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# Magnetic force on a charge

Sal shows how to find the size and direction of the magnetic force using F=qvB and the right hand rule. Created by Sal Khan.

## Want to join the conversation?

• what is a cross product? i dont understand
• Cross product is a special kind of vector product in which you get another vector by multiplying the two given vectors, and this resultant vector is perpendicular to both of them. The method to find the magnitude and direction of the resultant vector is as follows:
Suppose u have 2 vectors: A & B.
| A X B | = | A | | B | sin(t) where t is the angle b\w the 2 vectors. This is the magnitude.
to find direction, jst put the palm of ur right hand in the direction of vector A. Now curl ur fingers in the direction of vector B. Wherever ur thumb points, that's the direction of the resulltant!!
• So, what exactly is a magnetic field? Isn't it the region under the influence of a magnetic force? Then what did Sal mean by " the effect that magnetic fields have on moving charges" ?
• Magnetic fields are relativistic electric fields (ie. the creation of an electric field due to space contraction from moving charges), but this was not known until Einstein's theory of relativity. Before relativity was discovered, magnetic fields were known to be generated by current (ie. moving charges) or changing electric fields.
• so the earth's magnet at the top is really the south pole?
• Yes. The geographical north pole is actually the magnetic south pole, and vice versa.

Interestingly though, studies suggest that the polarity of Earth's magnetic field swaps over a very long period. Thus, it is correct to say at some point in time, like geographical and magnetic poles were in the same hemispheres.
• Why does F = Q(v * B)? How did we derive this equation?
• It's not derived, it's an empirical observation.
If you think about it, it should make sense, or at least parts of it should. if there's a force on a charge when it moves through a field, then it makes sense that the force gets twice as big if you:
a) make the charge twice as big
b) make the field twice as strong
c) move the charge twice as fast
• could you make a train with magnets that push the train wheels
(1 vote)
• Anything with an electric motor is pushed by magnets
• I noticed that in some videos you write charge with a capital Q and in others you write it with a lowercase q. Is there any significance to that or are they interchangeable?
• Khan is sloppy with conventions; Happens when you are a polymath.
• at sal said that both electricity and magnetism end up being the same thing and i'm curious to know how so if anyone can tell me how i'll appreciate that.
• Go to youtube and search for "Veritasium how do magnets work" and watch the two related videos.