Birmingham July 2012

Saturday July 7^th

The third lecture was given by Lynne Long, Schools Liaison Officer for the School of Physics and Astronomy at Birmingham University. She started her lecture by looking at something really cold.

Children are fascinated by liquid nitrogen because it looks hot, although steam vapour goes up.

Not terribly clear in the above picture but vapour is falling downwards from the cup of liquid nitrogen. This tells us the vapour is cold. The vapour is in fact water vapour condensing around the liquid nitrogen. Liquid nitrogen is as dangerous as something hot with a temperature 0f -196 degrees Celsius. It is dangerous to store and children are intrigued by this: 1) it is very cold but you can quickly pour it over your hand as it rapidly turns back into a gas;

2) The lid must not be screwed on to the container to allow for expansion if the liquid nitrogen turns back into a gas; 3) It can cause suffocation if it escapes into an enclose space as it displaces air. This is the reason why you can’t transport liquid nitrogen in a car.

The above picture shows liquid nitrogen being siphoned out of the container through a rubber tube. Atmospheric pressure pushes the liquid up the tube into a region of reduced pressure at the top of the tube. This reduced pressure means gravity pulling down on the liquid in the tube is not sufficient to keep the liquid stationary so it flows up and over the top of the tube (siphon). As an added extra the rubber becomes brittle and would easily shatter at such low temperatures.

An instant Popsicle with liquid nitrogen.

A balloon deflates when put into liquid nitrogen as the air inside cools and liquid oxygen can actually be seen. As soon as the temperature of the balloon increases it inflates again.

What Lynne is demonstrating here is something my year 13s do. If a small powerful magnet passes down a tube it takes a very long time to reach the end (much slower than free-fall). The reason is because the copper tube “sees” a changing magnetic field from the falling magnet. This changing magnetic field induces a current in the copper tube. The induced current in the copper tube creates its own magnetic field that opposes the magnetic field that created it (Lenz’s law). This produces a force that opposes gravity.

Now if the magnet and tube are cooled in liquid nitrogen then the magnet takes even longer to fall down the tube. The resistance of the system has decreased producing larger currents therefore a greater magnetic field opposing the movement downwards.

Lynne then demonstrated a magnet levitating above a high temperature superconductor cooled with liquid nitrogen.

Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (Faraday’s law of induction). This current effectively forms an electromagnet that repels the magnet. This process is used by certain Japanese trains.

The Yamanashi MLX01 MagLev train.

The temperature in space is about -269 degrees Celsius (although some of the satellites have even lower temperature than this inside to work properly).

Lynne then talked about the effect of high temperatures. Iron melts at 1808 kelvin. An exploding supernova’s temperature is 100 million degrees Celsius. We human’s have a very small range of temperature changes: from snow to a hot beach

Lynne then went on to outline the lecture that she gives to school students.

Average molecular kinetic energy = 3kT/2 (k is the Boltzmann constant and T is the temperature in Kelvin). There is more energy in a bath tub of warm water than one single white hot ember.