Charge and Electric Forces

Objective: Describe charge conservation and charging mechanisms.

Electric charge is a property of matter measured in coulombs. Protons carry positive charge and electrons negative charge of equal magnitude; most everyday charging involves transfer or redistribution of electrons rather than making new charge. Like charges repel and unlike charges attract. An object can be charged by contact, rubbing, or induction with suitable grounding, while total charge in an isolated system is conserved. A neutral object has zero net charge, not no charged particles. The simplified electron-transfer model does not cover every material detail, and a measured charge depends on the system boundary.

Objective: Predict the direction and relative magnitude of electrostatic forces.

For two idealized point charges, electric force magnitude obeys Coulomb’s law F = k|q₁q₂|/r², where r is their separation. Doubling separation at fixed charges reduces force to one quarter; doubling one charge doubles force. Force direction is repulsive for like charges and attractive for unlike charges. The charges exert equal-magnitude opposite forces on each other, consistent with Newton’s third law. In real objects charge can be distributed and polarizable, so the point-charge relation is a model with conditions. A neutral nearby object can still be attracted through induced charge separation, so net zero charge is not proof of zero electric interaction.

Objective: Distinguish force per charge from energy per charge.

An electric field describes force per positive test charge at a point, E = F/q, with direction defined by the force on a positive test charge. Field lines are diagrams, not physical strings; they point outward from positive sources and inward toward negative sources in simple static examples. Electric potential difference ΔV is change in electric potential energy per charge, measured in volts (J/C), and helps predict energy transfer when charge moves. A positive charge tends to accelerate along an electric field if electric force dominates, while a negative charge feels force opposite the field. Potential difference is not identical to electric field: voltage compares positions, whereas field describes a local force relationship.