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### Course: MCAT>Unit 8

Lesson 13: Magnetism

# Magnetism - Part 3

Explore the force on a moving charge from a magnetic field, using the cross product of velocity and the magnetic field. Discover how this force, perpendicular to the velocity, causes the particle to move in a circular path. Uncover the intriguing fact that the force depends on the particle's velocity and charge, not its mass. Created by Sal Khan.

## Want to join the conversation?

• I think I am having some hand issues. If my index finger is pointing in the velocity, my middle finger is pointing up, my thumb is actually pointing out, not down (as Sal said in the video). I never learned this trick.. so any help is appreciated!
• Hi everyone! There's a ivy league trick to the Right Hand Rule for magnetism.

Use your hand in relaxed form with your fingers forward and thumb pointed away from the fingers. The thumb is always the flow as it represents one vector . The fingers are always the field as there are multiple vectors usually in parallel. This leaves you with the palm to represent the force (we push with the palm) due to the system. No velocity means no thumb and no thumb means no right hand (at least as we know it anyways). So no hand means no force. Same goes for when the fingers are missing.

Hope this helps!
• how on earth @ are his middle finger and thumb able to bend in opposite directions? anybody else having issues?
• I can see how it would be easy to visualize it like your thumb would have to bend in the complete opposite direction of your middle finger. However, this is not the case. Since your middle finger should actually be pointing toward yourself, your thumb will be able to naturally point in the downward direction.

It should look like this:
Your index finger will be pointing in the same direction as the velocity (to the right).
Then, your middle finger will actually be pointing toward you, in the same direction and the same way that we visualized it coming out of the screen. (Your palm should also be facing you.)
This way, your thumb will be pointing in the downward direction.
• u all are really genius :) but im a looser :(
• Don't say that - anyone can learn anything :)
• Why do you use sine instead of cosine? I thought cross products were done with cosine and dot products with sine, am I incorrect?
• Other way around :) cross products are sine and dot products are cosine. (Just remember CD for Cosine-Dot)
• What is the definition of a Magnetic field
• It is impossible for my middle finger to point up while my thumb is pointing down.
If my middle finger is pointing up and my index finger is pointing in the direction of the charge, then my thumb is pointing to the right. I hate the right hand rule. I never know which way the magnetic force is pointing even when i follow the instructions.
• Agasha, The magnetic field is always at a right angle to the direction of your charge. For the right hand rule this is represented by your middle finger. In your case the middle finger is pointed up so that is the direction the magnetic field would be if the movement of your particle was straight forward as you have indicated with your pointer finger. The direction of force is perpendicular to both the magnetic field and the direction of your charge. that is why it is pointing to the right (your thumb) in you example.
(1 vote)
• Would a moving negative charge be acted upon in the opposite direction? (Since Q would be negative?)
(1 vote)
• Is the circle thing why magnetic field lines are drawn as circles?
Or is the circle thing in this case just describing the path of the positive charge, since the magnetic force acting on it is perpendicular to the charge's velocity?

And in that case, what defines magnetic field? What do the vector lines describe?
(Here it looks like the positive charge is going around a magnetic field line? Or the plane of the circle is perpendicular to the magnetic field line/magnetic field direction?)
(1 vote)
• I think the initial deflection is incorrect. It ought to go upwards ... correct?
(1 vote)
• isnt sin(90) 0? or is my life a lie
(1 vote)