current
Summary
Electric current is the rate at which charge flows past a point in a circuit, measured in amperes (1 A = 1 C/s). By convention it points the way positive charge would move, from high to low potential, even though electrons usually carry it in metals.
Detail
Meaning: Current is the net rate at which charge passes a cross-section of a conductor. By convention it points the way positive charge would move, from higher to lower potential, which is opposite to electron motion in metals. A potential difference across a resistance drives it. Charge is conserved, so current is the same through elements in series and splits at parallel junctions.
Equation: I = ΔQ/Δt = V/R, where I is current (A, equal to C/s), ΔQ is charge moved (C), Δt is time (s), V is potential difference (V), and R is resistance (Ω).
On the MCAT: Passages show a circuit and ask for the current through one resistor, or how current changes when a resistor is added. At fixed battery voltage, adding a resistor in series lowers the total current, and adding one in parallel raises it. Kirchhoff's junction rule (current in equals current out) and power, P = IV, also appear. Ions carrying current in solutions or across membranes can be used as the setting.
Common confusion: Students think a resistor uses up current. It does not, because charge is conserved: the same current leaves a resistor as enters it. What drops is electric potential, as electrical energy is dissipated, typically as heat.
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.