## PHYS208 Fundamentals of Physics II

### Answer for Ch. 30, 45P

If *l* is the length of the left side of the amperian loop, then the line integral
is *Bl*. Since neither *B* nor *l* is zero, Ampere's law would apparently
be violated since no current is enclosed by the loop.
Thus the sketch of the magnetic field lines must be in error.
The lines actually bulge outward and their density decreases gradually,
not abruptly as shown.

### Answer for Ch. 30, 47P

a)

b)

c)

d) zero

e)
When *r* = *c*, the formula for a) should give the same result as the formula for b).

When *r* = *b*, the formula for b) should give the same result as the formula for c).

When *r* = *a*, the formula for c) should give zero.

### Answer for Ch. 30, 48P

### Answer for Ch. 30, 49P

*i*_{wire} = (3/8) *i*

### Answer for Ch. 30, 51P

a) Suppose the field is not parallel to the sheet, as shown:

Reverse the direction of the current. By the Biot-Savart law, **B** should reverse direction.

Now rotate the sheet by 180 degrees about a line perpendicular to the sheet.
**B** should now point down rather than up, for the same current configuratino as first shown --
in contradication to our initial assumption.

The only direction of **B** which is preserved under these transformations is one directed horizontally.

b) Evaluate the line integral of Ampere's law around the rectangular loop shown.
By the symmetry of the current sheet, **B** will have the same magnitude at the same
distance above and below the sheet; the upper and lower sides of the amperian loop are
constructed to be the same distance from the sheet, each having length *a*.

If the line integral is evaluated counterclockwise, the contribution of each of these sides
is *Ba*. There will be no contribution from the other two sides, since **B** is
perpendicular at all points. So the line integral around the amperian loop is 2*Ba*.

The enclosed current is the linear current density *lambda* times the length *a*
of the side of the loop; *i*_{enc} = *lambda a*.

Thus by Ampere's law, *B* = *mu*_{0} lambda/2,
directed as shown.

"http://www.physics.udel.edu/~watson/phys208/exercises/answers/0403.html"

Last updated April 2, 1998.

Copyright George Watson, Univ. of Delaware, 1997.