7. Find the gradient for the function 𝑢 = 𝜑(𝑟), 𝑟 = √𝑥2 + 𝑦2 + 𝑧2.Answer: 𝛁𝑢 = 𝜑′(𝑟) 𝒓𝑟 .
Question
7. Find the gradient for the function
u = \phi(r), \quad r = \sqrt{x^2 + y^2 + z^2}.
Answer: \nabla u = \phi'(r) \mathbf{r}.
Solution
To find the gradient of the function u = φ(r), where r = √(x² + y² + z²), we need to use the chain rule for differentiation.
Step 1: Differentiate φ(r) with respect to r to get φ′(r).
Step 2: Differentiate r with respect to x, y, and z to get the vector (x/r, y/r, z/r). This is the unit vector in the direction of r, denoted as 𝒓𝑟.
Step 3: Multiply φ′(r) by 𝒓𝑟 to get the gradient of u, which is 𝛁𝑢 = φ′(r) 𝒓𝑟.
So, the gradient of the function u = φ(r), where r = √(x² + y² + z²), is 𝛁𝑢 = φ′(r) 𝒓𝑟.
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