Question

Find the divergence and curl of the given vector field. \(\vec{F}=\nabla f,\) where \(f(x, y)=\frac{1}{2} x^{2}+\frac{1}{3} y^{3}\)

Short answer

Divergence: \(1 + 2y\); Curl: \(0\).

Step-by-step solution

Compute the Gradient of f

The first step is to compute the gradient of the scalar function \(f(x, y) = \frac{1}{2}x^2 + \frac{1}{3}y^3\). The gradient \( abla f \) is found by calculating the partial derivatives of \(f\) with respect to each variable and forming a vector. \[ abla f = \left( \frac{\partial f}{\partial x}, \frac{\partial f}{\partial y} \right) \]Calculate the partial derivatives:- \( \frac{\partial f}{\partial x} = x \)- \( \frac{\partial f}{\partial y} = y^2 \)Thus, \(\vec{F} = abla f = (x, y^2) \).

Find the Divergence of Vector Field F

The divergence of a vector field \(\vec{F} = (P(x, y), Q(x, y))\) is given by \(abla \cdot \vec{F} = \frac{\partial P}{\partial x} + \frac{\partial Q}{\partial y}\).For \(\vec{F} = (x, y^2)\):- \( P(x, y) = x \) and \( Q(x, y) = y^2 \)Now compute the partial derivatives:- \( \frac{\partial P}{\partial x} = \frac{\partial x}{\partial x} = 1 \)- \( \frac{\partial Q}{\partial y} = \frac{\partial y^2}{\partial y} = 2y \)Therefore, the divergence is:\[ abla \cdot \vec{F} = 1 + 2y = 1 + 2y \].

Find the Curl of Vector Field F

In two dimensions, the curl of a vector field \(\vec{F} = (P(x, y), Q(x, y))\) is defined as \(abla \times \vec{F} = \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y}\).For \(\vec{F} = (x, y^2)\):- \( \frac{\partial Q}{\partial x} = \frac{\partial y^2}{\partial x} = 0 \)- \( \frac{\partial P}{\partial y} = \frac{\partial x}{\partial y} = 0 \)Thus, the curl is:\[ abla \times \vec{F} = 0 - 0 = 0 \].

Key concepts

Gradient

In vector calculus, the gradient plays a crucial role in understanding how a scalar field changes in space. Think of the gradient as a vector that indicates the direction and rate of the fastest increase of a function. When dealing with functions of two variables, like our example function \( f(x, y) = \frac{1}{2}x^2 + \frac{1}{3}y^3 \), the gradient is calculated by taking partial derivatives of \( f \) with respect to each variable.

• Partial Derivative Definition: A partial derivative of a function of several variables is its derivative with respect to one of those variables, with the other variables held constant.

To find the gradient, calculate the partial derivatives:

• \( \frac{\partial f}{\partial x} = x \)
• \( \frac{\partial f}{\partial y} = y^2 \)

This results in the gradient vector expressed as \( abla f = (x, y^2) \). The result, \( (x, y^2) \), shows the direction of steepest ascent on the graph of \( f \). In summary, the gradient provides valuable information about the tendency of change of a scalar field, offering insights into where and how the field is changing most dramatically.

Divergence

Divergence is another key concept in vector calculus. It measures the magnitude of a vector field's source or sink at a given point, essentially telling us how much the field spreads out or converges. It's a scalar value, indicating how a vector field's flow expands or contracts. For a two-dimensional vector field \( \vec{F} = (P(x, y), Q(x, y)) \), the divergence is calculated as follows:\[ abla \cdot \vec{F} = \frac{\partial P}{\partial x} + \frac{\partial Q}{\partial y} \]In our example, where \( \vec{F} = (x, y^2) \):

• \( P(x, y) = x \) with \( \frac{\partial P}{\partial x} = 1 \)
• \( Q(x, y) = y^2 \) with \( \frac{\partial Q}{\partial y} = 2y \)

Thus, the divergence of \( \vec{F} \) is \( 1 + 2y \). This result tells us that at any point \( (x, y) \) in the plane, the vector field \( \vec{F} \) diverges at a rate influenced directly by the \( y \)-coordinate. Essentially, depending on the value of \( y \), the strength at which the field diverges or spreads out is determined.

Curl

The curl of a vector field in two dimensions offers insights into the rotational characteristics of the field. It detects the tendency of a field to rotate around a point. Mathematically speaking, for a two-dimensional vector field \( \vec{F} = (P(x, y), Q(x, y)) \), the curl is defined by:\[ abla \times \vec{F} = \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \]In our specific example, the vector field is \( \vec{F} = (x, y^2) \):

• \( \frac{\partial Q}{\partial x} = \frac{\partial y^2}{\partial x} = 0 \)
• \( \frac{\partial P}{\partial y} = \frac{\partial x}{\partial y} = 0 \)

Thus, the curl evaluates to 0. This result implies that there is no rotational effect or circulation at any point in this particular vector field. In two dimensions, a zero curl generally indicates that the field is not 'twisting' or 'turning' and implies potential field characteristics. Overall, understanding the curl is essential for analyzing rotation in vector fields.