Alt+F4 said:
hmm can you explain more? Basically in lecture and discussion they taught us the most basic stuff
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/sp02
So For question 1, how would you know where the Force is at all?
Another Question
If i Just Curl my fingers based on the Current or B Field, my thumb will tell me where the force is right? if it is negative then i switch. So Can i just somhow ignore Velocity?
For any moving positive charge, the right hand rule will tell you the direction of the magnetic field. Point your thumb in the direction of motion (velocity) and your findgers will curl in the direction of the magnetic field. The field circulates around the line on which the charge is moving. If a positive charge is moving toward you, the field is CCW. If it is moving away from you, the field is CW. A current is just a group of charges all moving at the same time. For a straight wire they are all moving in the same direction, so point your thumb in the direction of the current.
For the watch in #1 the center of the watch is always the same distance from the moving charge. The question does not ask about force; it asks about the direction of the field produced by the charge. There will be no force on the moving charge unless it is moving through a field produced by other moving charges. It would be hard to hold your hand with your fingers curled around the direction of motion. But you can rotate the picture either physically or in your mind to move the charge to the bottom of the figure. Then it will be easy. Convince yourself that the field in the middle of the watch is the same for any time of day.
Once you realize that the charge on the watch hand always produces the same field in the center of the watch, imagine that you have a whole ring of charges moving together. Each charge produces the same field at the watch center, so all those fields add together to make a stronger field. What you have in this case is a current loop. This leads to a different right hand rule for a current loop. From doing this problem, you should be able to come up with this rule. If you curl your fingers in the direction of the loop of current, what is the direction of the field
within the loop. Actually, the field strength varies with distance from the center of the loop, but every point within the plane of a current loop has the same field direction.
The force on a moving positive charge from an existing magnetic field (created by other moving charges) has a different right hand rule. Hold your hand so that your fingers curl in the same sense as a smallest angle rotation from the v direction to the B direction. Then your thumb points in the direction of the force. Again, a current is a group of moving charges, so the direction of current is the direction of velocity. It's not that you are ignoring velocity, it is that velocity is in the same direction as the current.
You are correct that if the charges are negative, everything is reversed. You can use left hand rules for negatvie charges. If for some given configuration you reverse a velocity direction, the force is reversed. If you reverse a field direction the force is reversed. If you reverse both the velocity and the field what happens?
