$*$ P$2$A A D$-$shaped frame is made out of plastic of small square cross section and tightly wrapped uniformly with $N$ turns of wire as shown in Figure $21\mathrm{.}75,$ so that the magnetic field has essentially the same magnitude throughout the plastic. $(R$ is much bigger than $w.)$

With a current I flowing, what is the magnetic field inside the plastic? Show the direction of the magnetic field in the plastic at several locations.

Figure $21\mathrm{.}75$