Divergence

Calculus Level 3

If A = x 2 z i ^ 2 y 3 z 2 j ^ + x y 2 z k ^ \vec{A} = x^2 z \hat{i} - 2y^3z^2 \hat{j} +xy^2z \hat{k} , then determine the divergence at the point ( 1 , 1 , 1 ) (1,-1,1) ?

-5 -3 -4 -2

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1 solution

Nazmus Sakib
Sep 12, 2017

Given vector, A = x 2 z i ^ 2 y 3 y 2 j ^ + x y 2 z k ^ \vec{A} = x^2 z \hat{i} - 2y^3y^2 \hat{j} +xy^2z \hat{k} and alternative point ( 1 , 1 , 1 ) (1,-1,1) .

The divergence of A = A \vec{A} = \vec{\nabla} \cdot \vec{A}

= ( i ^ x + j ^ y + k ^ z ) ( x 2 z i ^ 2 y 3 z 2 j ^ + x y 2 z k ^ ) = \left( \hat{i} \dfrac{∂}{∂x} + \hat{j} \dfrac{∂}{∂y} + \hat{k} \dfrac{∂}{∂z} \right) \cdot \left(x^2z \hat{i} - 2y^3z^2 \hat{j} + xy^2 z \hat{k}\right)

= x ( x 2 z ) + y ( 2 y 3 z 2 ) + z ( x y 2 z ) = \dfrac{∂}{∂x} (x^2z) + \dfrac{∂}{∂y} (-2y^3z^2) +\dfrac{∂}{∂z} (xy^2z)

= 2 x z 6 y 2 z 2 + x y 2 =2xz - 6y^2z^2 + xy^2

Therefore, the divergence at point ( 1 , 1 , 1 ) : (1,-1,1):

= 2 × 1 × 1 6 ( 1 ) 2 ( 1 ) 2 + 1 × ( 1 ) 2 =2\times 1 \times 1 - 6(-1)^2 (1)^2 + 1 \times (-1)^2

= 2 6 + 1 = 3 =2-6+1 = -3

That is not correct! The answer is -3 for (x,y,z) = (1,-1,1).

tom engelsman - 3 years, 9 months ago

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Thanks I've edited the solution. @Calvin Lin please update the correct ans to -3

Nazmus sakib - 3 years, 9 months ago

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