Why Does E=mc2_ - Brian Cox [62]
As everyone who has spent time in a school chemistry lab with a box of matches and an inattentive teacher knows, chemical reactions can sometimes lead to the production of energy. A coal fire is a perfect, nicely controlled example; a little nudge from a lighted match and energy is released steadily for hours. More dramatic, an exploding stick of dynamite releases similar amounts of energy to a coal fire, albeit rather more quickly. The energy doesn’t come from the match that lit the fire or the fuse, but from the energy stored within. The bottom line is always that the combined mass of the products of the reaction must be less than the mass we started with if some energy has been lost.
A final example may serve to further illustrate the idea of energy release through chemical reactions. Imagine sitting in a room full of hydrogen and oxygen molecules. We would be able to breathe perfectly well, and at first sight it would appear quite safe and comfortable since it takes energy to pull apart two hydrogen atoms bound together in a molecule. This would seem to suggest that molecular hydrogen should be a stable substance. It can, however, be broken up via a chemical reaction that generates an impressive amount of energy; so impressive in fact that hydrogen gas is very dangerous stuff. It is highly flammable in air, needing only a tiny spark to trigger disaster. In our newfound language, we can analyze the process in a little more detail. Suppose we mix together a gas of hydrogen molecules (two hydrogen atoms bound together) and a gas of oxygen molecules (two oxygen atoms bound together). Now, you might well become very nervous sitting in your room when you discover that the combined mass of two hydrogen molecules and one oxygen molecule is bigger than the combined mass of two water molecules, each of which is made of two hydrogen atoms and an oxygen atom. In other words, the four hydrogen atoms and two oxygen atoms that started as molecules are more massive than two lots of H2O. The excess mass is approximately 6 eV/c2. The hydrogen and oxygen molecules would therefore quite like to be rearranged into two water molecules. All that will be different is the configuration of the atoms (and their associated electrons). At first glance the energy release per molecule is tiny, but a roomful of gas contains in the region of 1026 molecules,9 and that translates into around 10 million joules of energy, which is plenty enough to rearrange your own personal molecules as a side effect. Fortunately, if we are careful, then we are not destined to be incinerated because although the final products have a mass that is smaller than the initial products, it takes a bit of effort to put them, and their electrons, into the right configuration. It is a bit like pushing a bus over a cliff edge—it takes some effort to get it started but when it goes, there is no stopping it. That said, it would be very unwise to light a match, which would supply plenty enough energy to trigger the molecular rearrangement process and get the water production under way.
Liberating chemical energy by shuffling atoms around or gravitational energy by shuffling heavy things around (like huge volumes of water in hydroelectric plants) provides our civilization with a means to generate and harness energy. We are also becoming increasingly adept at harvesting the abundant resources of kinetic energy found in nature. As the wind blows, molecules of air rush along, and we can convert that wild kinetic energy into useful energy by putting a wind turbine in the way. The molecules bang into the blades of the turbine and as a result the molecules slow down, delivering their kinetic energy to the turbine, which starts to rotate (incidentally, that is another example of the conservation of momentum). In this way, the kinetic energy of the wind gets transformed into rotational energy of the turbine, and that in turn can be used to power a generator. Harnessing the power of the sea works in much the same way, except that it is the kinetic energy