The force exerted by q1 on q2 is equal in magnitude and opposite in direction to the force exerted by q2 on q1.

The force exerted by q1 on q2 is equal in magnitude and opposite in direction to the force exerted by q2 on q1.

• A. They are in the same direction.
• B. They are equal in magnitude and opposite in direction.
• C. They are both zero.
• D. They depend on the relative velocity.

Answer: (B)

The forces between two point charges form an action–reaction pair: they come in equal magnitudes and opposite directions. According to Coulomb’s law, the force on q2 due to q1 has magnitude k|q1 q2|/r^2 and lies along the line joining the charges. The force on q1 due to q2 has the same magnitude but points in the opposite direction, so F12 = -F21. The direction depends on the signs of the charges (repulsive if like charges, attractive if opposite signs), but the two forces always mirror each other. The forces do not depend on relative velocity, which would introduce magnetic effects only when charges are moving.

The forces between two point charges form an action–reaction pair: they come in equal magnitudes and opposite directions. According to Coulomb’s law, the force on q2 due to q1 has magnitude k|q1 q2|/r^2 and lies along the line joining the charges. The force on q1 due to q2 has the same magnitude but points in the opposite direction, so F12 = -F21. The direction depends on the signs of the charges (repulsive if like charges, attractive if opposite signs), but the two forces always mirror each other. The forces do not depend on relative velocity, which would introduce magnetic effects only when charges are moving.