How an inductor lose energy when pulling iron

Question

I'm confusing with a problem.
Consider an ideal inductor with a shorted circuit and no resistance and having a current through it. we know this current is constant while the inductor has no resistance because of its formula
The magnetic field of this inductor produces a force on a nearby iron block and pull it.
this action moves the iron closer to inductor and increase its inductance . right ?
so after that we have an inductor with same current and a higher inductance . so it have more stored energy ? also we have a kinetic energy in iron block.
where is my mistake ? the inductor must lose energy .

Puk · Accepted Answer

The energy stored in the magnetic field is
$$E=frac{1}{2}Li^2$$
where $L$ is the inductance and $i$ is the current. As the iron is pulled in, $L$ increases and part of this energy is converted to the kinetic energy of the iron. Both of these effects cause $i$ to be reduced. The total energy is still conserved.

How an inductor lose energy when pulling iron

One Answer

Add your own answers!

Ask a Question