GATE AE 2022 • Q48 NAT • 2 Marks Engineering Mathematics

Engineering Mathematics

Calculus

Subtopic: Multiple Integrals & Vector Calculus

Question Statement:

$\vec{v} = x^3 \hat{i} + y^3 \hat{j} + z^3 \hat{k}$ is a vector field where $\hat{i}, \hat{j}, \hat{k}$ are the base vectors of a Cartesian coordinate system. Using the Gauss divergence theorem, the value of the outward flux of the vector field over the surface of a sphere of unit radius centered at the origin is ______ (rounded off to one decimal place).
Question figure for GATE AE 2022 Q48

Scientific schematic diagram reproduced from official IIT paper

Official Answer Key:

7.55

Step-by-Step Solution & Explanation

### **Detailed Step-by-Step Solution (GATE AE 2022 Q.48)** **1. Divergence Theorem**: $$\Phi = \iint_S \vec{v} \cdot d\vec{A} = \iiint_V (\nabla \cdot \vec{v}) \, dV$$ $$\nabla \cdot \vec{v} = \frac{\partial(x^3)}{\partial x} + \frac{\partial(y^3)}{\partial y} + \frac{\partial(z^3)}{\partial z} = 3(x^2 + y^2 + z^2) = 3r^2$$ **2. Volume Integration in Spherical Coordinates**: $$\Phi = \int_0^{2\pi} d\phi \int_0^\pi \sin\theta \, d\theta \int_0^1 3r^2 \cdot r^2 \, dr = 4\pi \times 3 \left[ \frac{r^5}{5} \right]_0^1 = \frac{12\pi}{5} = 2.4\pi \approx 7.54$$ - **Official Answer Key**: **7.5** (Range: 7.4 to 7.6)
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