Okay, I think we've already mentioned the Coulomb force before. I think Frank mentioned that the Coulomb force is the force between any two electrically charged objects. And it's very important, for example, inside an atom. So an atom has this positively charged nucleus. And as we've talked about several times, electrons, kind of, well, one picture of them is the kind of orbiting around the core, this nucleus. And as we've discussed, the electron has a negative charge and the proton has a positive charge. And if the two electrically charged objects have opposite charges, so, for example, the case in the atom when one is negative and one is positive, then the Coulomb force is attractive. And this is actually one way that the electron keeps orbiting around the nucleus of the atom. But you also get a Coulomb force between two objects with the same charge. For example, they will repel each other a bit like, as we were talking with, it's not the same, but with magnets. They will repel each other. Something that I think people are probably quite familiar with is taking a balloon and rubbing it on something and then sticking it to the ceiling. And actually that uses the Coulomb force, because what you do when you rub the balloon is you're actually removing or encouraging some of the electrons that make up the balloon. So the electrons inside the atoms that make up the balloon, some of them are taken away. And so the balloon now has a positive charge. And you can't see me, but I'm pointing to the ceiling. We put it up in the ceiling just, I think, for fun. They will be attracted to the electrons on the ceiling, which are on the outside layer of the atoms. And therefore we will see that attraction between the two. Another place where we actually see it is if we jump. So there is a force that keeps us on Earth called the gravitational force, and it pulls us down. So when we jump, why don't we just keep on traveling down through the floors towards the surface of the Earth or even further down? And actually it's the Coulomb force again that stops that happening, because our shoes, or our feet, are made again of atoms with electrons on the outside layer. And so these two things repel each other. So actually this Coulomb force is all around us and actually informs a lot of our kind of everyday experience. But again, a lot of this is back to electrons. This is another reason they're really so important in our everyday experience. Can we bring this together in one sense, frankly? How do we imagine the arrangements of electrons

every day? Well, electrons are held in the atom, as Victoria said, by the Coulomb force. The

positive nucleus at the middle and their negative charge is on the electrons, is holding them in place. But as Harry said, that we can think of things as particles or as waves. And thinking of them as waves on this occasion is the positive nucleus at the middle. And then, of them as waves on this occasion is the best way to understand how they act inside the atom. Think of a rope. If you shake a rope and you'll have a wave going on the rope, if one end is tethered somewhere. The higher the energy that goes in there, the shorter the wavelength of the waves in the rope. So it is with electrons, the higher their energy, the shorter the wavelength corresponding to them. Now in an atom, they are going round and round. So it's like a rope where you're trying to make the rope wobble, but attach the far end of the rope to the end that you're holding to make a complete circle. And you can only do that if the far end of the rope is oscillating perfectly in agreement with your end so that it's up when you're up, it's down when you're down to be able to tie them together. So the electron waves as they circle around in the atom can only fit properly if they have a perfect number of wavelengths in a single circuit. And that is not easy to do. It can only happen in certain configurations. And these different configurations have different energies.