Halliday, Resnick & Walker — Induction and Inductance: Problem 60

60 A circular region in an $xy$ plane is penetrated by a uniform magnetic field in the positive direction of the $z$ axis. The field’s magnitude $B$ (in teslas) increases with time $t$ (in seconds) according to $B = at$, where $a$ is a constant. The magnitude $E$ of the electric field set up by that increase in the magnetic field is given by Fig. 30-49 versus radial distance $r$; the vertical axis scale is set by $E_s = 600 \text{ } \mu\text{N/C}$, and the horizontal axis scale is set by $r_s = 4.00 \text{ cm}$. Find $a$.