Consider two parallel conducting frictionless rails in a gravity free rails parallel to x axis. A movable conductor PQ( y direction) of length $l$ slides on those rails. The rails are also connected by a fixed wire AB with a resistor of resistance $R$. Suppose a magnetic field exists in region which varies as $$B = cx$$The magnetic field is perpendicular to the plane of the system. Initially PQ is given some velocity $v_0$ in the x direction. Let the velocity at any instant be $v$ and the distance from AB be $x$
According to the flux approach, $$\Phi=cx^2l$$ $$\frac{d\Phi}{dt}=2cxlv$$ Force on conductor $= 2c^2x^2l^2v$
According to motional EMF approach $$\epsilon = cxvl$$ Force on conductor $= c^2x^2l^2v$
What have I done wrong?