Charge is sprayed onto a large non-conducting belt above the left-hand roller in Fig. The belt carries the charge, with a uniform surface charge density s, as it moves with a speed v between the rollers as shown in the figure. The charge is removed by a wiper at the right-hand roller. Consider a point just above the surface on the moving belt. Find an expression for the magnitude of magnetic field at this point.

Explanation

Equilivalent current in the belt=sv(current per unit width)

`:.` Linear current density of belt is `j=sv`
Using Ampere's law we can calculate magnetic field near the
belt as
`B=(mu_0j)/2=(mu_0sv)/2`