# physics EM

posted by on .

Which of the following choices of path allow you to use Ampère’s law to find B(r).
1. The path must pass through the point r_vec.
2. The path must have enough symmetry so that B(r)x dl is constant along large parts of it.
3. The path must be a circle.
choices are....

a only
a and b
a and c
b and c

• physics EM - ,

You can use amperes closed form integral on ANY closed path. To get B, the current has to pass through the surface formed by the closed path.
Now, I don't like the answers. THe question is which choices of path allow you to ...

Then all the answers say path MUST....
a) I have no idea what rvec means, without some graphical reference.
b) The path does not have to have any symetry, but it is nice if it does. In either case, you are allowed to choose any path which encloses the current.
c) The path does not have to be a circle, but it is nice if it does.

Did a Graduate Student write this test question?

• physics EM - ,

no its a website. and that last choice doesnt make sense cause it asks which allows and that sats it must be...so is that one right? and the r_vec is is some point in the field.

• physics EM - ,

the answer was a and b wow that's retarded.