Specific resistance of a substance is measured in

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ohm-cm Explanation: Specific resistance, also known as electrical resistivity ($\rho$), is an intrinsic property of a material that quantifies how strongly it opposes the flow of electric current. It is defined as the resistance offered by a unit cube of the material, which leads to the units being expressed as ohm-meter ($\Omega\cdot m$) or ohm-centimeter ($\Omega\cdot cm$). Working Principle: According to the resistance formula $R = \rho \frac{l}{A}$, where $R$ is resistance, $l$ is the length, and $A$ is the cross-sectional area, rearranging for resistivity gives $\rho = R \frac{A}{l}$. In the SI system, if $R$ is in ohms, $A$ is in $m^2$, and $l$ is in meters, then $\rho$ is in $\Omega\cdot m$. In practical engineering, particularly for small components, centimeters are often used, resulting in $\Omega\cdot cm$. Key Formulas: $R = \rho \frac{l}{A}$ — Relation between resistance, resistivity, length, and area; $\rho = \frac{1}{\sigma}$ — Relation between resistivity and conductivity Key Points: Specific resistance is independent of the physical dimensions of the conductor.; It depends solely on the material composition and temperature.; Higher specific resistance materials are poor conductors.; Metals have very low resistivity, while insulators have extremely high resistivity. Advantages: Allows direct comparison of material conductivity regardless of sample shape.; Essential for calculating the resistance of any physical wire geometry. Disadvantages: Temperature dependence requires specification of the reference temperature.; Requires precise measurement of cross-sectional area for calculation. Applications: Selecting materials for electrical wiring (Copper vs Aluminum).; Designing heating elements using high-resistivity alloys like Nichrome. Additional Info: Resistivity is the reciprocal of conductivity, denoted by the Greek letter $\sigma$ (sigma).. Option A (ohms) represents simple resistance, not specific resistance.. Option B (mhos) represents conductance, the reciprocal of resistance.. Option D (cm/ohm) is the reciprocal of resistivity times area, having no standard utility. Answer: C is correct — Specific resistance is defined as resistance per unit length and unit area, yielding the unit $\Omega\cdot cm$. Exam Tip: Always verify if the question asks for resistivity ($\Omega\cdot m$) or conductivity (Siemens/m), as they are inversely related: $\rho = \frac{1}{\sigma}$.

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