Physic Labs

Electricity and magnetism

Coulomb force between two charges

Explore Coulomb's law qualitatively: force scales with |q₁q₂|, inversely with r², and its direction depends on the charge signs. Measure the force as charge and separation change, and verify F=k∣q1q2∣/r2F = k|q₁q₂|/r².

Middle school

⚠ The charge simulation is virtual; do not touch high-voltage sources in real experiments.

Equipment

  • Two point charges that can be dragged to new positions
  • q₁, q₂, and separation r sliders; field-line toggle

Procedure

  1. Test attraction and repulsion

    Use the q₁ and q₂ sliders to set opposite signs and observe the force vectors pointing toward each other; change one charge to the same sign to see repulsion. Keep q₁ and q₂ fixed and increase r, then reduce it, comparing the force magnitude in the readout. Turn on the field lines for a view of the field pattern; drag a sphere to change relative position directly. Compare the observed quantities with F=k∣q1q2∣/r2F = k|q₁q₂|/r².

  2. Measure the distance relationship

    Keep q₁ and q₂ fixed and record force at two separations r. Compare readings with F=k∣q1q2∣/r2F = k|q₁q₂|/r^2; doubling separation reduces the force to one quarter.

  3. Vary the charge magnitudes

    Keep r and charge signs fixed, then increase the magnitudes of q₁ and q₂ in turn. Observe the force readout, enable field lines for comparison, and record how force scales with ∣q1q2∣|q₁q₂|.

Simulation

Experiment history

Charles-Augustin de Coulomb (1736–1806), a French military engineer and physicist, investigated electrical and magnetic forces using a torsion balance that he refined. In memoirs published from 1785, he suspended a light fiber and measured its twist under the action of small charged spheres, making it possible to quantify interactions at short distances. Coulomb's measurements showed that the force between point charges decreases with the square of their separation and increases with the product of their charge magnitudes; their signs determine attraction or repulsion. These were experimental results under controlled conditions and helped establish quantitative electrical science in the late eighteenth century. The proportionality constant bears his name, although the law's modern formulation belongs to the later framework of electromagnetic theory.

Related physicists

Charles-Augustin de Coulomb

Related library topics

Electric charge and Coulomb’s law