--- title: "Coulomb's Law and the Electric Field of a Point Charge" description: "Two point charges push or pull with a force that also falls off as the inverse square, but this one comes with two signs. Field strength is the force per coulomb on a small positive test charge." canonical: https://lightmysky.com/learn/science/coulombs-law-and-the-electric-field-of-a-point-charge-mt_p7FuzIsQch source: https://lightmysky.com/learn/science/coulombs-law-and-the-electric-field-of-a-point-charge-mt_p7FuzIsQch.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Coulomb's Law and the Electric Field of a Point Charge Two point charges push or pull with a force that also falls off as the inverse square, but this one comes with two signs. Field strength is the force per coulomb on a small positive test charge. Subject: Science · Area: Energy · Ages 16 to 17 Page: https://lightmysky.com/learn/science/coulombs-law-and-the-electric-field-of-a-point-charge-mt_p7FuzIsQch ## Ready when they can - Calculates the force between two point charges and reads its direction from the signs - Finds E as force per coulomb and as kQ over r squared, and gives both units it is quoted in - Compares the electric and gravitational forces between two protons and comments on the ratio ## Lesson: Charges that push and pull by squares Point charges push or pull along the line joining them, with like signs repelling and opposite signs attracting. The size follows Coulomb's law: F equals k times each charge over r squared, with k about 9 times 10 to the 9. **Example.** An electron and a proton a tiny distance apart in hydrogen pull on each other electrically far harder than they pull by gravity. Each feels an equal and opposite force, just as Earth and the apple pulled equally hard on each other. Two protons show the same pattern: their electric push dwarfs their gravitational pull. Field strength E is the force on a tiny positive test charge, divided by the size of that charge. For a point charge Q it is k Q over r squared, pointing away from positive Q and toward negative Q. It is quoted in newtons per coulomb, the same as volts per metre. Lines leave positive charges and enter negative ones, and a map of them has no crossings, since one spot cannot have two directions. **Tip.** Read direction from the signs first, then compute size from the distances. **Recap.** Charge signs set the direction along the joining line, and the inverse square sets the size. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Static Charge and Electric Fields](https://lightmysky.com/learn/science/static-charge-and-electric-fields-mt_6ii6CVOTI5) - [Orbits: Period, Speed and Kepler's Third Law](https://lightmysky.com/learn/science/orbits-period-speed-and-keplers-third-law-mt_PLKLykGF8Z) - [Newton's Law of Gravitation and Gravitational Field Strength](https://lightmysky.com/learn/science/newtons-law-of-gravitation-and-gravitational-field-strength-mt_uUYXl3L0oW) ## Opens up - [The Electric Field of a Continuous Charge Distribution](https://lightmysky.com/learn/science/the-electric-field-of-a-continuous-charge-distribution-mt_IIcVqWapC1) - [Electric Potential and the Uniform Field Between Plates](https://lightmysky.com/learn/science/electric-potential-and-the-uniform-field-between-plates-mt_LKTfeYxNde) - [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY)