Which of the following is not an application of Ohm's Law?

• A. Finding the voltage drop across a resistor in a series circuit using $V=IR$
• B. Determining the resistance of a wire from its length, cross-sectional area, and resistivity using $R=\frac{\rho L}{A}$
• C. Predicting how the current changes if the resistance in a circuit is doubled while keeping voltage constant
• D. Calculating the current through a resistor when the voltage and resistance are known using $I=\frac{V}{R}$

Answer: (B)

Verified step by step guidance

1. Understand that Ohm's Law relates voltage (V), current (I), and resistance (R) in a simple linear relationship: \(V = I \times R\).
2. Identify that applications of Ohm's Law involve using this relationship to find one of the three quantities (V, I, or R) when the other two are known.
3. Recognize that calculating voltage drop across a resistor, predicting current changes when resistance changes, and calculating current from voltage and resistance are direct applications of Ohm's Law.
4. Note that determining resistance from a wire's length, cross-sectional area, and resistivity uses the formula \(R = \frac{\rho L}{A}\), which is derived from material properties and geometry, not from Ohm's Law itself.
5. Conclude that the application involving \(R = \frac{\rho L}{A}\) is not an application of Ohm's Law, but rather a use of the resistivity formula.