electric charge

Meaning: Electric charge is a basic property of matter that determines how it interacts with electric and magnetic fields. It comes in two kinds, positive and negative. Like charges repel and opposite charges attract. Charge is quantized, so any net charge is a whole-number multiple of the elementary charge e, and it is conserved, so the net charge of an isolated system does not change. A proton carries +e and an electron −e. In ordinary charging by friction, contact or induction, electrons move; protons stay fixed in nuclei.

Equation: Q = ne, where Q is net charge in coulombs (C), n is a signed integer (excess protons minus excess electrons), and e = 1.60 × 10⁻¹⁹ C.

On the MCAT: Expect Coulomb's law, F = kq₁q₂/r², where F is force in newtons (N), q₁ and q₂ are charges in C, r is separation in meters (m), and k ≈ 9.0 × 10⁹ N·m²/C². Doubling r cuts the force to one-quarter. A positive product of charges means repulsion and a negative product means attraction. A classic calculation has two identical conducting spheres touch and then separate. Each ends up with half the total charge. Passages also test conductors versus insulators and charging by induction.

Common confusion: A neutral object is not charge-free. It holds equal positive and negative charge, so its net charge is zero. Its charges can shift (polarize), which is why a charged object can attract a neutral one.