electric force

Meaning: Electric force acts along the line joining two charges. Like charges repel and opposite charges attract. The two charges exert equal and opposite forces on each other (Newton's third law), even when one charge is much larger. With several charges, the net force on one is the vector sum of the individual forces. A charge q in an electric field E feels a force of magnitude qE, along E for a positive charge and opposite E for a negative one.

Equation: F = k|q₁q₂|/r², where F is the force in newtons (N), k is Coulomb's constant, about 9.0 × 10⁹ N·m²/C², q₁ and q₂ are the charges in coulombs (C), and r is the separation in meters (m). In a field: F = qE, with E in N/C.

On the MCAT: Expect ratio reasoning. Doubling one charge doubles the force, and halving the distance makes it four times larger. Other items include adding force vectors from two or three point charges, predicting the direction of the force on a positive or negative charge, and comparing the electric force with gravity between particles. A charge in a uniform field also follows a parabolic path.

Common confusion: Students mix up force and potential energy. Force falls off as 1/r², but electric potential energy, U = kq₁q₂/r, falls off as 1/r. Also, the smaller charge does not feel a smaller force. Both feel the same magnitude.