0
$\begingroup$

I was recently given this question by a friend as part of her homework:

enter image description here

The gist of the question is: Say I have a laboratory induction coil with a primary coil which I can slide in and out of the secondary. The turns ratio on the primary and secondary is assumed to be constant.

So as the question states, when the coil is inserted all the way in, there is a spark produced. When it's only inserted say halfway in (not completely in), there is no spark produced.

However, high school physics tells us that V_1/V_2 is proportional to n_1/n_2, so the voltage ratio should be consistent and the spark should appear regardless of how far the secondary coil is in the primary. Why is it not?

$\endgroup$
  • $\begingroup$ Loss of magnetic flux produced by the primary which should be linked to the secondary. $\endgroup$ – Farcher Dec 20 '18 at 22:38
  • $\begingroup$ Yes the magnetic fields need to overlap strongly, also that's why an iron core is used, magnetic fields concentrate (or flow more freely) in iron, both items help to keep efficiency and energy transfer up. In theory you are correct about the turns ratio but the overlap is very poor compared to lots of windings. $\endgroup$ – PhysicsDave Dec 20 '18 at 22:59
1
$\begingroup$

Yes the magnetic fields need to overlap strongly, also that's why an iron core is used, magnetic fields concentrate (or flow more freely) in iron, both items help to keep efficiency and energy transfer up. In theory you are correct about the turns ratio but the overlap is very poor compared to lots of windings.

$\endgroup$
  • $\begingroup$ eek ok i realised that my original wording wasn't great - I meant comparing the coil halfway in vs all the way in. I know that the coil needs to be at least partially in haha $\endgroup$ – Thornkey Dec 21 '18 at 21:31
  • $\begingroup$ I would still guess its the overlap and efficiency that are making the difference. $\endgroup$ – PhysicsDave Dec 21 '18 at 23:02
  • $\begingroup$ alright :/ so no nice simple equation for it then? $\endgroup$ – Thornkey Dec 21 '18 at 23:55
1
$\begingroup$

the turns ratio formula assumes the two coils are coupled through a single magnetic core. If those coils are not coupled, the turns ratio formula gives a wrong answer. for example, if the two coils are on opposite sides of a room, there's no coupling (or nearly none) between them and what happens in one coil has no effect on the other.

$\endgroup$
  • $\begingroup$ eek ok i realised that my original wording wasn't great - I meant comparing the coil halfway in vs all the way in. I know that the coil needs to be at least partially in haha $\endgroup$ – Thornkey Dec 21 '18 at 21:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.