Math Problem Statement

Let C be the positively oriented circle centred at the origine with radius 2. Compute the circulation of the vector F = -yi + xj around a circle C using Green's theorem.

Solution

To compute the circulation of the vector field F=yi+xj\mathbf{F} = -y\mathbf{i} + x\mathbf{j} around the circle CC using Green's Theorem, we need to evaluate the line integral CFdr\oint_C \mathbf{F} \cdot d\mathbf{r}.

Green's Theorem states:

CFdr=D(QxPy)dA\oint_C \mathbf{F} \cdot d\mathbf{r} = \iint_D \left( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right) dA

where F=Pi+Qj\mathbf{F} = P\mathbf{i} + Q\mathbf{j}. In our case, P=yP = -y and Q=xQ = x.

First, compute the partial derivatives:

Qx=xx=1\frac{\partial Q}{\partial x} = \frac{\partial x}{\partial x} = 1 Py=(y)y=1\frac{\partial P}{\partial y} = \frac{\partial (-y)}{\partial y} = -1

So, we have:

QxPy=1(1)=2\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} = 1 - (-1) = 2

Now, we need to integrate this over the region DD, which is the disk of radius 2 centered at the origin. The area element in polar coordinates is dA=rdrdθdA = r \, dr \, d\theta.

The integral becomes:

D2dA=2DdA\iint_D 2 \, dA = 2 \iint_D dA

The area of the disk DD with radius 2 is:

Area(D)=π(2)2=4π\text{Area}(D) = \pi (2)^2 = 4\pi

Therefore,

2DdA=2×4π=8π2 \iint_D dA = 2 \times 4\pi = 8\pi

So, the circulation of the vector field F\mathbf{F} around the circle CC is:

CFdr=8π\oint_C \mathbf{F} \cdot d\mathbf{r} = 8\pi

Would you like more details on any part of this solution or have any other questions?

Here are 5 questions you might want to ask:

  1. How is Green's Theorem applied in more complex vector fields?
  2. Can Green's Theorem be used to compute area?
  3. What are the conditions for using Green's Theorem?
  4. How can the circulation be interpreted physically?
  5. Can you explain the divergence theorem in relation to Green's Theorem?

Tip: When working with Green's Theorem, always check the orientation of the curve and ensure that the region DD is simply connected and the vector field components have continuous partial derivatives.

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Math Problem Analysis

Mathematical Concepts

Vector fields
Green's Theorem
Line integrals
Partial derivatives
Polar coordinates

Formulas

Green's Theorem: \( \oint_C \mathbf{F} \cdot d\mathbf{r} = \iint_D ( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} ) dA \)

Theorems

Green's Theorem

Suitable Grade Level

Advanced undergraduate level