Past exam of the physics 2 paper of the University of Oxford / 2017 / 3 / Solution

Use a cross-section normal to the sheets' line of intersection: the positive sheet is $x=0$, and the negative sheet is $y=0$. A Gaussian pillbox gives a single infinite sheet's electric field magnitude $\sigma /(2{ϵ}_0)$ on either side. Superposition consequently gives, away from the sheets,The electric field points away from the positively charged sheet and towards the negatively charged sheet, bisecting each right angle. The electric field lines are straight within each quadrant: in the first quadrant, for example, $dy/dx=-1$, so $x+y$ is constant along a line. They start on $x=0$ and finish on $y=0$. At a sheet, the normal component has the usual surface-charge jump, so the quadrant formula specifies the appropriate one-sided limits rather than a unique electric field on the charged surface.