, and what makes some substance capable of being superfluids while other incapable,I would be grateful to a detailed easy explanation for a high school student, thanks in advance.

2. Mar 19, 2014

### dauto

1st: The fluid is NOT at zero Kelvin. It is at a low temperature above zero Kelvin. Zero Kelvin is unachievable according to the 3rd law of thermodynamics.

2nd: There isn't much of the actual explanation of that phenomenon that is accessible to a high school student. At those low temperatures a sizable fraction of the Helium gas molecules form a Bose-Einstein condensate which is a state where the molecules share a single quantum state. All the molecules become entangled with each other leading to a coordinated macroscopic behavior which eliminates viscosity (because viscosity relies on molecules randomly interacting with each other, spreading their energy around, and dissipating the flow. That doesn't happen when the atoms behave in a coordinated way due to their quantum entanglement).

3. Mar 19, 2014

### Entanglement

Thanks very much!!! pressure and volume would vanish at zero kelvin violating law of conservation of mass, so it's impossible as you said.

4. Mar 19, 2014

### Entanglement

I slightly understand that due to my physics experience as a high schooler, yet I appreciate your help what I want to know is what makes it move against gravity along the sides of the container how is that related to its zero viscosity ?

5. Mar 20, 2014

### Entanglement

And why gases never solidify even at temperatures very near to zero kelvin ??

6. Mar 20, 2014

### Staff: Mentor

Gases do freeze solid if you cool them enough and if the pressure is high enough - you can buy chunks of frozen carbon dioxide at some grocery stores where I live. Helium is unusual in that under a pressure of one atmosphere it won't freeze solid at any temperature.

7. Mar 20, 2014

### DrDu

It only flows upwards some centimeters. Hence it can also be kept in a container if the walls are higher than that.
Actually, flowing upwards is not uncommon for ordinary liquids, like water, too. Observe some red wine in the glass next time you drink one on my health!
So if the rim of a wine glass is low enough, it will also leak out.
The only difference to liquid helium is that in liquid helium, there is no surface contact effect involved.

8. Mar 20, 2014

### DrDu

No, this argument rules out only ideal gas behaviour at zero kelvin.

9. Mar 20, 2014

### dauto

No, the 3rd law of Thermodynamics has nothing to do with mass conservation. The law simply states that 0 K cannot be achieved in a finite number of steps. It doesn't claim it violates any physical principle such as mass conservation - that's nonsense.

10. Mar 20, 2014

### Entanglement

But If we are talking about ideal gases PV=nRT at zero kelvin, volume and pressure becomes zero which is impossible as it is a violation to conversation of mass.

But since there is no such thing called ideal gas, that isn't true, then why non-ideal gases can't achieve zero kelvin

11. Mar 20, 2014

### dauto

Again, you're being careless in your logic steps. The mass of the gas is proportional to n, not V. Also, T=0 does not imply that "volume and pressure becomes zero". It implies that volume OR pressure become zero, so there is a perfectly good solution with finite volume and density and zero pressure and temperature.
The 3rd law doesn't say that the zero K state is meaningless. It states that it is unachievable through a finite number of thermodynamic processes.

12. Mar 20, 2014

### dauto

Not even that. The argument is just plain wrong.

13. Mar 20, 2014

### Staff: Mentor

The laws of thermodynamics, including the third law, apply to all gases, whether ideal or not. And the third law says that you cannot cool anything down to absolute zero.

14. Mar 20, 2014

### Entanglement

Why??

15. Mar 20, 2014

### Entanglement

Ok ok guys, I'm still a freshman at physics,
What I know is that PV = nRT is only for ideal gases, let's make an well organized conversation so I don't get confused, If we suppose that ideal gases exist why couldn't it reach to zero kelvin ??

16. Mar 20, 2014

### dauto

It's a law. If there was a why it would be a theorem.

There are justifications for if from the point of view of statistical mechanics but for now you probably ought to concentrate on understanding it from the point of view of thermodynamics.

17. Mar 20, 2014

### dauto

there are two ways you can use to remove energy from an ideal gas

1) Heat: You would have to find something colder than the gas to absorb the heat given off by the gas. Nothing can be at zero Kelvin or below so that wouldn't work.

2)Work: The capacity of the gas to perform work as it expands is proportional to its pressure that also decreases as the temperature drops so that doesn't work either.

You're out of luck.

Last edited: Mar 20, 2014
18. Mar 20, 2014

### Staff: Mentor

Loosely speaking, an ideal gas is one which the variation in the amount of energy carried by a single particle is small compared with the total amount of energy contained in the volume of gas. As you cool the gas towards absolute zero, the total amount of energy contained in the gas becomes smaller while the particles stay the same size, so eventually the ideal gas assumption breaks down. Thus, any real gas will stop behaving like an ideal gas if you cool it enough - or equivalently, ideal gases only exist if you stay away from zero kelvin.

And as for your other question, why an non-ideal gas (or anything else, for that matter) cannot reach zero kelvin....

Well, how would I cool something down to zero kelvin? I'd put it in contact with something colder, and let heat flow from my object into the colder object (a heat sink), thereby reducing the temperature of my object. But that heat flow will warm the colder object, so even if it started at zero kelvin, it won't stay there and my two objects will reach equilibrium at a temperature somewhere slightly warmer than zero kelvin. Now, you may object that we just have to cool the heat sink back down to zero kelvin again - but that's exactly what we're trying to do with the first object, so we've just moved the problem around.