# Transformers and inverse square law

1. May 24, 2014

1. The problem statement, all variables and given/known data
This was in my test paper today:
A transformer is cut into half so that one half contains the primary coil and the other half contains the secondary coil. They are moved 30cm apart. Explain why the transformer would not work

3. The attempt at a solution
My answer: The magnetic field strength will be very weak so it would induce negligible emf in the coil.

Then my teacher gave me zero marks and said this is wrong.
His answer: The field lines will not reach the secondary coil. Therefore,not magnetic field lines are cut and emf is not induced.

I thought what the \$%^&?

I think what he said is wrong. The magnetic field lines will reach any distance.

2. May 24, 2014

### BvU

This is rather unpleasant and muddy. You and teacher have a different view on things and you are not teacher.

My impression is that teachers transformer in this question is pretty much ideal and the magnetic field outside the iron zero. So there. Cut it in 2 and any motor hanging from the secondary coil stops running. This is also true for a not-so ideal transformer. So for all practical purposes, chopping a transformer with an iron core in two and separating the parts by a foot stops it from working.

But, as you (and I second that) know, you can still easily detect the oscillating magnetic field with an oscilloscope etc. etc.

Sometimes the knowledge of being right is all the satisfaction you can reasonably expect out of such situations. Bear in mind that teacher also has to deal with less gifted students.

3. May 24, 2014

Thanks BvU!
This is the most frequent reason why I lose marks in the exam. I try really hard to gain as much knowledge as I can but it makes things worse sometimes.

4. May 24, 2014

### BvU

Part of being smart is to know when to play dumb. Sounds arrogant, but a lot of really very smart kids run into trouble early in life or in their schooldays by not taking that into account.
Key is to turn things around (aka outsmarting): why doesn't teacher acknowledge you're damn right? Is there a history to this? And the next step: how can you make him see you as an ally instead of a nuisance/threat ?

Oh well, PF is about physics, not about life. I know little about the former and hardly anything of the latter...

5. May 24, 2014

Why would he see me as a threat?
Oh really?

6. May 24, 2014

### dauto

I don't think either your answer or your teacher's answer is very good but I think the teacher's answer is better than yours. You said the magnetic field strength is weak but in fact it is not weak. It's just not going through the secondary coil.

7. May 25, 2014

I am very confused

8. May 25, 2014

### ehild

The B-field of the half coil will be be like that of a bar magnet. Some field lines reach the secondary coil,
but that field is very week to produce appreciable induced voltage. You were right but you should have emphasized that the field in the secondary coil would be very weak.

Now think of the working principle of the transformer. Primary and secondary coil share a common iron core. Almost the whole flux produced in the primary coil appears in the secondary one. The loss is small, the output power is close to the input. That can not be said about the halved coil. It is not a transformer any more.

Last edited: May 25, 2014
9. May 25, 2014

Thanks. I will be more careful from now on.

10. May 25, 2014

### Staff: Mentor

Why do you say the magnetic field strength will be very weak?

11. May 25, 2014

If you put it 30cm away and the transformer is not working, that means the magnetic field is weak. "Field lines will not pass through the secondary coil" does not make much sense to me because magnetic field follows the inverse square law which states that the intensity of the magnetic field will decrease with the square of the distance. That means as x approaches infinity, the magnetic field strength will approach zero. But it's not possible to reach infinity, right?

12. May 25, 2014

### Staff: Mentor

It would be better to say far fewer field lines will now pass through the secondary coil, rather than none.

I asked because I thought the transformer was under load. But on re-reading, I see it is not. (Had it been powering a load, then breaking the core would remove most of that load and the current in the primary of the transformer would accordingly fall to near zero as it became practically an inductor.)