electrostatic
Summary
Electrostatic describes electric charges at rest and the forces, fields and potentials they produce. The most testable point is Coulomb's law: the force between two point charges falls off as 1/r², repelling for like charges and attracting for opposite charges.
Detail
Meaning: Electrostatics is the physics of electric charges at rest, or of charge distributions not changing in time. Each charge sets up an electric field, and another charge in that field feels a force: like charges repel, opposite charges attract. The force is conservative, so it has an electric potential energy and an electric potential. In biochemistry, electrostatic interactions are attractions between charged groups, such as salt bridges.
Equation: F = kq₁q₂/r², where F is the force in newtons (N), k ≈ 9.0 × 10⁹ N·m²/C², q₁ and q₂ are the charges in coulombs (C), and r is the separation in meters (m).
On the MCAT: Expect point charges and a prediction: doubling r cuts F to one quarter; doubling one charge doubles F. Related forms are E = kq/r² (field, N/C), U = kq₁q₂/r (energy, J) and V = kq/r (potential, V). Questions also ask for field-line direction or the sign of work as a charge moves.
Common confusion: Force and field go as 1/r², but potential energy and potential go as 1/r. Also, "static" describes the source charges; a test charge released in the field still accelerates. Keep the signs of the charges in U: it is negative for opposite charges.
Sources
- Fundamentals of Physics
- University Physics
- Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.