Question

Determine whether or not the vector field is conservative. if it is conservative, find a function f such that f = ∇f. (if the vector field is not conservative, enter dne.) f(x, y, z) = ye−xi + e−xj + 2zk

Answer 1

A three-dimensional vector field is conservative if it is also irrotational, i.e. its curl is $\mathbf 0$. We have

$\nabla\times\mathbf f(x,y,z)=-2e^{-x}\,\mathbf k$

so this vector field is not conservative.

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Another way of determining the same result: We want to find a scalar function $f(x,y,z)$ such that its gradient is equal to the given vector field, $\mathbf f(x,y,z)$:

$\nabla f(x,y,z)=\mathbf f(x,y,z)$

For this to happen, we need to satisfy

$\begin{cases}f_x=ye^{-x}\\f_y=e^{-x}\\f_z=2z\end{cases}$

From the first equation, integrating with respect to $x$ yields

$f_x=ye^{-x}\implies f(x,y,z)=-ye^{-x}+g(y,z)$

Note that $g$ *must* be a function of $y,z$ only.

Now differentiate with respect to $y$ and we have

$f_y=-e^{-x}+g_y=e^{-x}\implies g_y=2e^{-x}\implies g(y,z)=2ye^{-x}+\cdots$

but this contradicts the assumption that $g(y,z)$ is independent of $x$. So, the scalar potential function does not exist, and therefore the vector field is not conservative.