# Magnetic field inside a box

1)A sensitive magnetic instrument can be shielded very effectively from outside fields by placing it inside a box of :
a)teak wood
b)plastic material
c)soft iron of high permeability
d)a metal of high of conductivity
The electric lines of force cannot penetrate a closed metal box. But I don’t know about magnetic lines of forces. Can someone clear my doubt? The answer would be either {c} or (d).

## Answers and Replies

Hootenanny
Staff Emeritus
Gold Member
What can you say about the magnetic flux in a substance with a high permeability when compared to air?

Hootenanny said:
What can you say about the magnetic flux in a substance with a high permeability when compared to air?

In a substance with high permeability the magnetic flux will be high. So do u mean that soft iron cannot be used as its permeability is high?

Hootenanny
Staff Emeritus
Gold Member
Amith2006 said:
In a substance with high permeability the magnetic flux will be high. So do u mean that soft iron cannot be used as its permeability is high?
Quite the reverse actually. The magnetic flux will be 'redirected' into the soft iron rather than the air and therefore the flux will be 'deflected' around whatever is inside the box. Does that make sense?

Hootenanny said:
Quite the reverse actually. The magnetic flux will be 'redirected' into the soft iron rather than the air and therefore the flux will be 'deflected' around whatever is inside the box. Does that make sense?
I didn't understand entirely what u said. With whatever I have understood, I have the drawn the lines of forces that would exist inside the soft iron box. Is it right?

#### Attachments

• lines of force.JPG
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berkeman
Mentor
Amith2006 said:
I didn't understand entirely what u said. With whatever I have understood, I have the drawn the lines of forces that would exist inside the soft iron box. Is it right?
Good first try, but you need to show a finite thickness to the box, and show how the external field lines get concentrated inside the walls of the box, thus bypassing the volume inside the box. Give it another shot and I'll approve the pending attachment as soon as I can.

Hootenanny
Staff Emeritus
Gold Member
Amith2006 said:
I didn't understand entirely what u said. With whatever I have understood, I have the drawn the lines of forces that would exist inside the soft iron box. Is it right?
Not entirely the magnetic flux should not extend inside the cavity in the box, just in the material of the soft iron core itself as in this image;

Taken from http://www.tpub.com/

Think about how the magnetic permittivity relates to the magnitude of flux through an object.

Edit: Guess berkeman beat me to it. Does mentorship come will free typing lessons? :tongue2:

Last edited:
I have understood now. Thanks.

berkeman
Mentor
Hootenanny said:
Edit: Guess berkeman beat me to it. Does mentorship come will free typing lessons? :tongue2:
I only beat you because you were taking the time to find a nice figure for the explanation. I looked briefly and gave up.