Polarity of the Rod

There is a very important topic in EMI called Motional emf. There is a specific use case where the Rod moves over between the Rails. We know that when this Rod moves in the Perpendicular Magnetic Field there is an induced current. Following sequence needs to be understood as the Basic Concept . It explains the Polarity of the Rod :-

1. The Rod ( which is different from Rails) moves in a magnetic field with velocity v.

2. The rod contains free electrons.

3. The Free electrons in the rod are also moving with the velocity of the rod v. 

4. Since the electrons are moving with Constant Velocity v (if the velocity is not Constant then it will be a decelarted motion -NEET AND JEE Students to Note) in the magnetic field B they experience a force F. The direction of this force (Bqv) makes the electrons move in a magnetic field. We can find the direction of this force using Fleming Right Hand Rule ( Go back to the chapter -Magnetic effects of current and read Fleming Left Hand Rule). 

5. Assume that the Electrons experience the force upwards. If this is the case the ELectrons will strat moving upwards thereby leaving a deficit of electrons at the other end. So the Upper side of Rod is becoming Positive and Lower side of the Rod is becoming positive because of the Deficit.

6. The above situation will mean that an electric field will start appearing in the rod from Positiive end to negative end.

7. This implies that the Rod's  Electric field will oppose the motion of electrons because the upper part of the Rod has become Negative. 

8. Thus the force eE (electric field) and Bqv are now anti parallel forces. The motion in external magnetic movement prouduced electric field in Rod and now that Electric field opposes the Motion itself. Note this point carefully.

9. At some point the eE and Bqv become equal and further flow of electrons stop.

10.Now we have the electric field in the rod ( lower part is positive and upper part is negative).

11. The existence of electric field implies we would have Potential Difference between Positive edge of the rod and the Negative edge of the Rod. And the Rod's length is also. ( Remember E=dV/dr , Potential Gradient)

12. So now we have Potential Gradient i.e the Polarised Rod has become the source of EMF ( Potential difference).

One needs to understand this concept indepth. Once this is clear you can attempt the numericals of motional emf.