Electrostatics: Charge, Coulomb's Law and Field Strength
Where electric forces come from, how strong they are, and how a charge fills the space around it with a field
Big Idea: Every electric effect — a crackling sweater, a lightning bolt over Issyk-Kul, the current in your phone — begins with electric charge. In grades 7–9 you learned that like charges repel and unlike charges attract. Now we make that quantitative. Coulomb's law tells you the exact size of the force between two charges, and the idea of the electric field (with its measure, the field strength) explains how one charge reaches across empty space to push or pull another.
By the end you will be able to:
- state what electric charge is, that it comes in two kinds and in whole multiples of the elementary charge e;
- apply the law of conservation of charge;
- calculate the force between point charges with Coulomb's law, F = k·q1q2 / r2;
- define the electric field and its strength E = F / q;
- find the field of a point charge and add fields with the superposition principle;
- read a picture of field lines.