Physics Olympiad quick help please - Is magnetic flux conserved?

I am just doing a physics olympiad paper (paper 1 - 2007) and have come a bit of a cropper with this part of a question:

[talking about a neutron star] as the core of the star collapses to form the neutron star, the elctrical conductivity becomes very high. In this case, the star's magnetic field lines become frozen into the material of the star and collapse down with the star, increasing the flux density. The neutron star will thus have a very strong magnetic field. If we take the flux

\Phi = BR^2

(r is the radius which has decreased by a factor of 5), with B being the magnetic field strength whose initial value is

10^{-2} T

, then determine the final magnetic field strength after the collapse.

I would understand how to do it, if the magnetic flux was conserved, and it was only the density that increases as the volume of the star decreases. Is that right?

thanks for your help :biggrin:

Reply 1

motoroller

Flux is ALWAYS conserved, from what I remember (although sometimes it's field lines that are conserved instead). I need to revise!

Reply 2

Mehh

motoroller

Flux is ALWAYS conserved, from what I remember (although sometimes it's field lines that are conserved instead). I need to revise!

Nope. Flux is not always conserved.
But there are no magnetic monopoles.

The question states that the field lines are frozen (ie the flux is constant)...