EMG-01

Coulomb's law

F = k q1 q2 / r² along the joining line.

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ElectrostaticsCoulomb 1785

Governing equation

F=\dfrac{1}{4\pi\varepsilon_0}\dfrac{q_1 q_2}{r^2}

where

q_1: Charge 1 (µC)
q_2: Charge 2 (µC)
r: Separation (m)
F: Force (N)

Lecture brief

Historical brief

Coulomb, Gauss, Ampère, Faraday and Maxwell (1861–65) unified charge, current and light. The lab computes fields, induction, Poynting flux and the electromagnetic wave in SI. This sheet (EMG-01 — Coulomb's law) is the form associated with Coulomb 1785. Working symbols: $q_1$ , $q_2$ , $r$ $\rightarrow$ $F$ . Coulomb's torsion-balance measurements established the inverse-square law of electric force, with k = 1/(4π ε0).

Purpose

Purpose: compute

F

from

q_1

q_2

r

in Electromagnetism via

F=\dfrac{1}{4\pi\varepsilon_0}\dfrac{q_1 q_2}{r^2}

F = k q1 q2 / r² along the joining line. Use it when a real electromagnetism question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

q_1 = 2.000\,\mathrm{\mu C}

q_2 = -3.000\,\mathrm{\mu C}

r = 0.400\,\mathrm{m}

, the governing relation

F=\dfrac{1}{4\pi\varepsilon_0}\dfrac{q_1 q_2}{r^2}

yields

F = -0.337\,\mathrm{N}

. Two point charges in vacuum. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Charge 1 q_12 µCCharge 2 q_2-3 µCSeparation r0.4 m

Outputs

Force F-0.337 N

Reading speed

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Narration of this film

Two point charges in vacuum.

Coulomb's torsion-balance measurements established the inverse-square law of electric force, with k = 1/(4π ε0).

Reading speed

Watch on YouTube