Question

The purpose in doing the following simple problems is to become familiar with the formulas we have discussed. So a good study method is to do them by hand and then check your results by computer. Compute the divergence and the curl of each of the following vector fields. $$\mathbf{V}=x^{2} y \mathbf{i}+y^{2} x \mathbf{j}+x y z \mathbf{k}$$

Short answer

The divergence is 5xy and the curl is xz\textbf{i} - yz\textbf{j}.

Step-by-step solution

Recall the Divergence Formula

The divergence of a vector field \(\textbf{V} = P\textbf{i} + Q\textbf{j} + R\textbf{k} \) is given by \(abla \bullet \textbf{V} = \frac{\text{\textdegree} P}{\text{\textdegree} x} + \frac{\text{\textdegree} Q}{\text{\textdegree} y} + \frac{\text{\textdegree} R}{\text{\textdegree} z} \).

Apply the Divergence Formula

For \(\textbf{V} = x^{2} y \textbf{i}+ y^{2} x \textbf{j} + x y z \textbf{k} \), identify \( P = x^{2} y \), \( Q = y^{2} x \), and \( R = x y z \). Then, compute each partial derivative: \(\frac{\text{\textdegree} P}{\text{\textdegree} x} = 2xy \), \(\frac{\text{\textdegree} Q}{\text{\textdegree} y} = 2yx = 2xy \), \(\frac{\text{\textdegree} R}{\text{\textdegree} z} = xy \). Add these to get the divergence: \( abla \bullet \textbf{V} = 2xy + 2xy + xy = 5xy \).

Recall the Curl Formula

The curl of a vector field \( \textbf{V} = P \textbf{i} + Q \textbf{j} + R \textbf{k} \) is given by \[ abla \times \textbf{V} = \begin{vmatrix} \textbf{i} & \textbf{j} & \textbf{k} \ \frac{\text{\textdegree}}{\text{\textdegree} x} & \frac{\text{\textdegree}}{\text{\textdegree} y} & \frac{\text{\textdegree}}{\text{\textdegree} z} \ P & Q & R \ \end{vmatrix} \].

Apply the Curl Formula

Substitute \( P = x^{2} y \, Q = y^{2} x \, R = x y z \) into the determinant form and calculate: \[abla \times \textbf{V} = \begin{vmatrix} \textbf{i} & \textbf{j} & \textbf{k} \ \frac{\text{\textdegree}}{\text{\textdegree} x} & \frac{\text{\textdegree}}{\text{\textdegree} y} & \frac{\text{\textdegree}}{\text{\textdegree} z} \ x^{2} y & y^{2} x & x y z \ \end{vmatrix} \ = \textbf{i}\bigg( \frac{\text{\textdegree} (x y z)}{\text{\textdegree} y} - \frac{\text{\textdegree} (y^{2} x)}{\text{\textdegree} z} \bigg) - \textbf{j}\bigg( \frac{\text{\textdegree} (x y z)}{\text{\textdegree} x} - \frac{\text{\textdegree} (x^{2} y)}{\text{\textdegree} z} \bigg) + \textbf{k}\bigg( \frac{\text{\textdegree} (y^{2} x)}{\text{\textdegree} x} - \frac{\text{\textdegree} (x^{2} y)}{\text{\textdegree} y} \bigg) \ = \textbf{i}(xz - 0) - \textbf{j}(yz - 0) + \textbf{k}(2yx - 2xy) \ = xz\textbf{i} - yz\textbf{j} \]

Key concepts

Vector Field

A vector field is a mathematical construct where a vector is assigned to every point in a specific space. Think of it like a weather map where each vector represents the wind speed and direction at a particular location. In the context of this exercise:
The vector field is given by \(\textbf{V} = x^{2} y \textbf{i} + y^{2} x \textbf{j} + x y z \textbf{k}\). Here,

• \textbf{i} is the unit vector in the x-direction,
• \textbf{j} is the unit vector in the y-direction,
• \textbf{k} is the unit vector in the z-direction.

Each component of the vector field can vary depending on the position (x, y, z) in space. Understanding vector fields is crucial as they often represent physical phenomena such as gravitational fields, velocity fields in fluid dynamics, and electromagnetic fields.

Divergence

The divergence of a vector field measures how much the vector field spreads out (or diverges) from a point. It's like measuring how air flows out of a balloon at different points. Mathematically, the divergence of a vector field \( \textbf{V} = P \textbf{i} + Q \textbf{j} + R \textbf{k} \) is given by:
\( abla \bullet \textbf{V} = \frac{\text{\textdegree} P}{\text{\textdegree} x} + \frac{\text{\textdegree} Q}{\text{\textdegree} y} + \frac{\text{\textdegree} R}{\text{\textdegree} z} \).
Here, the operators \( \frac{\text{\textdegree} P}{\text{\textdegree} x} \), \( \frac{\text{\textdegree} Q}{\text{\textdegree} y} \), and \( \frac{\text{\textdegree} R}{\text{\textdegree} z} \) are partial derivatives describing how each component of the vector field changes with respect to x, y, and z respectively.

• For \( P = x^{2} y \), \( Q = y^{2} x \), and \( R = x y z \), we get:
• \( \frac{\text{\textdegree} P}{\text{\textdegree} x} = 2xy \),
• \( \frac{\text{\textdegree} Q}{\text{\textdegree} y} = 2xy \),
• \( \frac{\text{\textdegree} R}{\text{\textdegree} z} = xy \).

Adding these up, the divergence of this vector field is \( abla \bullet \textbf{V} = 2xy + 2xy + xy = 5xy \). The result tells us how much the field is diverging at each point (x, y, z).

Curl

The curl of a vector field measures the tendency of the field to rotate or 'curl' around a point. It's like measuring how water swirls in a whirlpool. Mathematically, the curl of a vector field \( \textbf{V} = P \textbf{i} + Q \textbf{j} + R \textbf{k} \) is given by:
\[ abla \times \textbf{V} = \begin{vmatrix} \textbf{i} & \textbf{j} & \textbf{k} \ \frac{\text{\textdegree}}{\text{\textdegree} x} & \frac{\text{\textdegree}}{\text{\textdegree} y} & \frac{\text{\textdegree}}{\text{\textdegree} z} \ P & Q & R \ \end{vmatrix} \].
This determinant expands to:
\[ abla \times \textbf{V} = \textbf{i}\bigg( \frac{\text{\textdegree} R}{\text{\textdegree} y} - \frac{\text{\textdegree} Q}{\text{\textdegree} z} \bigg) - \textbf{j}\bigg( \frac{\text{\textdegree} R}{\text{\textdegree} x} - \frac{\text{\textdegree} P}{\text{\textdegree} z} \bigg) + \textbf{k}\bigg( \frac{\text{\text{\textdegree}}Q}{\text{\text{\textdegree}} x} - \frac{\text{\text{\textdegree}}P}{\text{\text{\textdegree}} y} \bigg) \].
For the given exercise, substituting \( P = x^{2} y, Q = y^{2} x, R = x y z \) into the determinant form gives:

• \textbf{i} component: \( \frac{\text{\text{\textdegree}} (x y z)}{\text{\textdegree} y} - \frac{\text{\text{\textdegree}} (y^{2} x)}{\text{\text{\textdegree}} z} = xz \),
• \textbf{j} component: \( \frac{\text{\text{\textdegree}} (x y z)}{\text{\text{\textdegree}} x} - \frac{\text{\text{\textdegree}} (x^{2} y)}{\text{\text{\textdegree}} z} = -yz \),
• \textbf{k} component: \( \frac{\text{\text{\textdegree}} (y^{2} x)}{\text{\text{\textdegree}} x} - \frac{\text{\text{\textdegree}} (x^{2} y)}{\text{\text{\textdegree}} y} = 0 \).

So, the curl of this vector field is \( abla \times \textbf{V} = xz\textbf{i} - yz\textbf{j} \). It shows the rotation tendency at each point (x, y, z).

Partial Derivatives

Partial derivatives are a core concept in multivariable calculus. They focus on how a function changes as only one of the variables changes, keeping the other variables fixed. For a function \( f(x, y, z) \), the partial derivative with respect to \( x \) is denoted as \( \frac{\text{\textdegree} f}{\text{\textdegree} x} \), similarly for \( y \): \( \frac{\text{\textdegree} f}{\text{\textdegree} y} \) and for \( z \): \( \frac{\text{\textdegree} f}{\text{\textdegree} z} \).
In this exercise, we take partial derivatives to find the divergence and curl of a vector field. For example:

• Given \( P = x^{2} y \), the partial derivative with respect to \( x \) is \( \frac{\text{\textdegree} P}{\text{\textdegree} x} = 2xy \).
• For \( Q = y^{2} x \), the partial derivative with respect to \( y \) is \( \frac{\text{\textdegree} Q}{\text{\textdegree} y} = 2yx = 2xy \).
• For \( R = x y z \), the partial derivative with respect to \( z \) is \( \frac{\text{\textdegree} R}{\text{\textdegree} z} = xy \).

These partial derivatives help us compute the exact behavior of the vector field at any point, aiding in understanding complex physical or mathematical systems.