sweet springs said:
Hi. Born2bwire.
Start up issue was cleared by your teaching. Thanks.
Energy of electric current is converted to kinetic energy of electrons and cores, i.e. the whole body, thus electric current dissipates. In order to keep the body accelerated under magnetic field, in case of wire we connect buttery to supply energy to maintain current.
In case of permanent magnet why we do not have to supply energy to maintain magnetic dipole currents to keep the magnet accelerated?
Maybe you already answered but I cannot find it. Thanks for further advice in advance.
On an atomic level, an atom can have its own magnetic dipole moment. This moment is created by the electrons orbiting the nucleus. There is also an additional contribution from the intrinsic moment of the electrons and protons themselves but we are ignoring that for the most part in our macroscopic analysis. In a very rough, and non-quantum way, think of the electron in a circular orbit about the proton. This causes a loop current which creates a magnetic dipole. The total magnetic dipole of an atom or molecule is going to be dependent upon the orbitals so not every atom exhibits a significant dipole moment. In addition, in most materials, the moments of the atoms are aligned randomly, thus giving rise to no net magnetic field. However, in permanent magnets, we find that in certain materials, large areas of the material will have most of their magnetic moments aligned. These areas of net alignment are called domains. If we apply a process to make all of these domains line up too, then we can create a strong magnet. One way to think of these microscopic atomic-level currents is as a net macroscopic loop bound current.
For example, let us think of a rectangle that has loop currents on its surface. If you look at the left-hand side of the attached picture, you will notice that on the interior, the currents of the top half of a loop will be canceled out by the currents in the bottom half of a loop above it. That is, the adjacent parts of neighboring current loops "cancel" each other out. What we are left then is a net current loop that runs along the outside edge of the rectangle. This can be shown to be true mathematically and is the conceptual idea of how all these atomic loop currents can add up to a large loop current. This large loop current can be treated as a magnetic field source and it can react to the Lorentz force from other magnets. This can be found under a discussion of magnetization in a textbook.
So that is how all these little loop currents can add up to a significant magnetic field. The main point is that we have to align a large number of them along the same direction to get them to work together. This can be done by applying a large magnetic field to our material (like how you can run a magnet along an iron nail and temporarily magnetize it). Heating and physical shock can undo this alignment, randomize the domains, and destroy the magnet. Since these currents are from the atomic orbitals, there is no need to keep supplying energy because the orbitals are stable themselves. On a very basic level, if we have an electron orbiting in a circle, we do not need to expend any energy to keep it orbiting, only a constant force. However, when we ourselves physically move the magnets around a magnetic field, we are inputting and taking out energy from these orbitals. That is, if I pull two magnets apart, I impart work that gets injected back into the magnetic fields. This would correspond to adding energy into the electron orbits. Likewise, if I release the magnets and they pull themselves together, they would release the energy I gave them. It is a conservative system, like gravity.
