Lecture 5 – Atoms and energy

From the 1979 lecture programme:

Most atoms are stable, carbon, hydrogen, oxygen, gold and many others. They stay the same from now to doomsday — except for swapping loose electrons in chemical compounds or in making ions. 

But some atoms are radioactive, waiting to explode and hurl out a small 'chip' thus becoming quite a different atom — an atom of a different chemical element. The 'chip' flies out with so much energy that it knocks electrons off hundreds of atoms in the surrounding air — using electrical forces as it flies past. That was how radioactivity was discovered, by the ionization it causes.

And that is how it can sometimes hurt — or cure — your body, by making ions. See a radioactive sample making ions which are driven by an electric field, in a tiny measurable current. Watch three 'mystery experiments' which prepare for radioactive measurements. See Geiger counters respond to three kinds of 'chips' from different radioactive materials — alpha particles (heavy but speedy), beta particles (just high speed electrons), gamma rays (fastest of all, very short X-rays).

Watch a stream of beta particles pulled into orbit by a magnetic field. The ions formed in wet air by flying 'chips' attract water molecules electrically and thus form starters for water drops in a cloud. A cloud chamber makes use of that and lets us see the track of a 'chip' as a thin line of tiny water drops. See a cloud chamber in action, then look at photographs of special events in it. Those show the clearest evidence for nuclear atoms. The tiny explosions of radioactive atoms seem to happen by pure chance, at random.

But if we have a vast quantity of them the tale of random events smooths out to a steady rate. If we start with a large stockpile, a Geiger counter will show many explosions each minute, then fewer as time goes on — fewer because the stockpile has grown smaller. After a certain time the stockpile has dwindled down to half what we started with; and we call that time the half-life of that radioactive material.

But the remaining half-sized stockpile is, so to speak, as healthy as ever — it shows no signs of old age. It too will dwindle to half in the same half-life from then; and the remainder after that, a quarter of the original stockpile, will dwindle to half in one more half-life; and so on indefinitely.

That law of decay, with a constant half-life, is characteristic of all radioactive materials; and it is essential for people to understand it before they discuss dangers and values of nuclear power. So we shall offer illustrations and show a real example of radioactive decay.

We can now manufacture radioactive isotopes ('twin brothers') of ordinary stable chemical elements. We use them to act as sensitive labels, or tracers, for those elements. See a tracer being used. In preparation for new atomic models in the final lecture we shall show another strange behaviour: a stream of light delivering its energy packaged in 'bullets' of energy — 'photons' as we call them.

That is useful in many practical devices such as an 'electric eye' or a camera's light meter, and it is revolutionary in the world of new physics. Revolutionary because it shows that light has a double nature: behaving as waves — as we shall also show — and (or is it or?) as a stream of compact bullets which can cause more fissions, in a chain reaction. That delivers terrific heat and radioactive products either explosively in a bomb or steadily in a powerstation reactor. You will see models of nuclear changes, fission, chain reactions.


Natural world




Eric M Rogers



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