Calculus 3 : Divergence

Study concepts, example questions & explanations for Calculus 3

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Example Questions

Example Question #1 : Line Integrals

Compute \(\displaystyle div \vec{F}\) for 

\(\displaystyle \vec{F}=yz\ln(x)\vec{i}+2xyz\vec{j}+z\vec{k}\)

Possible Answers:

\(\displaystyle div\vec{F}=-\frac{yz}{x}+2xz\)

\(\displaystyle div\vec{F}=\frac{yz}{x}+2xz+1\)

\(\displaystyle div\vec{F}=2xz+1\)

\(\displaystyle div\vec{F}=0\)

\(\displaystyle div\vec{F}=\frac{yz}{x}\)

Correct answer:

\(\displaystyle div\vec{F}=\frac{yz}{x}+2xz+1\)

Explanation:

In order to find the divergence, we need to remember the formula.

Divergence Formula:

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\), where \(\displaystyle P\)\(\displaystyle Q\), and \(\displaystyle R\) correspond to the components of a given vector field \(\displaystyle \vec{F}=P\vec{i}+Q\vec{j}+R\vec{k}\).

 

Now lets apply this to our situation.

 

\(\displaystyle div\vec{F}=\frac{\partial}{\partial x}\Big(yz\ln(x)\Big)+\frac{\partial}{\partial y}\Big(2xyz\Big)+\frac{\partial}{\partial z}\Big(z\Big)\)

\(\displaystyle div\vec{F}=\frac{yz}{x}+2xz+1\)

Example Question #1 : Line Integrals

Compute \(\displaystyle div\vec{F}\) for 

\(\displaystyle \vec{F}=x^2z\ln(y)\vec{i}+x2^{y^2}z\vec{j}+x^x\vec{k}\)

Possible Answers:

\(\displaystyle div\vec{F}=-2xz\ln(y)-2\log(2)xyz2^{y^2}\)

\(\displaystyle div\vec{F}=2xz\ln(y)\)

\(\displaystyle div\vec{F}=0\)

\(\displaystyle div\vec{F}=2\log(2)xyz2^{y^2}\)

\(\displaystyle div\vec{F}=2xz\ln(y)+2\log(2)xyz2^{y^2}\)

Correct answer:

\(\displaystyle div\vec{F}=2xz\ln(y)+2\log(2)xyz2^{y^2}\)

Explanation:

In order to find the divergence, we need to remember the formula.

Divergence Formula:

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\), where \(\displaystyle P\)\(\displaystyle Q\), and \(\displaystyle R\) correspond to the components of a given vector field \(\displaystyle \vec{F}=P\vec{i}+Q\vec{j}+R\vec{k}\).

 

Now lets apply this to our situation.

 

\(\displaystyle div\vec{F}=\frac{\partial}{\partial x}\Big(x^2z\ln(y)\Big)+\frac{\partial}{\partial y}\Big(x2^{y^2}z\Big)+\frac{\partial}{\partial z}\Big(x^x\Big)\)

\(\displaystyle div\vec{F}=2xz\ln(y)+2\log(2)xyz2^{y^2}+0\)

\(\displaystyle div\vec{F}=2xz\ln(y)+2\log(2)xyz2^{y^2}\)

Example Question #1 : Divergence

Compute \(\displaystyle div\vec{F}\) for 

\(\displaystyle \vec{F}=xy\vec{i}+xz\vec{j}+xyz\vec{k}\)

Possible Answers:

\(\displaystyle div \vec{F}=y\)

\(\displaystyle div \vec{F}=x\)

\(\displaystyle div \vec{F}=y(1+x)\)

\(\displaystyle div \vec{F}=1\)

\(\displaystyle div \vec{F}=(1+x)\)

Correct answer:

\(\displaystyle div \vec{F}=y(1+x)\)

Explanation:

In order to find the divergence, we need to remember the formula.

Divergence Formula:

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\), where \(\displaystyle P\)\(\displaystyle Q\), and \(\displaystyle R\) correspond to the components of a given vector field \(\displaystyle \vec{F}=P\vec{i}+Q\vec{j}+R\vec{k}\).

 

Now lets apply this to our situation.

 

\(\displaystyle div\vec{F}=\frac{\partial}{\partial x}\Big(xy\Big)+\frac{\partial}{\partial y}\Big(xz\Big)+\frac{\partial}{\partial z}\Big(xyz\Big)\)

\(\displaystyle div \vec{F}=y+0+xy\)

\(\displaystyle div \vec{F}=y+xy=y(1+x)\)

Example Question #4 : Line Integrals

Find \(\displaystyle div\vec{F}\), where \(\displaystyle \vec{F}=xyz\vec{i}+x^2z^{-4}\vec{j}+y^{-10}z^{3}\vec{k}\)

Possible Answers:

\(\displaystyle div\vec{F}=yz+\frac{3z^2}{y^{-10}}\)

\(\displaystyle div\vec{F}=yz+z^4+\frac{3z^2}{y^{10}}\)

\(\displaystyle div\vec{F}=\frac{3z^2}{y^{10}}\)

\(\displaystyle div\vec{F}=yz+\frac{3z^2}{y^{10}}\)

\(\displaystyle div\vec{F}=xyz+\frac{3z^2}{y^{10}}\)

Correct answer:

\(\displaystyle div\vec{F}=yz+\frac{3z^2}{y^{10}}\)

Explanation:

In order to find the divergence, we need to remember the formula.

Divergence Formula:

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\), where \(\displaystyle P\)\(\displaystyle Q\), and \(\displaystyle R\) correspond to the components of a given vector field \(\displaystyle \vec{F}=P\vec{i}+Q\vec{j}+R\vec{k}\).

 

Now lets apply this to our situation.

 

\(\displaystyle div\vec{F}=\frac{\partial}{\partial x}\Big(xyz\Big)+\frac{\partial}{\partial y}\Big(x^2z^{-4}\Big)+\frac{\partial}{\partial z}\Big(y^{-10}z^3\Big)\)

\(\displaystyle div\vec{F}={yz}+0+3y^{-10}z^2=yz+\frac{3z^2}{y^{10}}\)

Example Question #11 : Line Integrals

Compute \(\displaystyle div\vec{F}\), where \(\displaystyle \vec{F}=10x^2\ln(yz)\vec{i}+xyz\vec{j}+xz\ln(y)\vec{k}\)

Possible Answers:

\(\displaystyle div\vec{F}=20x\ln(yz)-xz+x\ln(y)\)

\(\displaystyle div\vec{F}=20x\ln(z)+yz+x\ln(y)\)

\(\displaystyle div\vec{F}=0\)

\(\displaystyle div\vec{F}=20x\ln(yz)-xz-x\ln(y)\)

\(\displaystyle div\vec{F}=20x\ln(yz)+xz+x\ln(y)\)

Correct answer:

\(\displaystyle div\vec{F}=20x\ln(yz)+xz+x\ln(y)\)

Explanation:

All we need to do is calculate the partial derivatives and add them together.

\(\displaystyle div\vec{F}=\frac{\partial}{\partial x}(10x^2\ln(yz))+\frac{\partial}{\partial y}(xyz)+\frac{\partial}{\partial z}(xz\ln(y))\)

\(\displaystyle div\vec{F}=20x\ln(yz)+xz+x\ln(y)\)

Example Question #11 : Line Integrals

Given the vector field

\(\displaystyle \vec{F}=yz\vec{i}+xz\vec{j}+xy\vec{k}\)

find the divergence of the vector field:

\(\displaystyle div \vec{F}\).

Possible Answers:

\(\displaystyle div \vec{F}=xy+yz+xz\)

\(\displaystyle div \vec{F}=0\)

\(\displaystyle div \vec{F}=x+z\)

\(\displaystyle div \vec{F}=x+y\)

\(\displaystyle div \vec{F}=x+y+z\)

Correct answer:

\(\displaystyle div \vec{F}=0\)

Explanation:

Given a vector field 

\(\displaystyle \vec{F}=P\vec{i}+Q\vec{j}+R\vec{k}\)

we find its divergence by taking the dot product with the gradient operator:

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}\)

We know that \(\displaystyle P=yz, Q=xz, R=xy\), so we have 

\(\displaystyle div\vec{F}=\frac{\partial P}{\partial x}+\frac{\partial Q}{\partial y}+\frac{\partial R}{\partial z}=0+0+0=0\)

Example Question #1 : Divergence

Suppose that \(\displaystyle \underset{F}{\rightarrow}\)\(\displaystyle =2x^2yz^2+y^3e^{2x}+\arctan z\). Calculate the divergence.

Possible Answers:

\(\displaystyle 4x+3y^2+\frac{1}{1+z^2}\)

\(\displaystyle 4xyz^2+2y^3e^{2x} + 2x^2z^2+3y^2e^{2x}+4x^2yz+\frac{1}{1+z^2}\)

\(\displaystyle 4xz^2+2y^2e^{2x} + 2x^2z+3y^2e^{2x}+4x^2yz+\frac{1}{1+z^2}\)

\(\displaystyle 4xyz+2x^2y^2z^2 +4x^2yz+6xy^2z+\frac{1}{1+z^2}\)

Correct answer:

\(\displaystyle 4xyz^2+2y^3e^{2x} + 2x^2z^2+3y^2e^{2x}+4x^2yz+\frac{1}{1+z^2}\)

Explanation:

We know,

\(\displaystyle div F(x,y,z)=\frac{\partial{F}}{\partial{x}}+\frac{\partial{F}}{\partial{y}}+\frac{\partial{F}}{\partial{z}}\)

Use this to obtain the correct answer

Example Question #2 : Divergence

Given that

\(\displaystyle \vec{F}=< 2x,y,2z>\)

calculate \(\displaystyle div(\vec{F})\)

Possible Answers:

\(\displaystyle 1\)

\(\displaystyle 5\)

\(\displaystyle 0\)

\(\displaystyle x^2+\frac{1}{2}y^2+z^2\)

Correct answer:

\(\displaystyle 5\)

Explanation:

\(\displaystyle div(\vec{F})=\frac{\partial{F}}{\partial{x}}+\frac{\partial{F}}{\partial{y}}+\frac{\partial{F}}{\partial{z}}\)

using this formula we have

\(\displaystyle div(\vec{F})=2+1+2=5\)

Example Question #9 : Divergence

Find \(\displaystyle div \vec{F}\), where F is given by the following curve:

\(\displaystyle \left \langle 2x, 3xy, z^2\cos(z)\right \rangle\)

Possible Answers:

\(\displaystyle 2+3x+2z\cos(z)-z^2\sin(z)\)

\(\displaystyle 2+3x+2z\cos(z)\)

\(\displaystyle \left \langle 2, 3x, 2z\cos(z)-z^2\sin(z)\right \rangle\)

\(\displaystyle 2+3xy+2z\cos(z)-z^2\sin(z)\)

Correct answer:

\(\displaystyle 2+3x+2z\cos(z)-z^2\sin(z)\)

Explanation:

The divergence of a vector is given by

\(\displaystyle div \vec{F}=\bigtriangledown \cdot \vec{F}\)

where \(\displaystyle \bigtriangledown = \left \langle f_x, f_y, f_z\right \rangle\)

So, we take the partial derivative of each component of our vector with respect to x, y, and z respectively and add them together:

\(\displaystyle f_x(2x)=2\)

\(\displaystyle f_y(3xy)=3x\)

\(\displaystyle f_z(z^2\cos(z))=2z\cos(z)-z^2\sin(z)\)

The derivatives were found using the following rules:

\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} ax=a\)\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}\)\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} f(x)g(x)=f'(x)g(x)+f(x)g'(x)\)

\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} \cos(x)=-\sin(x)\)

Example Question #3 : Divergence

Find \(\displaystyle div \vec{F}\) where F is given by

\(\displaystyle \left \langle 12y\cos(z), y^2e^x, z^2\right \rangle\)

Possible Answers:

\(\displaystyle 2ye^x+z^2\)

\(\displaystyle 12\cos(z)+2ye^x+2z\)

\(\displaystyle 2ye^x+2z\)

\(\displaystyle 0\)

Correct answer:

\(\displaystyle 2ye^x+2z\)

Explanation:

The divergence of a curve is given by

\(\displaystyle div \vec{F}=\bigtriangledown \cdot \vec{F}\)

where \(\displaystyle \bigtriangledown = \left \langle f_x, f_y, f_z\right \rangle\)

Taking the dot product of the gradient and the curve, we end up summing the respective partial derivatives (for example, the x coordinate's partial derivative with respect to x is found).

The partial derivatives are:

\(\displaystyle f_x=0\)

\(\displaystyle f_y=2ye^x\)

\(\displaystyle f_z=2z\)

The following rules were used to find the derivatives:

\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} ax=a\)\(\displaystyle \frac{\mathrm{d} }{\mathrm{d} x} x^n=nx^{n-1}\)

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