like-charges

Meaning: Like charges are two charges of the same sign. They repel each other with electric forces directed along the line joining them, while opposite charges attract. The two forces are equal in magnitude and opposite in direction, even when the charges differ in size (Newton's third law). Pushing like charges closer takes work from an outside agent, so the pair's electric potential energy rises as they approach and falls as they separate.

Equation: F = k|q₁q₂|/r², where F is the force magnitude (N), k is Coulomb's constant, about 9 × 10⁹ N·m²/C², q₁ and q₂ are the charges (C), and r is their separation (m). Potential energy is U = kq₁q₂/r (J), which is positive for like charges.

On the MCAT: Expect questions on how force changes when r or a charge changes: doubling r cuts F to one-fourth, and doubling one charge doubles F. Field-line drawings show lines from like charges bending away from each other without crossing. A common setup asks where a third charge feels zero net force: between two like charges, closer to the smaller one.

Common confusion: A larger charge does not push harder on a smaller one; the forces are equal and opposite. Also, "like" refers to the sign of the charge, not its size.