charge
Summary
Electric charge is a property of matter that produces and responds to electric force. It is positive or negative, measured in coulombs, with like charges repelling and opposite charges attracting. Net charge is conserved and comes in whole multiples of e = 1.60 × 10⁻¹⁹ C.
Detail
Meaning: Charge sets how an object exerts and feels electric force. Protons carry +e, electrons carry −e, and neutrons are neutral. Charge is quantized (q = ne, n an integer) and conserved in an isolated system. Objects become charged by gaining or losing electrons, through friction, contact, or induction. Charge moves freely in conductors but not in insulators.
Equation: F = k|q₁q₂|/r², where F is the magnitude of the electric force (N), k ≈ 9.0 × 10⁹ N·m²/C², q₁ and q₂ are the charges (C), and r is the separation of the centers (m).
On the MCAT: Expect Coulomb's law scaling. Doubling r cuts the force to one quarter, and doubling one charge doubles the force. Two identical conducting spheres that touch share their total charge equally, then separate. Other questions ask which way the force on a test charge points, or how many electrons were transferred (n = q/e).
Common confusion: A neutral object is not charge-free. It has equal amounts of positive and negative charge. Also, only electrons transfer in ordinary charging, so a positively charged object has lost electrons. Its protons have not been added.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.