Ohm's Law And Resistance

Question

When a potential difference of 12 volts is applied across a resistor, a steady current of 3 amperes flows, so what is its resistance value?

RankGuru · Accepted Answer

Ohm's law states that for a conductor at constant temperature, the current is directly proportional to the applied voltage, written as $V = IR$, and this fundamental relation appears in NCERT Class 12, Chapter 3 (Current Electricity). Rearranging to find resistance gives $R = \frac{V}{I}$. Substituting the given data, $R = \frac{12}{3} = 4$ ohm. The proportionality constant $R$ is a property of the material and geometry of the resistor. The option 36 ohm is wrong because it multiplies voltage by current rather than dividing, which actually yields power, not resistance. The option 0.25 ohm inverts the ratio by dividing current by voltage instead. The option 15 ohm wrongly adds the two quantities, an operation with no physical meaning here since they have different units. A plausibility check confirms the answer: a 4 ohm resistor across 12 V should indeed draw $12/4 = 3$ A, matching the stated current, and ohms as volts per ampere is dimensionally correct.

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