Average Equivalent Ring Resistance

There are two colored points (purely abstract entities) situated on a conducting circular ring.

The ring has a total electrical resistance R R , which is uniformly distributed over its circumference.

The green point is fixed at a particular location on the ring (the particular location doesn't matter).

The red point traverses over the ring's circumference at a constant angular speed ω \omega , and its location coincides with that of the green point at time t = 0 t = 0 .

The time-averaged (over an integer number of rotational periods beginning at t = 0 t = 0 ) equivalent resistance between the two points can be expressed as R α \large{\frac{R}{\alpha}} .

Determine the value of α \alpha .


The answer is 6.

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2 solutions

Guilherme Niedu
Dec 5, 2016

The resistance between the two points is, for one side of the ring:

R 1 = R θ 2 π R_1 = R \cdot \dfrac{\theta}{2\pi}

And, for the other side is:

R 2 = R ( 2 π θ 2 π ) R_2 = R \cdot \left(\dfrac{2\pi - \theta}{2\pi}\right)

The equivalent resistance is:

R e q = R 1 R 2 = R 1 R 2 R 1 + R 2 = R 4 π 2 ( 2 π ω t ω 2 t 2 ) R_{eq} = R_1 \mathbin{\|} R_2 \ = \dfrac{R_1\cdot R_2}{R_1 + R_2} = \dfrac{R}{4\pi^2} \cdot (2\pi\omega t - \omega^2 t^2)

The time-averaged value will be the integral of R e q R_{eq} over one period divided by one period. First evaluating the integral:

I = 0 2 π / ω R 4 π 2 ( 2 π ω t ω 2 t 2 ) d t I = \displaystyle\int_0^{2\pi/ \omega} \dfrac{R}{4\pi^2} \cdot (2\pi\omega t - \omega^2 t^2) \, dt

I = R 4 π 2 ( π ω 4 π 2 ω 2 ω 2 3 8 π 3 ω 3 ) I = \dfrac{R}{4\pi^2} (\pi \omega \dfrac{4\pi^2}{\omega^2} - \dfrac{\omega^2}{3}\dfrac{8\pi^3}{\omega^3} )

I = R π 3 ω I = \dfrac{R\pi}{3\omega}

The time-averaged value will be:

m = I 2 π ω m = \dfrac{I}{\frac{2\pi}{\omega}}

m = R 6 m = \dfrac{R}{6}

Then, α = 6 \color{#3D99F6} \fbox{α = 6}

maybe i am wrong.But at first glance, wouldnt averaging over two periods give a different answer?

basically, 0 2 n π ω R 4 π 2 ( 2 π ω t ω 2 t 2 ) \displaystyle \int_{0}^{\frac{2 n \pi}{\omega}}\frac{R}{4\pi^2} \cdot (2\pi\omega t - \omega^2 t^2)

I = R 4 π 2 ( π ω 4 n 2 π 2 ω 2 n 3 ω 2 3 8 π 3 ω 3 ) \large I = \frac{R}{4\pi^2} (\pi \omega \frac{4n^{2} \pi^2}{\omega^2} - \frac{n^{3} \omega^2}{3}\frac{8\pi^3}{\omega^3} )

division over 2 n π ω \frac{2n\pi}{\omega} gives R 6 \frac{R}{6} ( 3 n 2 n 2 3n-2n^{2} ) when averaged over n time periods

Rohith M.Athreya - 4 years, 6 months ago

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In this case you will also have to consider R 1 R_1 and R 2 R_2 as R θ 2 π n \large R \frac{\theta}{2\pi n} and R 2 π n θ 2 π n \large R \frac{2\pi n - \theta}{2\pi n} . Do this and you will see than n n cancels out. I've made it over one period for simplicity.

Guilherme Niedu - 4 years, 6 months ago

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yes,yes u are right! i overlooked that! thanks :)

Rohith M.Athreya - 4 years, 6 months ago

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@Rohith M.Athreya You're welcome!

Guilherme Niedu - 4 years, 6 months ago

Yeah, the trick is that when you're rotating, you can "go forever" and effectively stay in the same place on average. That's why it's advantageous to only try to average over one period. Averaging over more periods requires heightened caution.

Steven Chase - 4 years, 6 months ago
R G Staff
Dec 13, 2016

For the first cycle after time t,
R 1 = R 2 π ω t R_1 = \frac{R}{{2\pi }}\omega t
R 2 = R R 1 = R 2 π ( 2 π ω t R_2 = R-R_1=\frac{R}{{2\pi }}(2\pi - \omega t
R e q = R 1 R 2 R = R 4 π 2 ( 2 π ω t ω 2 t 2 ) R_{eq} =\frac{R_1R_2}{R}=\frac{R}{{4{\pi ^2}}}(2\pi \omega t - {\omega ^2}{t^2})


R e q R_{eq} vs t graph is a parabola for any one of the cycles. If A is the area of one of the parabolas as shown in the diagram, then the average resistance over n complete cycles would be
<R> = a r e a u n d e r R e q t g r a p h t \frac{area\,under\, R_{eq}-t \, graph}{t}
<R> = n A n 2 π ω \frac{nA}{\frac{n2\pi}{\omega}}
<R> = A 2 π ω \frac{A}{\frac{2\pi}{\omega}}

A = 0 2 π ω R e q d t A = \int_0^{\frac{{2\pi }}{\omega }} {{R_{eq}}} dt
<R> = R/6

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