Using Divergence and Curl

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.

If F is a vector field in R3, then the curl of F is also a vector field in R3. Therefore, we can take the divergence of a curl. The next theorem says that the result is always zero. This result is useful because it gives us a way to show that some vector fields are not the curl of any other field. To give this result a physical interpretation, recall that divergence of a velocity field v at point P measures the tendency of the corresponding fluid to flow out of P. Since div curl (v)=0, the net rate of flow in vector field curl(v) at any point is zero. Taking the curl of vector field F eliminates whatever divergence was present in F.

Theorem: divergence of the curl


Let F=P,Q,R be a vector field in R3 such that the component functions all have continuous second-order partial derivatives. Then, div curl (F)=(×F)=0.

Proof

By the definitions of divergence and curl, and by Clairaut’s theorem,

div curl (F)=div [(RyQz)i+(PzRx)j+(QxPy)k]=RyxQxz+PyzRyx+QzxPzy=0.

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Example: showing that a vector field is not the curl of another

Show that F(x,y,z)=exi+yzj+xz2k is not the curl of another vector field. That is, show that there is no other vector G with curl G=F.

try it

Is it possible for G(x,y,z)=sinx,cosy,sin(x,y,z) 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 F is a conservative vector field then its curl is zero, and if the domain of F 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 F=P,Q,R is conservative, then curl F=0

Proof

Since conservative vector fields satisfy the cross-partials property, all the cross-partials of F are equal. Therefore,

curl F=(RyQz)i+(PzRx)j+(QxPy)k=0.

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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 curl (f)=0 for any scalar function f. In terms of our curl notation, ×(f)=0. This equation makes sense because the cross product of a vector with itself is always the zero vector. Sometimes equation ×(f)=0 is simplified as ×=0.

theorem: curl test for a conservative field


Let F=P,Q,R be a vector field in space on a simply connected domain. If curl F=0, then F is conservative.

Proof

Since curl F=0, we have that Ry=QzPz=Rx, and Qx=Py. Therefore, F satisfies the cross-partials property on a simply connected domain, and Cross-Partial Property of Conservative Fields Theorem implies that F is conservative.

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The same theorem is also true in a plane. Therefore, if F is a vector field in a plane or in space and the domain is simply connected, then F is conservative if and only if curl F=0.

Example: testing whether a vector field is conservative

Use the curl to determine whether F(x,y,z)=yz,xz,xy is conservative.

We have seen that the curl of a gradient is zero. What is the divergence of a gradient? If f is a function of two variables, then div (f)=(f)=fxx+fyy. We abbreviate this “double dot product” as 2. This operator is called the Laplace operator, and in this notation Laplace’s equation becomes 2f=0. Therefore, a harmonic function is a function that becomes zero after taking the divergence of a gradient.

Similarly, if f is a function of three variables then

div (f)=(f)=fxx+fyy+fzz.

Using this notation we get Laplace’s equation for harmonic functions of three variables:

2f=0.

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 f(x,y)=x2+xy to be the potential function of an electrostatic field that is located in a region of R2 free of static charge?

try it

Is it possible for function f(x,y)=x2y2+x to be the potential function of an electrostatic field located in a region of R2 free of static charge?