Transformers and inverse square law

Gold Member

Homework Statement

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

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. How is my answer wrong? Answers and Replies Related Introductory Physics Homework Help News on Phys.org BvU Science Advisor Homework Helper 2019 Award 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. 1 person Gold Member 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. BvU Science Advisor Homework Helper 2019 Award 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... Gold Member how can you make him see you as an ally instead of a nuisance/threat ? Why would he see me as a threat? I know little about the former Oh really? 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. Gold Member What? Then what would be your answer? I am very confused ehild Homework Helper Homework Statement 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 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.
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:
1 person
Gold Member
Thanks. I will be more careful from now on.

NascentOxygen
Staff Emeritus
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

The Attempt at a Solution

My answer: The magnetic field strength will be very weak so it would induce negligible emf in the coil.
Why do you say the magnetic field strength will be very weak?

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

NascentOxygen
Staff Emeritus