Safety boundary
Use simulations, supplied data, and ordinary classroom-static examples only. Do not use mains electricity or improvised high-voltage devices.
Explicit physics instruction
Electric charge comes in positive and negative forms. Like charges repel and unlike charges attract; the force acts along the line joining the charges.
Coulomb interaction strength scales with the product of charge magnitudes and inversely with the square of separation. Doubling distance reduces ideal force to one-fourth.
The model is powerful but simplified: point-charge assumptions and surrounding materials can matter.
Vocabulary: Coulomb interaction; charge sign; inverse-square; separation; point-charge model.
Worked example
If separation doubles while charge magnitudes stay fixed, force becomes one-fourth as large because F∝1/r².
Guided question
Two charges move from distance r to 3r. What fraction of the original force remains?
Independent work
- Rank four charge pairs by attraction/repulsion and relative force strength.
- Solve three inverse-square ratio problems.
- State one assumption behind treating objects as point charges.
Misconception check
Doubling distance does not halve an inverse-square force; it quarters it.
Formative check
What determines whether electric force is attractive or repulsive?
Reveal response
The signs of the interacting charges.
Mentor evidence conference
Ask Archaemenes to audit sign conventions, vector/scalar distinctions, units, and model assumptions.
Teacher guidance
Foundation: Use sign diagrams and ratio tables.
Core: Solve force-direction and inverse-square comparisons.
Extended: Analyze where finite-size or medium effects limit the point-charge model.