electric current

Meaning: Current is charge moving through a conductor per unit time. A potential difference (such as a battery) sets up an electric field that pushes charges along. Conventional current is drawn as the flow of positive charge, so in a metal wire it points opposite to the actual motion of electrons. In an electrolyte solution, cations and anions both carry current.

Equation: I = ΔQ/Δt, where I is current in amperes (A = C/s), ΔQ is charge passing a point in coulombs (C), and Δt is time in seconds (s). Ohm's law gives I = V/R, where V is potential difference in volts (V) and R is resistance in ohms (Ω).

On the MCAT: Passages show circuits with resistors and ask how current changes when a resistor is added. Adding resistance in series lowers the total current. Adding a branch in parallel raises it. Current is the same through every series element and splits at a junction, because charge is conserved. A charge-versus-time graph has slope equal to current. In electrochemistry, current times time gives charge, and dividing by the Faraday constant gives moles of electrons.

Common confusion: Students think a resistor uses up current. It does not. The current entering a resistor equals the current leaving it, and what drops across the resistor is electric potential, as energy is dissipated. Also, do not reverse the arrow: conventional current is opposite to electron flow.