coulomb
Summary
The coulomb (C) is the SI unit of electric charge, equal to one ampere-second (1 C = 1 A·s). The elementary charge is e = 1.60 × 10⁻¹⁹ C. The most testable uses are Q = IΔt and the charge unit in Coulomb's law.
Detail
Meaning: The coulomb (C) is the SI unit of electric charge. One coulomb is the charge passing a point in one second at a current of one ampere, so 1 C = 1 A·s. Charge is quantized: a proton carries +e and an electron −e, with e = 1.60 × 10⁻¹⁹ C. One coulomb is about 6.2 × 10¹⁸ elementary charges, a large amount for static electricity.
Equation: Q = IΔt = ne, where Q is charge (C), I is current (A), Δt is time (s), n is the number of elementary charges (unitless), and e is the elementary charge (C).
On the MCAT: Charge enters Coulomb's law, F = kq₁q₂/r², with k ≈ 9.0 × 10⁹ N·m²/C². The force scales with the product of the charges and falls with the square of the distance. Charge also defines electric field (E = F/q, in N/C) and potential (1 V = 1 J/C). In electrochemistry, one mole of electrons carries about 9.65 × 10⁴ C (Faraday's constant), which links current and time to moles of metal plated.
Common confusion: The coulomb is not the ampere. The coulomb is an amount of charge; the ampere is a rate, one coulomb per second.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.