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Physics 208 Quiz 4

Solutions
February 13, 2008 (due: February 20, 2008)
Problem 1 (20 points)

A circular loop with a diameter d = 40 cm is rotated in a uniform electric field until the position of
maximal electric flux is found. The flux in this position is Φ = 5.2 · 105 Nm2 /C. What is the magnitude
of the electric field?
The maximum flux is reached when the surface-normal vector of the circle enclosed by the loop is parallel
to the electric field. Since the electric field is constant, we have
Φ 4Φ N V
ΦE = AE ⇒ E = = ≃ 4.14 · 106 = 4.14 · 106 .
A πd2 C m

A pyramid with horizontal square base, a = 6 m on each side, and a height, h = 4 m is placed in an
upward vertical electric field of magnitude E = 52 N/C. Calculate the electric flux through the pyramids
four slanted surfaces.
~
E h
a
Hint: Think about the total flux through the pyramid first, before you do a lot of unnecessary work!
The total electric flux through the pyramid vanishes since the electric field is constant. Thus the flux
through the four slanted surfaces is, up to the sign, the same as through the square base:
Nm2
ΦE = EA ≃ 1.872 · 103 = 1.872 · 103 Vm.
C
l ξ
x
60◦
h
~
E
~ = 7.8 · 104 N/C~iy (see figure).
Consider a closed triangular box resting within a horizontal electric field E
Calculate the electric flux, Φ, through
(a) the vertical rectangular surface,

The electric field is constant, and the surface-normal vector points in direction −~iy . Thus we have
ΦE,V = −lhE.
(b) the slanted surface,

I choose parameters x ∈ (0, l) and ξ ∈ (0, d) = (0, 2h) as indicated in the figure.
√
3 1
~ ~ ~
~r(ξ, x) = xix + ξ iξ , with iξ = − ~iy + ~iz
2 2
The surface-normal vector is
√ !
~ = ∂~r × ∂~r dξ dx = ~iξ × ~ix =
dA
1~
iy +
3~
iz dξ dx. (1)
∂ξ ∂x 2 2
Note that the order of the parameters, ξ and x, has been chosen such that we get the right orientation
of the surface vector, namely pointing upwards! The flux is
Z l Z 2h
E
Z
~
ΦE,S = dS · E =~ dx dξ = lhE.
0 0 2
(c) the entire surface of the box.

The total flux through the box is given by the sum of (a) and (b) since the normal vectors of the
~ The total flux is thus
other surfaces are perpendicular to E.
ΦE,tot = ΦE,V + ΦE,S = 0,
as to be expected for a constant electric field.
Problem 4 (30 Points)

A charge, q, is located in the center of a coordinate system. What is the total electric flux of its electric
field through the entire surface of a cylinder with its axis along the z-axis (reaching from z = −h/2 to
z = +h/2 and radius ρ = R (see figure)?
z
(a) Use Gauss’s Law to find the answer!
(b) Do the integrals to verify it! h y

q
R
x
Hint: Use cylinder coordinates, (ρ, ϕ, z)! The following integrals are needed to solve the problem:
ρ 1
Z
dρ 2 2 3/2
= −p ,
(ρ + z ) ρ2 + z 2
(2)
1 z
Z
dz 2 2 3/2
= p .
(ρ + z ) ρ2 ρ2 + z 2
Solution
(a) According to Gauss’s Law the flux through the cylinder is the enclosed charge ×1/ǫ0 :
q
ΦE = .
ǫ0
(b) To evaluate the flux we have to parametrize the three surfaces of the cylinder, i.e., the top and
bottom caps and the envelope. This is most easily done in our standard cylinder coordinates ρ, φ,
and z. In these coordinates the electric field reads
~ = 1 ~r = q
E ρ~iρ + z~iz . (3)
4πǫ0 r3 4πǫ0 (ρ2 + z 2 )3/2
For the upper cap the parameters are ρ ∈ (0, R) and φ ∈ (0, 2π), and z = h/2 = const. The
~ = ρ dρ dφ ~iz . Plugging all this into the flux integral, gives
surface-normal vector is dA
2π R
ρh q
Z Z
Φupper = dφ dρ , (4)
0 0 2 4πǫ0 (ρ + h2 /4)3/2
2
and using the first of the integrals, given in Eq. (2), leads to
" #
q h
Φupper = 2− p . (5)
4ǫ0 R2 + h2 /4
The lower cap gives the same result due to symmetry:
Φlower = Φupper . (6)
For the envelope of the cylinder the right parametrization is given by z ∈ (−h/2, h/2), φ ∈ (0, 2π),
while ρ = R = const, and the surface-normal vector is given by dA ~ = R dφ dz ~iρ . With help of the
second integral, given in Eq. (2), we find
2π h/2
q R2 q h
Z Z
Φenv = dφ dz = p . (7)
0 −h/2 4πǫ0 (R + z )
2 2 3/2 2ǫ0 R + h2 /4
2
For the total flux, we have to add our results (5-7):

q
Φtot = Φupper + Φlower + Φenv = 2Φupper + Φenv = , (8)
ǫ0
which shows that Gauss’s Law is correct.

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Name: ................................................ Section:.................

Physics 208 Quiz 4

Problem 1 (20 points)

Problem 2 (20 points)

(a) the vertical rectangular surface,

(b) the slanted surface,

(c) the entire surface of the box.

Problem 4 (30 Points)

(a) Use Gauss’s Law to find the answer!

(b) Do the integrals to verify it! h y

Φlower = Φupper . (6)

For the total flux, we have to add our results (5-7):

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