Two Parallel Square Metal Plates That Are $1.5 \mathrm {~cm}$ Apart And

Two parallel square metal plates that are $1.5 \mathrm {~cm}$ apart and $22 \mathrm {~cm}$ on each side carry equal but opposite charges uniformly spread out over their facing surfaces. How many excess electrons are on the negative surface if the electric field between the plates has a magnitude of $18,000 \mathrm {~N} / \mathrm { C } ? \left( k = 1 / 4 \pi \varepsilon _ { 0 } = 9.0 \times 10 ^ { 9 } \mathrm {~N} \cdot \mathrm { m } ^ { 2 } / \mathrm { C } ^ { 2 } , e = 1.6 \times 10 ^ { - 19 } \mathrm { C } \right)$