Physics 2 · Electric Circuits · 14 min read · Updated 2026-05-11
Ohm's Law, Kirchhoff's Loop Rule and Power — AP Physics 2
AP Physics 2 · Electric Circuits · 14 min read
1. Ohm's Law★★☆☆☆⏱ 4 min
On a V-I graph, ohmic materials produce a straight line passing through the origin, with slope equal to constant resistance. While $R = V/I$ can calculate instantaneous resistance for non-ohmic materials at a single operating point, this does not make the component ohmic.
Exam tip: When asked to identify an ohmic material from a V-I graph, remember the requirement is a straight line passing through the origin, not just any straight line. A non-zero intercept means the component is not ohmic.
2. Kirchhoff's Loop Rule★★★☆☆⏱ 5 min
Kirchhoff's Loop Rule is a direct application of conservation of energy to closed circuits. It states that the net change in electric potential around any closed loop in a circuit is zero:
Physically, this means the total energy gained from energy sources (like batteries) equals the total energy lost to resistive components when moving a charge around the full loop. Correct application depends entirely on following a consistent sign convention:
Choose any traversal direction (clockwise or counterclockwise; direction does not change the final result if you are consistent).
For batteries: moving from negative to positive terminal gives $
Delta V = +\varepsilon$ (potential gain); moving from positive to negative gives $
Delta V = -\varepsilon$ (potential drop).
For resistors: moving in the same direction as conventional current gives $
Delta V = -IR$ (potential drop); moving opposite current gives $
Delta V = +IR$ (potential gain).
Exam tip: If your final current comes out negative, do not change the magnitude. The negative sign only indicates your assumed direction was wrong; AP Physics 2 accepts negative current values as correct if reasoning is accurate.
3. Electric Power in Circuits★★☆☆☆⏱ 3 min
Electric power is the rate of energy conversion in a circuit component. The general formula applies to any component, whether it supplies power (like a battery) or dissipates power (like a resistor):
Positive $P$ means the component absorbs or dissipates power; negative $P$ means the component supplies power to the circuit. For ohmic components, substitute Ohm's Law to get two alternative useful forms:
$P=I^2 R$ is most convenient for series circuits (all components share the same current), while $P=V^2/R$ is easiest for parallel circuits (all components share the same voltage). Always use the potential difference across the specific component you are analyzing, not the total battery emf, unless the component is connected directly across the battery.
Exam tip: Always use the potential difference across the specific component you are calculating power for, not the total emf of the battery, unless the component is connected directly across the battery with zero internal resistance.
4. AP-Style Worked Practice Questions★★★☆☆⏱ 2 min
Common Pitfalls
Why: Students forget $V$ in power formulas refers to the component's own potential difference, not the source voltage. Series components split the source voltage.
Why: Students associate resistors with voltage drops and forget the sign depends on traversal direction relative to current direction.
Why: Students confuse the general formula $R=V/I$ (valid for any component) with Ohm's Law, which requires constant resistance across all voltages.
Why: Students memorize a simplified rule for single-battery circuits and generalize it incorrectly.
Why: Students generalize the behavior of temperature-dependent filaments to all metallic conductors.