Download electric forces and electric fields

MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 11: ELECTRIC FIELDS AND FORCES
WORKSHEET SOLUTIONS
1. At what point is the electrical field associated with a uniformly-charged, hollow, metallic sphere
greatest?
(A) center of the sphere
(B) at the sphere’s inner surface
(C) at infinity
(D) at the sphere’s outer surface
(E) within the metal shell
1A. (D) at the sphere’s outer surface
2. Two point charges are placed along a horizontal axis with the following values and positions: +3 C
at x = 0 cm and -7 C at x = 20 cm ( = “micro” = 10-6). What is the magnitude of the electric field at
the point midway between the two charges (at x = 10 cm)?
2A.
(1) E = E1 + E2
(2) E1 = kq1 / r2
(3) E1 = (9.0 x 109 N.m2/C2)(3 x 10-6 C) / (0.10 m)2
(4) E1 = 2.7 x 106 N/C
(5) E2 = kq2 / r2
(6) E2 = (9.0 x 109 N.m2/C2)(7 x 10-6 C) / (0.10 m)2
(7) E2 = 6.3 x 106 N/C
(8) E = 2.7 x 106 N/C + 6.3 x 106 N/C
(9) E = 9.0 x 106 N/C
3. Initially, a net charge of – 5.0 C is placed on a 5.0 cm radius metallic hollow sphere. Next, a +10
C charge is carefully inserted at the center through a hole in the latter’s surface. What electric field is
present at a point 10 cm from the center of the sphere?
3A.
(1) E = k(QEFF) / r2
(2) E = (9.0 x 109 N.m2/C2)(5 x 10-6 C) / (0.1 m)2
(3) E = 4.5 x 106 N/C
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
4. Two point charges are separated by 5 cm and have charges of + 3.0 C and – 4.0 C, respectively.
What is the electric field at a point midway between the two charges?
4A.
(1) E = E1 + E2
(2) E1 = kq1 / r2
(3) E1 = (9.0 x 109 N.m2/C2)(3 x 10-6 C) / (0.025m)2
(4) E1 = 4.32 x 107 N/C
(5) E2 = kq2 / r2
(6) E2 = (9.0 x 109 N.m2/C2)(4 x 10-6 C) / (0.025m)2
(7) E2 = 5.76 x 107 N/C
(8) E = 4.32 x 107 N/C + 5.76 x 107 N/C
(9) E = 1.01 x 108 N/C
5. A Van De Graaff generator has a spherical dome of radius 42 cm. Operating in dry air, where
“atmospheric breakdown” is Emax = 3.00 x 106 N/C what is the maximum charge that can be held on
the dome?
5A.
(1) E = kq / r2
(2) q = Er2 / k
(3) q = (3.00 x 106 N/C)(0.42 m)2 / (9.0 x 109 N.m2/C2)
(4) q = 5.88 x 10-5 C
(5) q = 58.8 C
6. A glass rod rubbed with a silk cloth acquires a charge of + 7.40 x 10-9 C. How many electrons were
transferred from the glass to the silk?
6A.
(7.40 x 10-9 C)(1 electron / 1.6 x 10-19 C)
= 4.63 x 1010 electrons
7. A positive point charge is brought near a neutral conducting sphere. The sphere is isolated from
ground. Sketch the induced charge on the sphere.
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
7A. The positive charge attracts negative charge to the left side of the sphere leaving positive charges
on the right side:
8. A positive point charge is brought near a grounded conductor. Sketch the induced charge on the
conductor’s surface:
8A. The positive charge attracts electrons from ground to the sphere’s surface – the closer the charge
is to the sphere the more negative it becomes:
9. What happens to the charge on the sphere when the positive charge is removed?
9A.
It runs back to ground through the wire joining it to the earth. A charged conductor is made neutral by
grounding it.
10. Three point charges are on the x-axis as shown below. Compute the net force on the 30 micro
(30x) coulomb charge.
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
10A.
(1) Force from the left: F = kq1q2 / r2
(2) F = (9.0 x 109 N.m2/C2)(20 x 10-6 C)
(30 x 10-6 C) / (0.4 m)2
(3) F = 33.75 N (to the right)
(4) Force from the right: F = kq1q2 / r2
(5) F = (9.0 x 109 N.m2/C2)(25 x 10-6 C)
(30 x 10-6 C) / (0.6 m)2
(6) F = - 18.75 N (to the left)
(7) FNET = 33.75 N – 18.75 N
(8) FNET = 15 N to the right
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