Worksheet: Using Coulomb's Law to Determine an Electrostatic Force

In this worksheet, we will practice determining the force a charge exerts on charges around it using Coulomb's law.


Protons in an atomic nucleus are typically 10 m apart. What is the electric force of repulsion between nuclear protons? The charge of a proton is 1.6021×10 C.


Two small balls, each of mass 5.0 g, are attached to silk threads 50 cm long, which are in turn tied to the same point on the ceiling, as shown. When the balls are given the same charge 𝑄, the threads hang at 5.0 to the vertical. What is the magnitude of 𝑄?

  • A2.8×10 C
  • B8.7×10 C
  • C1.3×10 C
  • D6.1×10 C
  • E4.4×10 C


Two point particles have charges of 8.0 µC and 12 µC respectively. Each particle is held at rest by a force of magnitude 45 N, applied in the opposite direction to the electrical force exerted on it by the other particle. What is the magnitude of the displacement from one charge to the other?


In a salt crystal, the distance between adjacent sodium and chloride ions is 3.42×10 m. What is the force of attraction between the two singly charged ions?

  • A2.36×10 N
  • B2.03×10 N
  • C693×10 N
  • D2.23×10 N
  • E1.97×10 N


If both the Earth and the Moon were each given a particular net charge 𝑄, this would produce a repulsive force between them equal in magnitude to the attractive gravitational force between them. In considering the relationship of gravitational to electrical forces, model Earth and the Moon as point masses of 5.95×10 kg and 7.35×10 kg respectively.

Determine the magnitude of 𝑄.

  • A5.69×10 C
  • B3.56×10 C
  • C5.69×10 C
  • D5.69×10 C
  • E56.3×10 C

How many electrons would be required to produce the net charge sufficient to counteract the gravitational attraction between Earth and the Moon?

  • A3.56×10
  • B9.36×10
  • C9.11×10
  • D421×10
  • E3.65×10


Two small identical conducting spheres repel each other with a force of 0.01 N when they are 0.45 m apart. After a conducting wire is connected between the spheres and then removed, they repel each other with a force of 0.10 N. What is the largest possible value of the total charge on the spheres before the conducting wire is connected between them?

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