RE-NEET 2024 Physics Surface Charge Density MCQ Question

Topic: Surface Charge Density Difficulty: 🟢 Easy

✅ Ans: D — $0.4\times10^{-3}\ \text{C m}^{-2}$

For a metal cube, charge resides on its surface. (CloudFront)

Given:

• $Q=6\,\mu C=6\times10^{-6}C$
• Side $a=5\,cm=0.05\,m$

Total surface area:

$$A=6a^2=6(0.05)^2=0.015\,m^2$$

Surface charge density:

$$\sigma=\frac{Q}{A} =\frac{6\times10^{-6}}{0.015}$$ $$\boxed{\sigma=0.4\times10^{-3}\ C\,m^{-2}}$$

❌ Why other options are wrong?

They result from using an incorrect surface area or calculation.

📌 NCERT: Surface charge density, $\sigma=\frac{Q}{A}$.

🧠 NEET Trick: Cube → 6 faces → $A=6a^2$. Always convert cm → m first.