flow of electric charge
Summary
Electric current is the rate at which net charge passes a point in a conductor, I = ΔQ/Δt, measured in amperes (C/s). Conventional current points the way positive charge would move, which in a metal wire is opposite to electron motion.
Detail
Meaning: Electric current is the rate at which net charge passes a cross-section of a conductor. A potential difference creates an electric field that pushes the carriers: electrons in metal wires, and both cations and anions in electrolyte solutions. By convention, current points the way positive charge would move, from high to low potential.
Equation: I = ΔQ/Δt, where I is current in amperes (A = C/s), ΔQ is charge in coulombs (C), and Δt is time in seconds (s). With Ohm's law, I = V/R, where V is potential difference in volts (V) and R is resistance in ohms (Ω).
On the MCAT: Expect ΔQ = IΔt problems, often converted to a number of electrons using e = 1.60 × 10⁻¹⁹ C, or to moles of electrons in electrolysis using F ≈ 96,500 C/mol. Circuit questions ask how current changes when V or R changes. Current is the same through elements in series and divides among parallel branches (junction rule).
Common confusion: Conventional current is drawn opposite to electron motion in a metal, so the arrow does not show where electrons go. Also, a resistor does not use up current. Charge is conserved, so the same current leaves a resistor as enters it, while electrical energy is dissipated as heat.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young, Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.