electrostatic force vector
Summary
The electrostatic force vector is the force one charge exerts on another, directed along the line joining them: away for like charges (repulsion), toward for opposite charges (attraction). Its magnitude is k|q₁||q₂|/r², so doubling the distance cuts it to one-fourth.
Detail
Meaning: The electrostatic force vector is the push or pull one charge exerts on another, given as a magnitude and a direction. The direction lies along the line joining the charges: like charges repel and opposite charges attract. The two forces are a Newton's third law pair, equal in size and opposite in direction. With several charges, the net force on one is the vector sum of the separate forces.
Equation: F = k|q₁||q₂|/r², where F is the force magnitude in newtons (N), k ≈ 9.0 × 10⁹ N·m²/C², q₁ and q₂ are charges in coulombs (C), and r is the separation in meters (m). Get direction from the signs, not the formula.
On the MCAT: Passages place charges on a line or at square or triangle corners and ask for the net force direction on one charge. Draw each arrow, split into x and y components, then add. Doubling r cuts F to one-fourth; doubling one charge doubles F.
Common confusion: The larger charge does not feel the larger force; both feel equal magnitudes. Adding magnitudes without regard to direction is another error. On a negative charge the force points opposite the electric field; on a positive charge, along it.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway, Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.