Learning Objectives
- Use the properties of curl and divergence to determine whether a vector field is conservative.
Now that we understand the basic concepts of divergence and curl, we can discuss their properties and establish relationships between them and conservative vector fields.
Theorem: divergence of the curl
Let be a vector field in such that the component functions all have continuous second-order partial derivatives. Then, .
Proof
By the definitions of divergence and curl, and by Clairaut’s theorem,
.
Example: showing that a vector field is not the curl of another
Show that is not the curl of another vector field. That is, show that there is no other vector with curl .
try it
Is it possible for to be the curl of a vector field?
Watch the following video to see the worked solution to the above Try It
With the next two theorems, we show that if is a conservative vector field then its curl is zero, and if the domain of is simply connected then the converse is also true. This gives us another way to test whether a vector field is conservative.
theorem: curl of a conservative vector field
If is conservative, then .
Proof
Since conservative vector fields satisfy the cross-partials property, all the cross-partials of are equal. Therefore,
.
The same theorem is true for vector fields in a plane.
Since a conservative vector field is the gradient of a scalar function, the previous theorem says that for any scalar function . In terms of our curl notation, . This equation makes sense because the cross product of a vector with itself is always the zero vector. Sometimes equation is simplified as .
theorem: curl test for a conservative field
Let be a vector field in space on a simply connected domain. If , then is conservative.
Proof
Since , we have that , , and . Therefore, satisfies the cross-partials property on a simply connected domain, and Cross-Partial Property of Conservative Fields Theorem implies that is conservative.
The same theorem is also true in a plane. Therefore, if is a vector field in a plane or in space and the domain is simply connected, then is conservative if and only if .
Example: testing whether a vector field is conservative
Use the curl to determine whether is conservative.
We have seen that the curl of a gradient is zero. What is the divergence of a gradient? If is a function of two variables, then . We abbreviate this “double dot product” as . This operator is called the Laplace operator, and in this notation Laplace’s equation becomes . Therefore, a harmonic function is a function that becomes zero after taking the divergence of a gradient.
Similarly, if is a function of three variables then
.
Using this notation we get Laplace’s equation for harmonic functions of three variables:
.
Harmonic functions arise in many applications. For example, the potential function of an electrostatic field in a region of space that has no static charge is harmonic.
Example: analyzing a function
Is it possible for to be the potential function of an electrostatic field that is located in a region of free of static charge?
try it
Is it possible for function to be the potential function of an electrostatic field located in a region of free of static charge?
Candela Citations
- CP 6.45. Authored by: Ryan Melton. License: CC BY: Attribution
- Calculus Volume 3. Authored by: Gilbert Strang, Edwin (Jed) Herman. Provided by: OpenStax. Located at: https://openstax.org/books/calculus-volume-3/pages/1-introduction. License: CC BY-NC-SA: Attribution-NonCommercial-ShareAlike. License Terms: Access for free at https://openstax.org/books/calculus-volume-3/pages/1-introduction