Induced EMF in a rotating ring

[gochaka]

[Thread moved from the technical forums to the schoolwork forums]

Let's suppose we have a ring with radias R and radius of the cross section r. We start rotating with an angular velosity w. We will there be an induced EMF in the ring and how can i find it's value?

 

[Baluncore]

Welcome to PF.

Let's suppose we have a ring with radias R and radius of the cross section r. We start rotating with an angular velosity w. We will there be an induced EMF in the ring and how can i find it's value?

What material is the ring made from?
About what axis does the ring rotate?
Is there a magnetic field?
What orientation field?

 

[osilmag]

Hello gochaka. To answer your question straight up, that would be a no. Questions about this concept are posted repeatedly on PF, so you are not alone. To get an induced voltage, you must have a changing magnetic flux through the cross- sectional area.

 

[berkeman]

Let's suppose we have a ring with radias R and radius of the cross section r. We start rotating with an angular velosity w. We will there be an induced EMF in the ring and how can i find it's value?

Whelp, if there is a magnet nearby, what do you think? What are the Relevant Equations for this type of calculation? Is this question for schoolwork?

 

[gochaka]

There isn't an external magnetic field. The material of which there ring is made of the doesn't matter. You only know it's resistance . Also there surely is an induced EMF since this question was used in a very prestigious competition four years ago.

 

[Vanadium 50]

Maybe you should post the question verbatim.

 

[osilmag]

There isn't an external magnetic field. The material of which there ring is made of the doesn't matter. You only know it's resistance . Also there surely is an induced EMF since this question was used in a very prestigious competition four years ago.

That's hard to believe that there isn't an external field. You can't simply spin something and there magically be a voltage.

 

[gochaka]

Problem 5. A copper ring of cross-section S = 1 mm2 and radius r = 5 cm starts rotating
around its axis with a constant angular acceleration α = 1000 rad/s2
. Find the magnetic field
B at the centre of the ring. The resistivity of copper is ρ = 1.68 × 10−8 Ωm.
Hint: You may want to work in the reference frame of the rotating ring. What is the force that
gives rise to an EMF in the ring?

That's the original question used in the olympiad.

 

[Vanadium 50]

First, Olympiad problems are not B-level.
Second, you omitted a lot of relevant information in the OP.
Third, you miswrote the problem - you wrote angular velocity when it fact there is an angular acceleration.

So, how far did you get?

 

[gochaka]

I don't even know where to start from

 

[berkeman]

Problem 5. A copper ring of cross-section S = 1 mm2 and radius r = 5 cm starts rotating
around its axis with a constant angular acceleration α = 1000 rad/s2
. Find the magnetic field
B at the centre of the ring. The resistivity of copper is ρ = 1.68 × 10−8 Ωm.
Hint: You may want to work in the reference frame of the rotating ring. What is the force that
gives rise to an EMF in the ring?

That's the original question used in the olympiad.

You are leaving out the part about there being a B-field in the region of the ring. As mentioned already, without a changing magnetic flux in the ring, there will be no EMF induced.

And since this is a schoolwork-type question, I'll move the thread to the schoolwork forums.

 

[berkeman]

I don't even know where to start from

That is not acceptable in a schoolwork-type thread. You have to show your efforts before we can offer tutorial help.

** Where is the B-field coming from? There has to be one somewhere in the problem statement or diagram.

** What are the relevant equations for induced EMF for a changing flux through a conducting ring?

 

[Vanadium 50]

I think the B-field is internally generated. There is something in the actual question (but not the OP) that hints at it. But I don't want to mention it until the OP puts in at least a smidgen of work.

 

[Gavran]

I don't even know where to start from

(https://en.wikipedia.org/wiki/Stewart–Tolman_effect)

 

[berkeman]

Zero-effort poster @gochaka has not been back to this thread for several days, so I will tie it off. @gochaka -- if you do return and are willing to show some effort on this problem, please send me a Personal Message (PM) by clicking twice on my avatar and "Start a Conversation". If you are willing to show some effort I can reopen this thread.

Thanks all for trying to help the OP.