unit of charge
Summary
The SI unit of electric charge is the coulomb (C), the charge delivered by 1 ampere in 1 second. The smallest free charge is the elementary charge, e = 1.60 × 10⁻¹⁹ C, so the net charge on an ordinary object is a whole-number multiple of e.
Detail
Meaning: Charge is quantized: it comes in whole-number multiples of the elementary charge, e = 1.60 × 10⁻¹⁹ C. A proton carries +e and an electron −e. The SI unit is the coulomb (C), defined through current: 1 C = 1 A·s. One coulomb equals about 6.24 × 10¹⁸ elementary charges, so charges on everyday objects are usually microcoulombs or nanocoulombs.
Equation: Q = ne = It, where Q is net charge (C), n is the number of excess protons or electrons (a dimensionless integer), e is the elementary charge (C), I is current (A), and t is time (s).
On the MCAT: Coulomb's law, F = kq₁q₂/r², needs charges in coulombs, with k = 9.0 × 10⁹ N·m²/C², so convert μC (10⁻⁶ C) and nC (10⁻⁹ C) first. Other questions ask how many electrons were transferred (n = Q/e) or how much charge a current delivers in a given time. In electrochemistry, one mole of electrons carries about 9.65 × 10⁴ C (the Faraday constant).
Common confusion: The ampere is the SI base unit and the coulomb is derived from it, not the reverse. Also, e is not 1 C. A coulomb is an enormous amount of charge compared with a single electron or typical static charges, so answers in C are often tiny powers of ten.
Sources
- Physics (Giancoli)
- University Physics (Young and Freedman)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.