Lesson 4 of 13 · 6 min
Forces between multiple charges
NCERT §1.6
A tiny bead carrying +0.1 μC sits 30 cm from both balloons, at the top corner of an equilateral triangle. Two balloons push it at once. Which way does it go?
The lesson in notes
In short
Principle of superposition: the force on a charge due to several others is the vector sum of the forces each would exert alone.
The force between any two charges is unaffected by the presence of other charges; each pair obeys Coulomb's law as if it were alone.
F₁ = F₁₂ + F₁₃ + … + F₁ₙ, added by the parallelogram (or component) method, never by adding magnitudes.
All of electrostatics follows from two ideas: Coulomb's law and the superposition principle.
Symmetry often gives the answer: equal charges at the corners of an equilateral triangle exert zero net force on a charge at its centroid.
For any set of charges held only by their mutual forces, the forces on all of them add to zero, because Coulomb forces come in equal and opposite pairs.