Electrostatics Practice Questions

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Updated on Sep 27, 2011

Review these concepts at: Electrostatics Study Guide

Practice Questions

  1. Two strings are hooked to the ceiling and form two close pendulums that have two identical small metal objects hanging on one end and touching as shown in Figure 13.1. The objects are initially electrically neutral. One will be isolated from the second and charged. The charged object is then placed in contact with the other object. What will happen to the charge of the two pendulums after contact? Explain.
  2. Electrostatics

  3. For the information given in practice problem 1, 120 electrons are initially charging one metal object. After contact, what is the charge on each object?
  4. After equilibrium is established, an insulator is brought in close proximity with one of the objects in the system but not touching either A or B objects. Explain what happens in the new situation of equilibrium.
  1. Two amber beads are brought in proximity after being charged with 10 and 50 electrons. They are placed at a distance of 20 cm from each other. The force acting between the two charges was calculated in the example before. If the second object is charged with a charge that is three times larger, find the force acting between the new charges.
  2. In practice problem 4, the distance between the charges is increased to 2 m. What is the new force and what is the ratio between this force and the force you calculated in practice problem 4?
  3. Three amber beads are brought in proximity after being charged with 10.0 and 20.0 and 30.0 electrons, respectively. They are placed, as shown in Figure 13.5. Find the total force acting on charge A.


  1. In practice problem 6, the 30-electron charge is replaced by a charge of 30 protons. Find the new force on charge A.
  2. Consider the setup of charges in Figure 13.6. Find the net force on charge A on the x direction, on the y direction and the net force.


  1. In practice problem 8, is the net force the same on the other two charges? Explain.
  1. If a charge q produces a field E at a distance r, what is the electric field at a distance twice as large? Explain.
  2. If the distance is kept constant, 2 · r, but the charge is replaced by a charge of the initial one, what happens to the value of the electric field?
  3. What is the electric field created by a point-like charge very close to the charge? Explain.
  4. What is the electric field created by a point-like charge far away from the position where the charge is located? Explain.
  5. Three point-like charges arranged as in Figure 13.12 create an electric field. Determine the net field at position A. Charges are, from left to right: 1.0, 2.0, amd 3.0 pC. Electrostatics
  6. Can three electric charges create a zero electric field? Explain.
  7. An electron is moving between two positions in a constant electric field such that the potential difference is –25 V. Find the work done by the field to move the electron between the two points.


  1. Half the charge from the charged metal object will transfer to the neutral object once contact is established.
  2. 60 electrons and –96 · 10–19 C
  3. The insulator will charge with a charge of 96 · 10–19 C.
  4. F = 3 · Fl = 3 · 28.8 · 10–23N = 86.4 · 10–23N
  5. F* = F/100 = 86.4 · 10–23 N/100 = 0.864 · 10–23N and F*/F = 1/100
  6. FBA + FCA = 57 · 10–27N
  7. FBAFCA = 35 · 10–27N
  8. Fx = –2.8 · 10–3 N, Fy = 22 · 10–3 N, Fresultant = 22.2 · 10–3N
  9. Not necessarily, since now we will have different charges and different distances in between.
  10. The field will be four times smaller since E ~ 1/r2,
  11. The field decreases by 3 since E ~ q,
  12. Since E ~ 1/r2, the smaller the distance, the larger the field.
  13. Since E ~ 1/r2, the larger the distance, the smaller the field.
  14. E = 1.2 · 10–3 N/C
  15. The electric field is a vector and three vectors can be added to create no result.
  16. W = 40 · 10–19 J
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