Question

) each plate of a parallel-plate air-filled capacitor has an area of 0.0020 , and the separation of the plates is an electric field of is present between the plates. what is the surface charge density on the plates? (ε0 = 8.85 × 10-12 c2/n • m2)

Answer 1

I attached the full question.
We know that for a parallel-plate capacitor the surface charge density is given by the following formula:
$\sigma=\varepsilon_0 \frac{V}{d}$
Where V is the voltage between the plates and d is separation.
Voltage is by definition:
$V=Ed$
Voltage is analog to the mechanical work done by the force.
Above formula is correct only If the field is constant, and we can assume that it is since no function has been given.
The charge density would then be:
$\sigma=\varepsilon_0 \frac{Ed}{d}=\varepsilon_0E\\ \sigma= 8.85\cdot10^{-12}\cdot 2.1\cdot 10^6= 0.0000185\frac{c}{m^2}$
Please note that elecric permittivity of air is very close to  elecric permittivity of vacum, it is common to use them interchangeably.