resistor
Summary
A resistor is a circuit element that opposes current, producing a potential drop across it and dissipating electrical energy as heat. The most testable point is V = IR combined with the series and parallel rules: adding a resistor in series raises total resistance, but adding one in parallel lowers it.
Detail
Meaning: A resistor opposes the flow of charge, so a potential drop appears across it and electrical energy is dissipated as heat. A wire's resistance rises with its length and falls with its cross-sectional area. In series, every resistor carries the same current and the voltage drops add. In parallel, every branch has the same voltage and the branch currents add.
Equation: V = IR, where V is the potential difference (volts), I is the current (amperes) and R is the resistance (ohms, Ω). Power: P = I²R = V²/R (watts). Series: Req = R₁ + R₂ + R₃. Parallel: 1/Req = 1/R₁ + 1/R₂ + 1/R₃. Wire: R = ρL/A, with resistivity ρ (Ω·m), length L (m) and area A (m²).
On the MCAT: A passage gives a circuit diagram and asks how current, voltage or power changes when a resistor is added, removed or changed. With a fixed battery, a series resistor lowers the total current. A parallel resistor raises it, because Req falls below the smallest branch. Questions may also ask for the slope of an I–V graph (1/R for an ohmic resistor), or what happens when a wire is stretched or thickened.
Common confusion: Students say a resistor "uses up" current. Charge is conserved, so the current leaving equals the current entering. What drops across the resistor is potential, and energy is dissipated as heat.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway, Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.