Medium MCQ +4 / -1 PYQ · JEE Mains 2025

The electric flux is $\phi=\alpha \sigma+\beta \lambda$ where $\lambda$ and $\sigma$ are linear and surface charge density, respectively. $\left(\frac{\alpha}{\beta}\right)$ represents

  1. A displacement Correct answer
  2. B charge
  3. C electric field
  4. D area

Solution

<p>Let's analyze the given expression step by step.</p> <p>We are given the electric flux:</p> <p>$\phi = \alpha \sigma + \beta \lambda$</p> <p>where:</p> <p><p>$\sigma$ is a surface charge density with units of charge per unit area (C/m²).</p></p> <p><p>$\lambda$ is a linear charge density with units of charge per unit length (C/m).</p></p> <p>For the equation to be dimensionally consistent, the two terms on the right must have the same units as the flux $\phi$.</p> <p>For the term $\alpha \sigma$:</p> <p>Since $\sigma$ has units $\text{C/m}^2$, the constant $\alpha$ must have units that convert $\sigma$ into the same units as the flux.</p> <p>For the term $\beta \lambda$:</p> <p>Since $\lambda$ has units $\text{C/m}$, the constant $\beta$ carries its own units to ensure the term matches the flux’s dimensions.</p> <p>Since both terms add to give the flux, they must share the same overall dimension. Thus, the dimensions of $\alpha \sigma$ and $\beta \lambda$ must be equal:</p> <p>$[\alpha] \cdot \frac{Q}{L^2} = [\beta] \cdot \frac{Q}{L}$</p> <p>Here, $Q$ represents the unit of charge and $L$ represents a unit of length. Canceling the common factors, we find:</p> <p>$\frac{[\alpha]}{[\beta]} = L$</p> <p>That is, the ratio $\frac{\alpha}{\beta}$ has the dimension of length.</p> <p>Now, let’s match this with the given options:</p> <p><p>Option A: Displacement (commonly measured as a length)</p></p> <p><p>Option B: Charge (has the unit Coulomb)</p></p> <p><p>Option C: Electric Field (has units of V/m or N/C)</p></p> <p><p>Option D: Area (has units of length squared, $L^2$)</p></p> <p>Since $\frac{\alpha}{\beta}$ has the dimensions of a length, it logically corresponds to a displacement.</p> <p>Thus, the correct answer is:</p> <p>Option A – displacement.</p>

About this question

Subject: Physics · Chapter: Electrostatics · Topic: Gauss's Law

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