Mutual Inductance - Wavy Loop 2

Loop 1 1 is a circle:

x 1 = 3 + cos θ 1 y 1 = 2 + sin θ 1 z 1 = 0 0 θ 1 2 π x_1 = 3 + \cos \theta_1 \\ y_1 = 2 + \sin \theta_1 \\ z_1 = 0 \\ 0 \leq \theta_1 \leq 2 \pi

Loop 2 2 is a "wavy circle":

x 2 = cos θ 2 y 2 = sin θ 2 z 2 = 5 + 5 sin θ 2 0 θ 2 2 π x_2 = \cos \theta_2 \\ y_2 = \sin \theta_2 \\ z_2 = 5 + 5 \sin \theta_2 \\ 0 \leq \theta_2 \leq 2 \pi

If Loop 1 1 carries 1 1 unit of electric current, what is the magnetic flux through Loop 2 2 ?

Details and Assumptions:
1) Magnetic permeability μ 0 = 1 \mu_0 = 1
2) This link may be helpful (hyperphysics website)
3) Give your answer as a positive number


The answer is 0.017.

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1 solution

Karan Chatrath
May 10, 2020

Consider a point on loop 1:

r 1 = ( 3 + cos θ 1 ) i ^ + ( 2 + sin θ 1 ) j ^ + 0 k ^ \vec{r}_1 = (3+\cos{\theta_1}) \ \hat{i} + (2+\sin{\theta_1}) \ \hat{j} + 0 \ \hat{k}

And that on loop 2:

r 2 = cos θ 2 i ^ + sin θ 2 j ^ + ( 5 + 5 sin θ 2 ) k ^ \vec{r}_2 = \cos{\theta_2} \ \hat{i} + \sin{\theta_2} \ \hat{j} + (5+5\sin{\theta_2}) \ \hat{k}

The arc length elements on each loop can be computed by evaluating the total differentials:

d r 1 = ( sin θ 1 i ^ + cos θ 1 j ^ + 0 k ^ ) d θ 1 d\vec{r}_1 = \left(-\sin{\theta_1} \ \hat{i} + \cos{\theta_1} \ \hat{j} + 0 \ \hat{k} \right) \ d\theta_1 d r 2 = ( sin θ 2 i ^ + cos θ 2 j ^ + 5 cos θ 2 k ^ ) d θ 2 d\vec{r}_2 = \left(-\sin{\theta_2} \ \hat{i} + \cos{\theta_2} \ \hat{j} + 5\cos{\theta_2} \ \hat{k} \right) \ d\theta_2

Finally, we can compute the elementary magnetic vector potential at any general point due to loop 1:

d A = μ o I 4 π ( d r 1 r r 1 ) d\vec{A} = \frac{\mu_o I}{4 \pi}\left(\frac{d\vec{r}_1}{\lvert \vec{r} - \vec{r}_1\rvert}\right)

The total magnetic potential vector due to loop at any point can be evaluated as such:

A = μ o I 4 π L 1 d r 1 r r 1 \vec{A} = \frac{\mu_o I}{4 \pi} \oint_{L1} \frac{d\vec{r}_1}{\lvert \vec{r} - \vec{r}_1\rvert}

Now, we know that the magnetic field at that point can be computed as such:

B = × A \vec{B} = \nabla \times \vec{A}

The elementary surface area through the loop is say: d S d\vec{S} . This means that elementary flux is:

d Φ = B d S d\Phi = \vec{B} \cdot d\vec{S}

The total flux can be computed by integrating over the surface area of loop 2 as such:

Φ = S 2 B d S \Phi = \oint_{S2}\vec{B} \cdot d\vec{S} Φ = S 2 ( × A ) d S \implies \Phi = \oint_{S2} \left(\nabla \times \vec{A}\right) \cdot d\vec{S}

Applying Stoke's theorem leads to:

Φ = S 2 ( × A ) d S = L 2 A d r 2 \Phi = \oint_{S2} \left(\nabla \times \vec{A}\right) \cdot d\vec{S} = \oint_{L2} \vec{A} \cdot d\vec{r}_2 Φ = L 2 L 1 μ o I 4 π d r 1 r 2 r 1 d r 2 \Phi = \oint_{L2} \oint_{L1} \frac{\mu_o I}{4 \pi} \frac{d\vec{r}_1}{\lvert \vec{r}_2 - \vec{r}_1\rvert}\cdot d\vec{r}_2

Substituting expressions and simplifying the integrand gives a function f ( θ 1 , θ 2 ) f(\theta_1,\theta_2) and the integral looks as such:

Φ = 0 2 π 0 2 π f ( θ 1 , θ 2 ) d θ 1 d θ 2 \Phi = \int_{0}^{2 \pi} \int_{0}^{2 \pi} f(\theta_1,\theta_2) d\theta_1 \ d\theta_2

I performed this entire calculation process using a script of code and have only provided an outline. The answer evaluates to:

Φ 0.017 \boxed{\Phi \approx 0.017}

@Karan Chatrath very nice solution. I just upvoted it. And please forgive me. Sorry.

A Former Brilliant Member - 1 year, 1 month ago

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