A large parallel plate capacitor with uniform surface charge '$\sigma$' on the upper and '$-\sigma$' on the lower is moving with constant speed v as shown in figure. Then

 

Magnetic field between the plates is $B={\mu }_0\sigma vandB=0$ else where 
 

Magnetic field between the plates is $B=2{\mu }_0\sigma vandB=0$  else where 
 

Magnetic force per unit area on upper plate is $f_m=\frac{{\mu }_0{\sigma }^2{v}^2}{2}$   
 

Magnetic force per unit area on upper plate is  $f_m=\frac{{\mu }_0{\sigma }^2{v}^2}{4}$