# Is there an effect of magnetic field on stationary charge

Why does a magnetic field only exert a force on moving charges and not on stationary charges as electric field does?

Why questions are not simple to answer. Physics is not an explanation of why something happens, but merely a description of natural phenomena. Searching for reasons in physical laws is (in my opinion) not very fruitful. Thus, I won't answer your why-question.

In fact your statement is not really true. A magnetic field does interact with "charges" -- at least it interacts with an isolated electron and an isolated proton. I believe a simple picture is as follows:

• A magnetic field $$\vec B$$ "interacts" with magnetic moments $$\vec \mu$$. This can be seen by the magnetic energy $$E = -\vec \mu \cdot \vec B$$.
• Now, if the magnetic field is position dependent, the energy of the magnetic moment changes, if it moves though space. Since the change of energy is a force, $$F = \frac{dE}{ds}$$, the moving magnetic moment experiences a force in this scenario. However, even if the magnetic moment is at rest and we are simply changing the $$B$$-field, there is an "interaction" between $$B$$ and $$\mu$$, because the magnetic energy changes.
• Now to charges. An electron as well as a proton possesses a magnetic moment $$\vec \mu$$. Since the energy level of these charges changes with the magnetic field strength, there exists an effect between charge an B-field. Nevertheless, if the B-field is homogeneous and the charge is not moving it does not experience a force.

There are a few things to clarify:

1. the magnetic field (B) is defined as the Lorenz force acting on moving charges

2. there are basically permanent magnets and magnets induced by a moving electric current

Now the magnetic field is defined as acting on moving charges, though, when you have a permanent magnet, and a stationary (relative to the magnet) piece of metal, the magnet will have an effect on the electrons inside the metal. The effect will be aligning the spins (magnetic dipole moment) of the electrons inside the metal.