 But we have not yet addressed what magnetic fields do. So we saw that we made electric fields from charges and we made magnetic fields from moving charges. We discovered that electric fields apply forces to charges and so it might be a little bit unsurprising to discover that what magnetic fields do is they apply forces to moving charges. So if I have a charge in a magnetic field then if that charge isn't moving it doesn't actually see any force due to that magnetic field. So I have to have a moving charge and if I have a moving charge in a magnetic field then it does see a force. So you get the direction from another right hand rule. You take your right hand, you point your fingers in the direction of the velocity that the charge is moving. You swing your fingers round to the magnetic field and then your thumb is the direction of the force. So V, swinging around to B, thumb is F. So we look at the velocity, I put the fingers of my right hand in the direction of the velocity. I swing them round to the magnetic field and my thumb is pointing straight up at my face and so the force is coming up at us. And we describe that by an arrow with a tip. So the force is coming out of the screen there, straight up at us. And the strength of that force is given by the charge times the velocity times the magnetic field and there's a sign of the angle between the velocity and the magnetic field. So when they're at right angles, it's just charge times velocity times magnetic field. And the current is just a whole bunch of charges moving together and so if we know what happens to one charge, we can figure out what happens to a current. So if we have a current traveling along in a magnetic field, then we should be able to figure out what the force is on that. So is there moving charges in a current? Absolutely, that's exactly what a current is. So if this distance is a length L, so if it's a length L of Y, I say, then we know that the current is telling us how much charge is going along that length of wire every second. So if we want to know the charge times the velocity, well, velocity is just length divided by time and so that's just going to be the charge times that length of wire divided by the time it takes for charges to travel along that wire. And if we rearrange and note that the charge per unit time is just exactly what we meant by the current, then instead of having a charge times the velocity, we'll end up with the current times the length. And if you look at that, that has exactly the same unit. So we expect the strength of the force to be, and indeed it is. And if we want to get the direction, we can get it from the same right-hand rule we had before. Before, remember we have our right-hand fingers in terms of the velocity and we swing it round to B. So now we have them in the direction of the current, which is the same as the velocity of the charges and swing it round to B. And so the current, we swing them round I through to B and I have to have my right-hand thumb pointing down into the screen. And so the direction of the force is down into the screen. From our right-hand rule. So if the current is perpendicular to the magnetic field, if they're at right angles, then the force is just the current times the length of the wire times the magnetic field.