So that's one half of the story of electromagnetic charges,
electromagnetic attractions and repulsions.
That was attraction of opposites.
Now we've got the repulsions of light charges.
And that creates a problem for us.
Let's just think. We're all sitting here quite happily.
Nothing untoward happening to us.
Not exploding or anything.
But inside us are atoms and in the middle of the atoms are nuclei.
And here is an enlarged picture of a nucleus.
With lots of neutral particles in it.
But also lots of protons. Each one of which is positively charged.
Now like charges repel.
So each of these protons is repelling all the other protons.
The nucleus then should be exploding apart.
Yet somehow it stays together.
Now that's a puzzle.
We'll come to that in a moment and find the answer to it
but first of all, let me give you an idea of actually how strong that repulsive force is.
Bryson's next demonstration is going to show us the repulsive power
of electromagnetic forces.
So what we're going to do now is first of all cool down this ring
so that we make the effect rather more dramatic.
And we've got a magnet here which we'll turn on when Bryson throws the switch.
When everything is ready.
OK, don't spill it over me.
This is about as cold as, well, the universe out there now.
You all ready?
Oh, this is going to be really dramatic by the time it gets going.
Right, here it is.
Ready to go, put it on the magnet.
Now, turn the magnet on.
[Audience gasps]
OK, you reckon we'll try it again?
[Audience laughs]
[Audience gasps in amazement]
Well done.
[Applause]
So you saw there the power that magnets have got when you can make things repel.
In fact it sent the thing almost as high as I came down from at the beginning.
And then it came down rather faster than I did.
So that's the repulsive power.
That you saw there with a large scale magnet.
Just imagine now then the strength of the repulsive power buried in the nucleus.
These things are really small. Pretty well touching.
And each of them repelling with at least as much power as you just saw.
So how come we are still sitting here happily without our nuclei exploding apart.
There must be something very, very strong here, another force,
a very strong force gripping the nucleus together and overcoming that repulsive power.
Let me show you what we believe today that's caused by.
So here is a picture of a nucleus opened up.
And you see the three quarks buried inside one of them.
And the strong force that binds the quarks together,
we've shown here by these little wiggly lines
to give the impression of the things really being gripped.
Now this force is so strong that it leaks out very slightly.
We show this on the outside here.
And grips the individual neutrons and protons together.
So it's that very strong force, rooted in the quarks, that holds the nucleus together
and resists that outward thrust that we saw the demonstration of.