A very large flat metal slab is $4.0\ \text{cm}$ thick. A steady current flows through it, parallel to its faces, with a uniform density of $5.0 \times 10^{4}\ \text{A/m}^2$, as in the figure. The slab is so wide and long that its edges are far away, and the field near its middle is parallel to its faces and at right angles to the current. The metal is non-magnetic. Distances are measured from the middle plane of the slab, halfway between its faces. Take $\mu_0 = 4\pi \times 10^{-7}\ \text{T m/A}$. Which of the following statements are correct?

Fig. 1: Edge view of a very large flat slab 4.0 cm thick. Crosses all over it show a current going into the page with a uniform density of 5.0 times 10 to the 4 amperes per square metre. A dashed line marks the middle plane, halfway between the faces.