 Let's look at the rotation of a particle in a magnetic field. Now, I've already covered some of this in a previous video, so if you haven't seen that one, you do want to go back and watch it. Remember from that video that a magnetic field on a moving particle creates a force given by the cross product with Q times V cross B. And we saw that if the particle enters it at a perpendicular direction of the field, it will cause a circular path. And I have a lot more detail in that video. But what I didn't discuss in a lot of detail was all the possible directions. Now, since we're in the digital age, I do have to actually remind students what clockwise and counterclockwise means. So clockwise means the direction you go around is the direction of the numbers on a clock. And counterclockwise is the opposite direction. If I have a magnetic field, and in this case I'm using magnetic fields that are perpendicular to the screen, either pointed inward, shown by the X's, or as dots pointed outwards, then if I shoot a charged particle in from, I'm using from the left side in this case, but it actually doesn't matter which direction you shoot it in from. We can look at what happens to the charges. Now I've got two colored arrow here. Up at the top I say for positive charges. See, the positive charges will go one direction, and the negative charges will go in the other direction. So how do we know which path is for the positive charge and which path is for the negative charge? And similarly, if the field points outwards, how do we know which one is for the positive charge and which one is for the negative charge? This is where we want to use our right hand rule. And the right hand rule I'm using with my students has your fingers pointing in the direction of the magnetic field, your thumb pointing in the direction of the velocity, and then the palm of your hand points in the direction of the force on a positive charge, while a negative charge would be pushed in the opposite direction off the backside of your hand. And again, there's another video that does more about that. So I'm going to go through one of these, and then you can think a little bit more about how the rest of them make sense. So if we start with the field pointed inwards, pick up your right hand, point your fingers in towards the screen. And if you rotate around your wrist until your fingers are still pointed in the screen but your thumb is pointed towards the left, you'll notice that your palm is facing up. And that means my charge when it comes in would be deflected upwards. So this is the path a positive charge would flow. And then by symmetry, the negative charge would go in the other direction. So I'm using my purple here for the positive charges. Now if the field is pointed outwards, you'd have to sort of rotate your wrist around outwards. That's a little uncomfortable, but just kind of make sure that you're doing it with a loose elbow and rest. And you rotate it around so that your thumb is pointed to the left. You'll see that your palm is facing downward. So in this case, the positive charge would curve downward, and then the negative charge would curve upward. Now I say down and up, I mean relative to what we have on the screen. Now you can go back and think about the definitions of clockwise and counterclockwise to figure out whether it's rotating clockwise or counterclockwise for the positive and negative charge, depending on if the field is pointed inwards or outwards. Might take you just a little bit more practice to really get the hang of it, but it's a good exercise in doing the right hand rule for our force law.