8/12/2019 Quiz 2 Corrections
1/2
Quiz 2[10] Two parallel, infinite sheets of charge are a
distance d apart. The right one carries acharge density and the
other -. The electric field in regions I, II and III will be (+ if
towardthe right)
Answer: c) 0, -, 0
Reason:
As shown by the diagram above, we can use Gaussian surfaces to
find the answer to thisproblem. The red arrows show the direction
of the electric field for each separate sheet of
charge. With these surfaces, we know that our equation
is equal to
()
for the sheet on the right and ()
for the sheet on the left. This gives us
for the positively charged sheet and
for the negatively charged sheet. Using
superposition, we can see that the electric field in regions I
and III will be zero. This is alsoshown by the purple arrows
canceling out the red arrows in the above diagram. In region II,the
electric field will be negative since the electric field from both
sheets separately arepointing to the left, which is negative.For
questions 11-13 match the power of r dependence of the electric
field with the correctcharge distribution:
[11] Charged Sphere
Answer: e) 1/r2
Reason: We know that the surface area of a sphere is . Using
Gauss Law:
we can solve it for the case of a charged sphere. This gives us
()
and
. Therefore, we can see that the r dependence of the electric
field is 1/r
2
.
[12] Infinite line of chargeAnswer: d) 1/rReason: The electric
field of a line of charge can be found by superposing the point
chargefields of infinitesimal charge elements. The radial part of
the field from a charge element is
