The emf across ends of a conductor of length l moving with velocity v in region of magnetic feild B , is vBl , when v, B and l are mutually perpendicular. But v is velocity with respect to which frame of reference? Different observers can observe rod moving with different velocity and deduce different emf being generated. But if we connect ends of rod to an ammeter and resistance, it will show some current but that will not match current calculated by different observers. So v is measured wrt what?

Dear Student ,
Why different observer will have different velocity of the rod ?Actually all the observer must have same velocity .Frame of reference can change but that does not mean that the velocity will also change .
Hence v is the velocity of the object with respect to the frame of reference from where the observer is observing the motion of the rod or any other object .


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