ampere
Summary
The ampere (A) is the SI unit of electric current, the rate of charge flow: 1 A = 1 coulomb per second. The most testable point is that current in amperes follows I = V/R, so it depends on the voltage and the resistance in the circuit.
Detail
Meaning: The ampere (A) is the SI base unit of electric current, the rate at which charge passes a point in a conductor. One ampere is one coulomb per second. By convention, current points in the direction positive charge would move, from high to low potential, even though electrons in metal wires move the opposite way. At a junction, current in equals current out.
Equation: I = ΔQ/Δt, where I is current in amperes (A), ΔQ is charge in coulombs (C), and Δt is time in seconds (s). Also I = V/R, with V in volts (V) and R in ohms (Ω).
On the MCAT: Passages give batteries and resistors and ask for the current in a branch, or how it changes when a resistor is added. With the same battery, adding a resistor in series lowers the total current, and adding one in parallel raises it. Charge questions use ΔQ = IΔt, and the number of electrons is ΔQ/e, with e = 1.60 × 10⁻¹⁹ C.
Common confusion: A resistor does not use up current. The same current leaves a resistor as enters it, by conservation of charge. What drops across the resistor is electric potential, and electrical energy is dissipated as heat.
Sources
- Physics (Giancoli)
- Physics (Cutnell and Johnson)
- 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.