Ohm’s law basics — the “voltage drop” across a resistor is simply the potential difference measured between its two terminals. In circuit analysis, is this definition appropriate?

Introduction / Context:
The phrase “voltage drop” appears constantly in electronics. Students sometimes overcomplicate it, thinking it requires current flow or special conditions. In standard circuit theory, the voltage drop across any element is the electric potential difference between its two terminals, regardless of whether the element is a source, a resistor, or a reactive component.

Given Data / Assumptions:

• Lumped, ideal circuit elements are used for analysis.
• Voltage is defined as potential at one node minus potential at another node.
• Measurement is taken directly across the two terminals of the resistor.

Concept / Approach:
Voltage is a state variable associated with nodes. For any two points A and B, V_AB = V(A) - V(B). When the two points are the terminals of a resistor, this difference is colloquially called the “voltage drop across the resistor.” Ohm’s law then relates current through the resistor and the drop across it: V = I * R. The concept does not require that current be nonzero at the instant of measurement; with I = 0, the drop could be zero, but the definition still stands.

Step-by-Step Solution:

Verification / Alternative check:
A high-impedance voltmeter placed across the resistor reads this potential difference directly. SPICE node-voltage results confirm the same definition numerically for DC, AC, and transient simulations.

Why Other Options Are Wrong:

• Incorrect: Conflicts with standard definitions of potential difference.
• Applies only in DC: The definition holds for AC and time-varying signals.
• Applies only when current is nonzero: Voltage can be defined even with I = 0.
• Depends on power rating: Power rating is unrelated to the definition of voltage.

Common Pitfalls:
Confusing the definition of voltage with conditions for power dissipation; assuming “drop” implies energy loss only (sources can also have terminal voltages).

Final Answer:
Correct — the voltage drop across a resistor is the potential difference between its terminals.