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Coulomb's Law Revision Notes

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14.2Coulomb's Law


In these revision notes for Coulomb's Law, we cover the following key points:

  • The meaning of electrostatic force.
  • What are the factors affecting the electrostatic force?
  • What does the Coulomb's Law say?
  • How to find the electrostatic force?
  • How to find the equilibrium position between two charges?
  • How to calculate the resultant electrostatic force on a charge in 1-D?
  • How to calculate the resultant electrostatic force on a charge in 2-D?

Coulomb's Law Revision Notes

During his famous experiment, Coulomb discovered that:

  1. The magnitude of force between two electric charges is directly proportional to the product of magnitudes of charges |Q1| and |Q2| on both particles;
  2. The magnitude of force is inversely proportional to the product of distance between the charged particles;
  3. The force is attractive for unlike charges and repulsive for like ones; and
  4. The direction of force is along the line that joins the two charges.

Putting all together, Coulomb obtained the following equation:

F = k × |Q1| × |Q2|/r2

where k is a constant of proportionality.

The above equation is the mathematical expression of Coulomb's Law, which states that:

The electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects.

Accurate measurements have shown that the value of constant k is

k = 8.988 × 109 N × m2/C2

For simplicity, in most exercises we take k ≈ 9 × 109 N × m2 / C2.

The resultant force acting on a trial charge is zero when

F1-0 = F2-0

where F1-0 and F2-0 are the forces applied by the charges Q1 and Q2 on the trial charge Q0. Therefore, the condition to have equilibrium at a point between two like charges is

Q1/d2 = Q2/(r-d)2

where d is the distance of the trial charge from Q1 and r - d its distance from Q2.

If three or more charges are collinear, the resultant force acting on one of them lies according one dimension only. We simply add or subtract the individual forces as needed.

When charges are not collinear, the corresponding forces are not collinear either. In this case, we must consider their components according the basic directions separately and then, calculate the resultant force by using the Pythagorean Theorem.

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  2. Electrostatics Physics tutorial: Coulomb's Law. Read the Coulomb's Law physics tutorial and build your physics knowledge of Electrostatics
  3. Electrostatics Practice Questions: Coulomb's Law. Test and improve your knowledge of Coulomb's Law with example questins and answers
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  5. Continuing learning electrostatics - read our next physics tutorial: Electric Field

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