I have encountered the problem:
Find the magnetic energy stored inside a 1m length of wire carrying
10 amp. The wire is 1 mm in radius, and the current density is uniform.
I have found a solution where the energy density is used, or:
$$ \frac{U}{\textrm{ vol}}=\frac{B^2}{2μ_0} $$
Now I have two questions:
1) The energy density is derived for the case of magnetic field of an inductor (solenoid), but the single wire is not a solenoid and how could the same formula for energy density be used?;
2) I think it could be that the problem is about the wire of the solenoid, I mean is it possible to be implied that the wire in question is wound in the form of solenoid but it is not written explicitly?
After all, I solved the problem without knowing what exactly is meant. I applied the energy density expression derived for a solenoid (as given above) and got wrong answer. In matter of fact the final formula I obtained, and also find it in another solution in the web, is this: $U=\frac{μ_0i^2}{16π}=2.5\times 10^{-6}$ joules. The correct answer in the book is claimed to be $U=3.3(3)\times 10^{-3}$ joules!