Why must part of the circuit be moving relative to other parts, to have usable motional emf? Consider, for example, that the rails in Figure 23.11 are stationary relative to the magnetic field, while the rod moves.

The magnetic field is the area around a magnetic material or a moving electric charge where the force of magnetism acts.

The induced electromotive force is caused by a change in flux, which is as follows:

\[\varepsilon = \frac{{\Delta \Phi }}{{\Delta t}}\]

The flux is calculated by multiplying the area by the magnetic field strength and the cosine of the angle formed by the field and perpendicular to the area. In other words,

\[\Phi = BA\cos \theta \]

As we can see, it is the change in the area that causes the flux to change, as the position and magnetic field cannot change. There must be relative motion between the system's components, the rod, and the rails, for the area to change.

There is relative motion when the rod moves while the rails remain stationary, which means that the area of the loop changes. As a result, e.m.f. will be generated.