 |
|
|
 |
An everyday demonstration of Einstein's most famous equation: fission reactions are occurring in the uranium fuel of Reed College's Research Reactor.
(Note the blue glow is caused by Cerenkov radiation)
© Reed College Research Reactor
|
|
|
|
|
|
Bits & pieces |
|
|
|
| |
How much energy?
The mass of an atom of U235, plus the incoming neutron, is roughly 3.94 x 10-25 kilos. This is a small number (394 with a decimal point and 24 zeros in front of it) however, the speed of light is a large number (2.997925 x 108 metres per second) and it gets squared (multiplied by itself). For an equivalent amount of material, it works out that fission of U235 releases about 26 million times more energy than burning natural gas.
C is for...
It is thought that Einstein chose the letter c to represent the speed of light because the Latin word for swiftness is celeritas.
What a square.
It's one of the peculiarities of the world: Things tend not to double, instead they increase or decrease by squares. For example, driving three times faster than the speed limit means it will take you nine times longer to stop than it would have if your were travelling at the speed limit. If you're twice as far from a lamp or a speaker, the light or sound is not half as strong, it's a quarter.
|
|
|
|
|
 |
E=mc2 explained |
|
 |
|
|
|
|
If there were such a thing as the top ten equations of all time, the number one spot would have to go to Einstein's famous equation: E=mc2. Here, E stands for energy, m is for mass and c is the speed of light (more on that later).
There's no high-falutin' maths, but the underlying concept can be tricky to get you're head around, if you're not a physicist like Dr Greg Storr, manager of reactor operations at the Australian Nuclear Science and Technology Organisation (ANSTO). He describes E=mc2 as: "Simple but subtle, like a lot of good physics."
It's subtle because E=mc2 says that energy and mass are equivalent. Not 'can be converted to', like you might change Aussie dollars into US greenbacks, but 'is the same thing as'.
"One of the difficulties people have in understanding what Einstein was getting at is because we humans tend to confuse weight and mass," Greg said. "Mass is invariant: An object has the same mass on Earth, the Moon and in deep space."
Mass is also always conserved, it can't just vanish. For example, when something is said to 'disappear in a puff of smoke', it hasn't really gone, it's just been converted to smoke.
Energy is also something that's always conserved. It may change type (from electricity to heat in a radiator, for example), but it'll be there, somewhere.
"The conceptual leap Einstein made is that mass and energy are two expressions for the same thing," Greg explained.
Most of us conjure up a wild-haired old man when we imagine Einstein, but he was just 26 when he came up with this idea in 1905. At the time, he was doing a lot of thinking about light. He found that, as things near the speed of light, what scientists refer to as energy-mass equivalence, became clear, so he chose the speed of light to be a kind of conversion factor in the equation.
It wasn't until the 1930s that the first physical example of E=mc2 was recognised, and that was nuclear fission.
One of the naturally occurring elements that will fission, or split, spontaneously, is uranium. An atom of U235 has 143 neutrons and 92 protons in its nucleus, which makes those atoms unstable (stable atoms have equal, or almost equal, proportions of neutrons and protons).
"THE U235 nucleus wobbles a bit on its own, and sometimes it wobbles enough to split apart," Greg said. "The thing that splits it in a nuclear reactor is a free neutron coming in and getting absorbed into the nucleus, which makes it wobble even more. There are forces holding the nucleus together but it will split if the wobble gets larger than these."
U235 splits into lighter elements, such as barium and krypton (which are radioactive and emit gamma rays and beta particles) and highly energetic neutrons.
"If we add up the mass of U235, plus the neutron coming in, then add up the mass of all the stuff after fission, there's a little bit of mass lost and that's what we see as energy," Greg said.
If you think it's hard thinking about energy and mass as the same thing, try pretending that you don't know about neutrons or fission because, in 1905, Einstein didn't either.
|
|
|
|
|
|
|
© Copyright 2005 - Australian Nuclear Science and Technology Organisation (ANSTO)
| Privacy policy | Unsubscribe | Subscribe | Enquiries | |
|
|
