3. Consider an infinite parallel-plate capacitor (air-filled, ε 0 ) with a lower plate (at z=−d/2 ) carrying surface charge density −σ, and the upper plate (at z=+d/2 ) carrying surface charge density +σ. a) Determine all nine components of the stress tensor, in the region between the plates. b) Using the stress tensor, determine the electromagnetic force (per unit area) on the top plate.