We can further illustrate the effect of the motional electric field. When a conducting rod sees a magnetic field from a moving magnet (see figure 3), each electron in the rod experiences a force due to its relative motion through the field. If the direction of the motion of the magnet is such that a component of the force on the electrons is parallel to the conductor, the free electrons will move along the conductor. The electrons will move until they are balanced by equal and opposite electrostatic forces. This is because electrons collected at one end of the conductor, will leave a deficit of electrons at the other.


Figure 3. The moving source of a magnetic field produces an induced motional electric field Em, which is balanced by the electrostatic field, Es. The electric field is seen by an observer stationary with respect to the rod. An electrostatic shield around the rod does not influence the experiment.

While the motion continues, an observer inside the rod sees a zero electric field because of

where Es is the electrostatic field.

A remarkable observation is that this experiment can be done with or without electrostatic shielding around the conductor. It is worth noting that the Em field is quite different from the Es field in that the boundary condition for Em is equal to the boundary conditions for the magnetic field. (More on this later.)

In the equilibrium state, the observer in the reference frame of the moving rod will not feel any forces due to electric fields, either Es or Em. This conclusion has some profound effects on our experiments. For example, one cannot connect a voltmeter to the moving rod (that is stationary with respect to the rod) and expect to see a motional electric potential, Em. All wires of the voltmeter and the voltmeter itself will be equally polarized, in a manner similar to the rod. Understanding this concept is important, as it may be one of the fundamental reasons why the motional electric field often goes undetected.