Download Electric Fields / Potential Energy /

Practice Questions XIX
- Electric Charges, Forces&Fields
Name :
Date :
Level : Physics I
Instructor : Kim
1electron(e-) = -1.6×10-19C
Q=ne
Fe = k
|𝑸𝒒|
𝒓²
[N]
𝑸
1proton(e+) = +1.6×10-19C
𝑭
E = k 𝒓² [N/C] or E = 𝒒 [N/C] where k= 9×109 N·m²/C
1. Each of three objects has a net charge. Objects A and B attract each another. Objects B and C also attract
one another, but objects A and C repel one another. Which one of the following table entries is a possible
combination of the signs of the net charges on these three objects?
A
B
C
(a)
+
+
–
(b)
–
+
+
(c)
+
–
–
(d)
–
+
–
(e)
–
–
+
2. A conducting sphere has a net charge of –4.8×10-17C. What is the approximate number of excess
electrons on the sphere?
(a) 100
(b) 200
(c) 300
(d) 400
(e) 500
3. Complete the following statement: When a glass rod is rubbed with silk cloth, the rod becomes positively
charged as
*~there is only one answer~
(a) positive charges are transferred from the silk to the rod.
(b) negative charges are transferred from the rod to the silk.
(c) positive charges are created on the surface of the rod
(d) negative charges are transferred from the silk to the rod
(e) positive charges are transferred from the rod to the silk
hint : only the electrons move, protons stay put
4. An aluminum nail has an excess charge of +3.2×10 -6C. How many electrons must be added to the nail to
make it electrically neutral?
(a) 2×1013
(b) 2×1019
(c) 3.2×10-6
(d) 3.2×106
(e) 5×10-14
1electron(e-) = -1.6×10-19C
Q=ne
Fe = k
|𝑸𝒒|
𝒓²
[N]
𝑸
1proton(e+) = +1.6×10-19C
𝑭
E = k 𝒓² [N/C] or E = 𝒒 [N/C] where k= 9×109 N·m²/C
5. Two very small spheres are initially neutral and separated by a distance of 0.5m. Suppose that 3×10 13
electrons are removed from one sphere and placed on the other. i) What is the magnitude of the electrostatic
force that acts on each sphere? (ii) Is the force attractive or repulsive? Why?
a) 0.83N, repulsive
b) 0.43N, repulsive
c) 0.83N, attractive
d) 0.43N, attractive
6. In a vacuum, two particles have charges of q 1 and q2, where q1 = +3.5×10-6C. They are separated by a
distance of 0.26m, and particle q1 experiences an attractive force of 3.4N. Find the charge of q2.
a) +7.3×10-6C
b) –7.3×10-6C
c) +4.8×10-6C
d) –4.8×10-6C
7. A –4×10-6C charge is located 0.3m to the left of a +6×10 -6C charge. What is the magnitude and direction
of the electrostatic force on the positive charge?
(a) 2.4N, to the right
(b) 2.4N, to the left
(c) 4.8N, to the right
(d) 4.8N, to the left
(e) 7.2N, to the right
8. An electron traveling horizontally enters a region where a uniform
electric field is directed upward. What is the direction of the force
exerted on the electron once it has entered the field?
(a) to the left
(b) to the right
(c) upward
(d) downward
(e) out of page, toward the reader
E
electron!
9. An electric field with a magnitude of 160N/C exists at a spot that is 0.15m away from a charge. At a
place that is 0.45m away from this charge, what is the electric field strength?
a) 17.8N/C
b) 13.6N/C
c) 10.4N/C
d) 3.5N/C
1electron(e-) = -1.6×10-19C
Q=ne
Fe = k
|𝑸𝒒|
𝒓²
[N]
𝑸
1proton(e+) = +1.6×10-19C
𝑭
E = k 𝒓² [N/C] or E = 𝒒 [N/C] where k= 9×109 N·m²/C
10. Three point charges, each of the same magnitude, with varying
signs are arranged at the corners as shown. Which of the arrows
labeled A, B, C and D gives the correct direction of the net forces
that acts on the charges at the upper right corner?
(a) A (b) B (c) C (d) D (e) the net force on that charge zero
B
D
–q
+q
A
C
–q
11. Finding the Resultant Force
Consider three point charges located as shown, where q 1 = q3 = +5×10-6C,
q2= –2×10-6C, and r = 0.1m. Find the resultant force exerted on q 3. Also
the direction of the resultant force
(a) 24N, 112° from positive x-axis
(b) 12N, 112° from positive x-axis
(c) 24N, 68° from positive x-axis
(d) 12N, 68° from positive x-axis
q2
q3
q1
**12. Two spheres are mounted on identical horizontal
springs and rest on a frictionless table. When the spheres are
uncharged, the spacing between them is 0.05m, and the
0.05m
springs are unstrained. When each sphere has a charge of
-6
+1.6×10 C, the spacing doubles. Assuming that the spheres
have a negligible diameter, determine the spring constant of
the springs.
hint; there are only two forces acting on the spheres, electrical force(F e) and spring
force(Fs= –kx : here 'k' is the spring constant. Do not be confused with the 'k' in Fe=kQq/r2)
a) 109.5N/m
b) 92.2N/m
c) 64.1N/m
d) 36.8N/m
Fe
Fs