It’s October, so time to press on with the Physics part of my OU degree. Unit 6 deals with Linear Momentum which involves looking at collisions. First diagram is from Ernest Rutherford’s famous gold leaf experiment where the nucleus of the atom was discovered. Previous to this, protons and electrons were thought to be scattered in an atom rather like raisins in a cake, sometimes referred to as the Plum Pudding model.
Interpreting the results gave a Rutherford a bit of a headache. In the days before computers and automatic recording equipment, his two assistants sat in the dark for hours recording impacts of alpha particles on a screen. The prediction was that in the event of a collision, they would pass through and not be deflected by very much. Strangely, some of the particles (about 1 in 8,000) bounce back.
Rutherford eventually realises that most of the mass is concentrated in the nucleus and that the Plum Pudding model is wrong. This is embarrassing as this model was first proposed in 1904 by J J Thompson Lord Kelvin – a very famous scientist. Rutherford’s surprise was famously noted as: “it was as if a 15 inch naval shell bounced back after hitting a piece of tissue paper”. Why did he mention 15 inch shells? This is the early Twentieth century and navies’ power largely depends on who has the biggest guns. If you visit the Imperial War Museum, there is a pair of 18 inch guns at the entrance.
Show me, don’t tell me
While the course material is fine, I decide to see what is on YouTube for Rutherford’s gold leaf experiment and come across a new unit – a barn? In physics there are many internationally recognised SI units including 7 Base Units. But the barn is a new one. First two Google searches bring up a barn building but only the third search reveals the answer to what a barn unit is.
The name arises from the Manhattan Project when scientists were designing the first atomic bomb. To make an atomic bomb you need a chain reaction and to start a chain reaction, you bombard a uranium nucleus with atomic particles. But if you are trying to hit something, you need to have an idea how big your target is, in this case the cross sectional area of a nucleus of uranium. Very tiny but much, much larger than say, the hydrogen atom.
This is a top secret project so choosing a name is critical. There is an American saying about someone whose aim is terrible “He couldn’t hit the side of a barn“. Thus if anyone heard scientists talking about barns, they wouldn’t have a clue that it was related to the atom bomb.
While still not recognised as an official or SI unit, it is still used in atomic physics .
