Question

a wire is carrying a current (+) south. Under the wire the magnetic field is : a)south b)down c)east d)west

please explain

question 2: a proton is moving east in a magnetic field that is south. the force on the proton is directed: a) up b) down c) east d) north ???

THANKS

Answer 1

For 2 v ~ east and B ~ north, as the current determines the direction of the speed of the electrons flowing. Again we invoke f ~ v X B; so we curl from east to north. And this time the RH thumb is pointing out of the map or up. ANS C )

Give yourself a thumbs up with your right hand. The direction of the thumb is the resultant force direction. The curl of the four fingers as they form a fist follows the direction from one vector, e.g., v, to the other vector, e.g., B, that result in that force direction.

So, for 1, v ~ east and B ~ south. Then from f = q(v X B) you curl your RH fingers from east to south. (Always curl from the first vector to the second one in a cross product.) And whichever direction the thumb is pointing, that's the direction of the force f. For imagery, imagine north, south, east, and west on a map surface. We keep the map oriented so that the equator is at the bottom of the page. So when you curl your RH fingers from east to south, with a bit of twisting around, your thumb is pointing down or into the map. ANS b.