The Demonstrations

Cryogenics and Superconductivity Demonstrations

The workshops incorporate a number of demonstrations from the following list. Whether each demonstration is performed or not depends on the interests of the group and time limitations.
 

Effects on material properties: Ductile to brittle transition/ hardening

A flexible object such as a piece of rubber hose or a banana will be frozen in liquid nitrogen and its hardness demonstrated e.g. by using it to hit a nail into a piece of ply wood; a soft material such as a bunch of flowers is frozen in liquid nitrogen and crumbled by hand; a tennis ball is demonstrated to no longer bounce after being submerged in liquid nitrogen.
 

Effects on material properties: Thermal contraction

A metal plate with a hole through which a metal ball fits is frozen in liquid nitrogen and it is demonstrated that the ball no longer fits through the hole.
 

Thermal contraction and expansion of a gas

A balloon filled with air is frozen in liquid nitrogen – it shrinks and shrivels as the gas inside contracts and expands as it warms up.
 

Effects of containing an expanding gas

A small amount of liquid nitrogen is placed in tube with a loosely-fitting lid (e.g. an empty Pringles tube), the lid is replaced, the tube is agitated to initiate rapid boiling of the nitrogen. As the nitrogen turns from liquid to a rapidly expanding gas the lid is blown off with a loud popping sound.
 

Gas-liquid phase change expansion effects

Washing-up liquid and water (at room temperature) are mixed together in a plastic bowl; liquid nitrogen is poured in. The relatively high temperature of the water (compared to the nitrogen) causes the liquid nitrogen to change to the gas phase, resulting in a large expansion in volume and lots of bubbles.
 

Difference in boiling points of substances in the air

Liquid nitrogen is poured into a copper conical container suspended above a test tube. Air, due to the presence of oxygen in it, has a higher boiling point than that of liquid nitrogen and so condenses on the outside of the container and is collected in the test tube in which it reforms as a very small amount of oxygen-concentrated gas. A test for oxygen (re-lighting a glowing splint) proves the oxygen concentration of the gas in the test tube. The implications of this in the design of cryogenic systems is discussed. After the demonstration the used splint is put in a container of water. The lighter or matches used to light the splint initially are kept about the person of the demonstrator at all times.
 

Decrease in electrical resistance with temperature

A magnet is slid down/dropped on a selection of metal plates of varying electrical resistivities (e.g. Aluminium, Copper, Stainless Steel) and the impedance of inductive motion noted; the materials are then cooled with liquid nitrogen and the increased impedance noted and explained.
 

Levitation of a magnet using a high temperature superconductor (HTS)

The HTS pellet is cooled using liquid nitrogen and the effects of magnetic levitation and magnetic flux pinning demonstrated and discussed.