A coil of wire is placed along the axis of a solenoid?

[UnD3R0aTh]

http://img42.imageshack.us/img42/6336/14vu.png please explain your answer! also if u know of similar problems or where this problem is taken from do tell me coz i have an admission exam coming up soon

please explain the following:

1. what is the flux like?

3. how the coil spins?

z. how is the rotation about x or y differ from z?

 

[sophiecentaur]

What do you know about the laws of induction? What do you get when you apply them to this question?
(BTW - did you see the notice about No Homework of Course Questions to be posted here? There is a whole forum dedicated to this sort of thing)

 

[UnD3R0aTh]

that's not homework! that's a question from a sample test of an admission exam that i will take soon! sorry I'm new here I'm not familiar with the forum rules, did i post in the wrong forum?

my trial at the solution, this looks similar to the dynamo, when the "thing that rotates, sorry can't find the word here English is not my first language" is perpendicular to the flux, as it is when it's rotating around the z axis, emf will be zero because the nagle between the perpendicular on the "thing that rotates" and the flux is zero, and emf = NBAW sin theta, sin 0 = zero emf = zero, is that it? btw i already know the answer but this is my explanation that fits the answer!

 

[sophiecentaur]

The question to ask yourself is "does the magnetic flux change when you make the movement or change in each part of the question".

 

[jtbell]

that's not homework!

Even though it is not literally "homework", it is a "homework-like" problem, that is, you are trying to find the solution to a specific exercise. Such exercises belong in one of our "Homework and Coursework Questions" forums. I've moved this one for you. The next time you post a similar question, take note of the rules that are posted at the top of this forum (the one I've moved you to).

 

[UnD3R0aTh]

The question to ask yourself is "does the magnetic flux change when you make the movement or change in each part of the question".

exactly i asked myself that, two problems though i don't know how high the coil above the solenoid, so I'm not sure what the flux direction will be, assume it's downwards to upwards, problem two is that I'm not really sure if that movement will cause change in flux, I'm not even sure how it's rotated, does it rotate like a dynamo, or does it "spin" around the z axis in a circular fashion, these kinds of questions are unclear but if they gave enough information, i will solve it with ease coz i understand induction, so please break it down for me, thx

 

[voko]

Don't think about dynamos, they are more complex than this problem, so it makes no sense whatsoever to talk about them when you just need to solve this problem.

You need to understand what magnetic field the solenoid generates to solve this problem. Do you understand what it looks like?

 

[UnD3R0aTh]

yes i have an idea of what it looks like, it's complicated! btw guyz just exp[lain the solution and save me all this suspense xD

 

[voko]

"just explain the solution" is not how this forum works. We can help you solve a problem, but we need that you make an effort on your own.

Is there any symmetry in the field of the solenoid? How can you move the solenoid, using that symmetry, so that its field does not change?

 

[UnD3R0aTh]

i don't know, please explain :( believe me i spent days thinking about this i can't imagine it, i need clear description!

 

[voko]

Look here. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

 

[UnD3R0aTh]

ok so the flux is directed from downwards to upward, the coil is spinning along the Z axis which is in the same direction as the flux, what does this mean then? is there a change in flux?

 

[voko]

What would change in the magnetic field if the solenoid were rotated about the z axis?

 

[UnD3R0aTh]

i'm sorry but this is no way to help me, i have an exam coming I'm studying this is taking too much time, if i know that then i know the answer, is this how u really handle questions here or are u lazy to type? plus the coil rotates not the solenoid! so far u haven't explained what the flux look like, how the coil spins or rotates, i repeat the problem is not my understanding of induction, the problem is to understand the problem, please somebody help me! I'm low on time!

 

[voko]

You cannot prepare for your exam by being given answers to sample question. That will get your nowhere. You can only get prepared if you study the subject and learn how you can reason and apply your knowledge. What we do here is help you with reasoning.

You say that you understand induction. That is fine. Then you should have no problem answering whether rotating the coil (or the solenoid, which is the same thing) about the z-axis would change the magnetic field. Do you understand that the magnetic field is radially symmetric with respect to the axis of the solenoid?

 

[UnD3R0aTh]

radially?! u do understand that i don't speak English for a first language don't you? unless u going to give me explanation in simple english or rephrase the problem for me in a simple way to make it easy please don't bother, and please don't feel offended but i have no time for this

 

[UnD3R0aTh]

i apologize about yesterday, i was studying and i kept getting the notifications which interrupted me and it seemed like u didn't want to help me! let's give your method a try, but so far u haven't commented on my attempts at the solutions! so again the flux points upwards, both rotation about x, y and z will be perpendicular to the flux, your comment?!

btw if u explain to me what i need to know, i will be able to solve any problem like this, it's not just the solution to this particular problem, which is useless really in itself! but the skills that i will learn from solving it is what matters!

 

[sophiecentaur]

i apologize about yesterday, i was studying and i kept getting the notifications which interrupted me and it seemed like u didn't want to help me! let's give your method a try, but so far u haven't commented on my attempts at the solutions! so again the flux points upwards, both rotation about x, y and z will be perpendicular to the flux, your comment?!

btw if u explain to me what i need to know, i will be able to solve any problem like this, it's not just the solution to this particular problem, which is useless really in itself! but the skills that i will learn from solving it is what matters!

I do not understand what's going on here. You have been given a question yet you, apparently, have not been taught the basics of the topic. The field around a solenoid is very well known. The laws of induction are usually taught early on. Anything that changes the flux (or, as it is often stated, 'when lines of force are cut') will induce an emf. Those situations in the questions are not very difficult to interpret. Were you given any examples on your course of where induction can take place?
If I were you, I would complain, if not.

 

[voko]

so far u haven't commented on my attempts at the solutions! so again the flux points upwards, both rotation about x, y and z will be perpendicular to the flux, your comment?!

My comment is that rotations about x and y will indeed be perpendicular to the flux. But the rotation about z is not. So will it change the magnetic field sensed by the coil?

 

[sophiecentaur]

Rotating the coil about the axis of the solenoid doesn't change the area enclosing the magnetic flux so you would not expect an induced emf. The other cases seem to imply either the area changes or the flux changes - hence an induced emf.

 

[UnD3R0aTh]

thx for all your replies guyz, i think i have a problem interpreting the problem, is the problem saying that the coil rotates like a tire around an axis? or rotates like the coil of a dynamo or a motor?

p.s. i have a skype we could discuss this problem there it will be faster than this!

 

[sophiecentaur]

thx for all your replies guyz, i think i have a problem interpreting the problem, is the problem saying that the coil rotates like a tire around an axis? or rotates like the coil of a dynamo or a motor?

p.s. i have a skype we could discuss this problem there it will be faster than this!

If you replace the word 'Axis' with spindle or axle, then the Axis of a circle is the same as the axle of a wheel - the normal to the plane of the circle that passes through its centre.

 

[TSny]

... is the problem saying that the coil rotates like a tire around an axis? or rotates like the coil of a dynamo or a motor?

Rotation about the z-axis is like a tire rotating on its axle. Rotation about the x or y-axis flips the coil around a diameter of the coil (like a dynamo).

 

[UnD3R0aTh]

Rotation about the z-axis is like a tire rotating on its axle. Rotation about the x or y-axis flips the coil around a diameter of the coil (like a dynamo).

if that is true then my answer here should be correct :

this looks similar to the dynamo, when the coil is perpendicular to the flux, as it is when it's rotating around the z axis, emf will be zero because the angle between the perpendicular on the coil and the flux is zero, meaning they are in the same direction, and emf = NBAW sin theta, sin zero = zero, emf = zero

 

[TSny]

You are correct that at the instant you start rotating about the x or y axis, the emf will be zero because θ = 0 at that instant. But I believe that the question is asking about not just the instant when you start rotating the coil, but also during a period of time after the rotation has begun. In other words, if the coil is rotating about the y-axis, will there ever be any induced emf in the coil?

So, part of the difficulty that you are having with the question is just an interpretation problem.

 

[UnD3R0aTh]

You are correct that at the instant you start rotating about the x or y axis, the emf will be zero because θ = 0 at that instant. But I believe that the question is asking about not just the instant when you start rotating the coil, but also during a period of time after the rotation has begun. In other words, if the coil is rotating about the y-axis, will there ever be any induced emf in the coil?

So, part of the difficulty that you are having with the question is just an interpretation problem.

wrong, at the instant when it starts rotating about x and y, the coil will be parallel to the lines of flux and the "perpendicular on the coil" will be perpendicular to the flux thus 0 = 90 and there is emf max

 

[TSny]

I disagree. When I look at the diagram, I see the coil initially lying in the xy plane.

What is the direction of the magnetic field at the location of the coil?

 

[UnD3R0aTh]

Rotating the coil about the axis of the solenoid doesn't change the area enclosing the magnetic flux so you would not expect an induced emf. The other cases seem to imply either the area changes or the flux changes - hence an induced emf.

i think this is wrong because the movement seems to be the same kind of movement in all three cases, please guyz that's enough if anyone knows the direct explanation this is the time to spill the beans

 

[UnD3R0aTh]

guyz come on why are u not helping me? why are u not explaining anything? please i have an importatnt exam coming up, i learned induction on my own, if u think I'm missing something please point it out to me! this is urgent!

key question: how does flux changes?