Field Force
The magnetic field of the earth... What is the direction of the magnetic force in this case?
The magnetic field of the earth at a certain location is directed vertically downward. A proton is moving horizontally toward the west in the field. What is the direction of the magnetic force the field exerts on this charge?
The answer is south but I don't know why.
The force F on the proton (a positively charged particle) is F = qV x B
where qV is the charge of the proton times its velocity and B is the magnetic field strength. All three capitalised quantities are vectors and the right side of the above equation is a cross product which immediately gives you the southerly direction for F.
If you don't understand cross products you can take refuge in the right hand rule. Put the thumb, forefinger and middle finger of your right hand all mutually at right angles. qV is your thumb; B your forefinger; F your middle finger.
Alternatively you can use the right hand (normal) screw rule. Rotate from qV to B and imagine this to be a quarter turn of a normal screw. The direction the screw moves is the direction of F.
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