Charge is sprayed onto a large non-conducting belt above the left-hand roller in Figure P30.64. The belt carries the charge with a uniform surface charge density / as it moves with a speed v between the rollers as shown. The charge is removed by a wiper at the right-hand roller. Consider a point just above the surface of the moving belt.
(a) Find an expression for the magnitude of the magnetic field B at this point.
(b) If the belt is positively charged, what is the direction of B? (Note that the belt may be considered as an infinite sheet.)