If the magnetic fields outside a copper box are suddenly changed, what happens to the magnetic field inside the box? Such low-resistivity metals are used to form enclosures which shield objects inside them against varying magnetic fields.

Question

Philoid · Accepted Answer

If the magnetic field is suddenly changing, it will induce eddy currents on the walls of the copper box. And because of these eddy currents there will be a magnetic field and that will be in the opposite direction. Copper is a good conductor of electricity, so a magnetic field due to eddy currents will have strong strength. This newly generated magnetic field will cancel the original magnetic field. So the magnetic field inside the box will become zero. In this way, the copper box will become a shield and protect the objects inside in it form varying magnetic fields.