Rotation + Magnetism = Rotatism

Consider an Thin Conducting Rod of Length L which start's Rotating about an fixed Point P(L,0) in X-Y plane. Initially Rod is on X-axis at an distance of L from origin.(As shown)

There is an infinitely long wire on entire Y-axis. And current is flowing Through this wire is I 0 { I }_{ 0 }

This Rod is rotated By an external agent with Constant angular velocity ω \omega . Then Find the Magnitude of Motional EMF develop between the Point 'P' and centre of the rod at the instant when rod is rotated by an angle θ = 60 0 \theta ={ 60 }^{ 0 }


Details and Assumptions

\bullet Long wire and Conducting Rod are Placed in X-Y plane.

\bullet Rod is Rotated By an external agent so that it can definitely move with constant angular velocity.

\bullet EMF means electro motive Force.


Use following data

I 0 = 4 × 10 7 A m p . ω = 4 r a d / s L = 4 m \bullet { I }_{ 0 }\quad =4{ \times 10 }^{ 7 }\quad Amp.\\ \bullet \omega =4\quad rad/s\\ \bullet L=4\quad m .


This is Original
This is Part of set mixing of concepts


The answer is 13.7505.

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2 solutions

Deepanshu Gupta
Oct 22, 2014

Let at any Time t=t rod is rotated by an angle ' θ \theta ' So Consider an element of length 'dr' which is at an distance of 'r' from the point of Pivot ( Point P ).

Distance of This element from the wire is

x = L + r cos θ x\quad =\quad L+r\cos { \theta } .

Now emf induced between Point P and Centre O is :

\Ep P 0 = r = 0 r = L / 2 μ 0 I 0 ω r 2 π ( L + r cos θ ) ( d r ) \Ep P 0 = μ 0 I 0 ω 2 π r = 0 r = L / 2 r L + r 2 ( d r ) ( cos 60 = 1 / 2 ) \Ep P 0 = μ 0 I 0 ω π r = 0 r = L / 2 2 L + r 2 L 2 L + r ( d r ) \Ep P 0 = μ 0 I 0 ω π r = 0 r = L / 2 ( 1 2 L 2 L + r ) ( d r ) \Ep P 0 = μ 0 I 0 ω π ( [ r 2 L 2 L + r ln ( 2 L + r ) ] 0 L / 2 ) \Ep P 0 = μ 0 I 0 ω L 4 π cos θ ( 1 1 cos θ ln 25 16 ) { \Ep }_{ P0 }\quad =\int _{ r=0 }^{ r=L/2 }{ \frac { { \mu }_{ 0 }{ I }_{ 0 }\omega \quad r }{ 2\pi (L+r\cos { \theta } ) } (dr) } \\ \\ { \Ep }_{ P0 }\quad =\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ 2\pi } \int _{ r=0 }^{ r=L/2 }{ \frac { r }{ L+\cfrac { r }{ 2 } } (dr) } \quad \quad \quad \quad (\because \quad \cos { 60 } \quad =\quad 1/2\quad )\\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } \quad \int _{ r=0 }^{ r=L/2 }{ \frac { 2L\quad +\quad r\quad -\quad 2L }{ 2L+r } (dr) } \\ \\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } \quad \int _{ r=0 }^{ r=L/2 }{ (1\quad -\quad \frac { 2L }{ 2L+r } )(dr) } \\ \\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } (\quad { [r\quad -\quad \frac { 2L }{ 2L+r } \ln { (2L+r) } ] }_{ 0 }^{ L/2 }\quad )\\ \\ { \Ep }_{ P0 }\quad =\frac { { \mu }_{ 0 }{ I }_{ 0 }\omega L }{ 4\pi \cos { \theta } } (1-\frac { 1 }{ \cos { \theta } } \ln { \frac { 25 }{ 16 } } ) .

Here I used The Fact the magnetic field due to wire at an distance of x will be B = μ 0 I 0 2 π x B=\frac { { \mu }_{ 0 }\quad { I }_{ 0 } }{ 2\pi \quad x } .

And also If an rod of length 'l' moving with velocity 'v' then emf developed in it will be = B l v =Blv .

Deepanshu Gupta - 6 years, 7 months ago

I got the same answer but was marked incorrect. I sent the same as clarification also . It said it accepts only integer answers

jatin yadav - 6 years, 7 months ago

I have updated the answer to 13.7505.

Those who previously entered 13 or 14 have been marked correct.

Calvin Lin Staff - 6 years, 7 months ago

How did you integrate?

Tushar Gopalka - 6 years, 7 months ago

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\Ep P 0 = r = 0 r = L / 2 μ 0 I 0 ω r 2 π ( L + r cos θ ) ( d r ) \Ep P 0 = μ 0 I 0 ω 2 π r = 0 r = L / 2 r L + r 2 ( d r ) ( cos 60 = 1 / 2 ) \Ep P 0 = μ 0 I 0 ω π r = 0 r = L / 2 2 L + r 2 L 2 L + r ( d r ) \Ep P 0 = μ 0 I 0 ω π r = 0 r = L / 2 ( 1 2 L 2 L + r ) ( d r ) \Ep P 0 = μ 0 I 0 ω π ( [ r 2 L 2 L + r ln ( 2 L + r ) ] 0 L / 2 ) \Ep P 0 = μ 0 I 0 ω L 4 π cos θ ( 1 1 cos θ ln 25 16 ) { \Ep }_{ P0 }\quad =\int _{ r=0 }^{ r=L/2 }{ \frac { { \mu }_{ 0 }{ I }_{ 0 }\omega \quad r }{ 2\pi (L+r\cos { \theta } ) } (dr) } \\ \\ { \Ep }_{ P0 }\quad =\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ 2\pi } \int _{ r=0 }^{ r=L/2 }{ \frac { r }{ L+\cfrac { r }{ 2 } } (dr) } \quad \quad \quad \quad (\because \quad \cos { 60 } \quad =\quad 1/2\quad )\\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } \quad \int _{ r=0 }^{ r=L/2 }{ \frac { 2L\quad +\quad r\quad -\quad 2L }{ 2L+r } (dr) } \\ \\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } \quad \int _{ r=0 }^{ r=L/2 }{ (1\quad -\quad \frac { 2L }{ 2L+r } )(dr) } \\ \\ { \Ep }_{ P0 }=\quad \cfrac { { \mu }_{ 0 }{ I }_{ 0 }\omega }{ \pi } (\quad { [r\quad -\quad \frac { 2L }{ 2L+r } \ln { (2L+r) } ] }_{ 0 }^{ L/2 }\quad )\\ \\ { \Ep }_{ P0 }\quad =\frac { { \mu }_{ 0 }{ I }_{ 0 }\omega L }{ 4\pi \cos { \theta } } (1-\frac { 1 }{ \cos { \theta } } \ln { \frac { 25 }{ 16 } } ) .

Hopes this helps You ! @Tushar Gopalka

Deepanshu Gupta - 6 years, 7 months ago

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Bro how to integrate!! Hope this is correct dE=uiwxdx/π(2L+x) How to integrate this

Ace Pilot - 5 years, 9 months ago

General solutions is given by μ 0 i ω 2 π cos θ ( l a cos θ ( ln ( a + l cos θ a ) ) ) W h e r e : a = D i s t a n c e o f p i v o t f r o m w i r e l = l e n g t h o f r o d θ = a n g l e m a d e b y r o d w i t h h o r i z o n t a l ω = A n g u l a r f r e q u e n c y i = C i r r e n t i n w i r e \frac { { \mu }_{ 0 }i\omega }{ 2\pi \cos { \theta } } \left( l-\frac { a }{ \cos { \theta } } \left( \ln { \left( \frac { a+l\cos { \theta } }{ a } \right) } \right) \right) \\ \\ Where\quad :\\ a\quad =\quad Distance\quad of\quad pivot\quad from\quad wire\\ l\quad =\quad length\quad of\quad rod\\ \theta \quad =\quad angle\quad made\quad by\quad rod\quad with\quad horizontal\\ \omega \quad =\quad Angular\quad frequency\\ i\quad =\quad Cirrent\quad in\quad wire

I also wrote the exact same integral eqn but I also took the component of the element dx i.e. (dxsin(theta)) And I think we SHOULD take the element or else the integral will be wrong. By the way my answer was coming out to be 11.91( by taking the component of dx)

manansh kulshreshtha - 3 years, 10 months ago

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