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E-Fields and Coulomb’s Law
1. A source charge (3 C) is surrounded by an electric field.
a. What force would an electron (-1.6x10-19 C) experience at a location with a 1.25x1018 N/C
electric field? (Ans: 0.2N)
b. Replace the electron with a proton. What force would the proton experience?
c. How would the forces experienced by the electron and proton differ?
2. A negative source charge (Q) is shown in the diagram below which has created an electric field.
Various locations within the field are labeled. Imagine a positive test charge is placed at each location.
Draw a vector on each test charge showing the direction and magnitude of the force/E-field experienced
by the each charge.
3. Two charged spheres with respective values of 4 C and 2.5 C exert a repulsive force of
0.06N on each other. How far apart are the spheres? (Ans: 1.22m)
4. The symbol r in Coulomb's law equation represents the distance from ___.
a. A to B b. A to D c. B to C d. B to D
e. C to D f. A to G g. B to F h. C to E
5. Two charged Styrofoam cups are separated a distance of 20 cm. One cup has a charge of
3x10-6 C. If the repulsive electrical force between the cups is 0.5 N, what is the charge of the
second cup? (Ans: 7.4x10-7 C)
6. Suppose that two charges attract each other with an electrical force of 12 Newtons. If the
distance between the two charges is doubled, the charge of Q is double, and the charge of q is
tripled, then what is the new electrical force of attraction between the two charges? (Ans: 18N)
7. A source charge, -3.4 C, is located 0.8m east of a test charge. Determine the electric field
strength and its direction as experienced by the test charge. (Ans: 4.78x104 N/C East)
8. A source charge, 2 C, is located 0.02m south of a test charge. Determine the electric field
strength and its direction as experienced by the test charge. (Ans: 4.5x107 N/C North)
Superposition of Charges
9. Three point charges are placed in a horizontal line. The first charge has a value of 6 μC. The
second charge is placed 0.03 meters to the right of the first charge and has a charge of 1.5 μC.
The third charge is placed 0.02 meters to the right of the second and has a value of –2 μC.
Calculate the net electric force acting on EACH of the three charges due to the other two. (ANS:
46.75 N left, 157.5 N right, 110.7 N left)
a. Charge #1
b. Charge #2
c. Charge #3
10. Source charge A (3 C) is located 0.6m west of Source charge B (-2.1C). A test charge is
located directly in between the 2 source charges. Determine the net electric field strength at the
location of the test charge. (Ans: 5.1x105 N/C East)
11. Charged spheres A and B are fixed in position with B being 0.2 meters to the right of A.
Charge A is 4 μC and charge B is – 0.25 μC. What is the net electric force acting on a third
charged sphere C if it is placed 0.1 meters to the right of charge B. The charge on sphere C is
6.4 μC. (ANS: 1.12 N right)
12. Source charge A (-2.6 C) is located 0.4m north of Source charge B (-5.1C). A test charge
is located directly in between the 2 source charges. Determine the net electric field strength at
the location of the test charge. (Ans: 5.63x105 N/C South)
Mixed Problems:
13. An electric field of 2.0x104 N/C is directed along the positive x-axis. What is the electric force on an
electron in this field? (Hint: qelectron = -1.6x10-19 C) (Ans: 3.2x10-15 N, negative x direction)
14. Suppose that two charges attract each other with an electrical force of 12 Newtons. If the
distance between the two charges is tripled, the charge of Q1 is 1/2, and the charge of Q2 is
doubled, then what is the new electrical force of attraction between the two charges? Ans: 1.33N
15. A balloon has been rubbed with wool to give it a charge of -1.0 x 10-6 C. A glass rod with a
charge of +4.0 x 10-6 C localized at a given position is held a distance of 50.0 cm above the
balloon. Determine the electrical force of attraction between the rod and the balloon. (Ans:
0.144N)
16. A source charge, -15 C, is located 0.06m south of a test charge. Determine the electric field
strength and its direction as experienced by the test charge. (3.75x107 N/C South)
17. Three point charges are arranged on the x-axis. Charge #1 is located at the origin and has a
value of -6.1 μC. A second charge is located at 0.1 meters and has a charge of 1.1 μC. Charge
#3 is located at 0.4 meters and has a value of 1.9 μC. What is the net force acting on charge #3?
(Ans: 0.44N left)
18. Source charge A (4.8 C) is located 0.09m west of Source charge B (6.1C). A test charge
is located 0.02m west of charge A. Determine the net electric field strength at the location of the
test charge. (Ans: 1.13x108 N/C West)