Feynman and His Physics by Jörg Resag

Author:Jörg Resag
Language: eng
Format: epub
ISBN: 9783319968360
Publisher: Springer International Publishing

It is therefore quite difficult to confine superfluid helium within a vessel, since it can penetrate even the finest pores and it can also creep out over the vessel walls and escape into the open. In both processes it shows no viscosity.

However, it would be a simplification to say in general that helium II has no viscosity at all. For example, if a metal cylinder is set in rotation in helium II, it seems that after some time the liquid is drawn along with it and gradually set in rotation. This can be seen, for example, by the fact that the co-rotating liquid is able to set a small propeller wheel in rotation. If helium II had no internal friction at all, this would not be possible – the rotating metal cylinder would have no way of setting the surrounding liquid in motion.

How can these contradictory results about viscosity be explained? In order to solve this problem, the Hungarian-born American physicist Laszlo Tisza came up with the following idea in 1938. Helium II could in some sense be a mixture of two liquids. One of them is superfluid and can creep through the finest pores and glide up the walls of vessels without any friction. The other component, on the other hand, has the properties of a normal liquid and has a certain – albeit low – viscosity.

This simple model can actually explain many of the properties of helium II. It is found that the superfluid component behaves like a medium without any thermal energy. The helium atoms in this component do not appear to perform any random motions or oscillations to which a thermal energy and temperature could be assigned. If they move at all, they clearly do so only in a strictly ordered manner, hence in step, as it were.

The normal component, on the other hand, behaves like a gas whose particles reside in the suprafluid component and can move independently through it. Therefore, this component is perfectly capable of storing and transporting thermal energy.

In this model, the extremely effective heat conduction of helium II can be explained like this. At the heat source, the normal component absorbs thermal energy and moves away from the source to colder areas to distribute the heat. In order to keep the total helium density constant, the superfluid component flows in turn to the heat source, where it is partly converted into the normal component by the heat. Thus, there are two opposing flows of normal and superfluid components, and their velocity is limited only by the low viscosity of the normal component – the heat energy of the source is therefore dissipated and dispersed very quickly.

When all these phenomena are analyzed in detail, it is found that the closer the temperature comes to absolute zero, the greater the proportion of the superfluid component will be (Fig. 4.8). However, this picture should not be overinterpreted either. There are not really two different liquids in helium II that could somehow be separated from each other.

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