Surface Charge Density of shower

[popo902]

Homework Statement

When a shower is turned on in a closed bathroom, the splashing of the water on the bare tub can fill the room's air with negatively charged ions and produce an electric field in the air as great as 1000 N/C. Consider a bathroom with dimensions 2.3 m x 3.6 m x 1.6 m. Along the ceiling, floor, and four walls, approximate the electric field in the air as being directed perpendicular to the surface and as having a uniform magnitude of 647 N/C. Also, treat those surfaces as forming a closed Gaussian surface around the room's air. What are (a) the volume charge density ρ and (b) the number of excess elementary charges e per cubic meter in the room's air?

Homework Equations

E dot DA = q
q = \phi\epsilon

The Attempt at a Solution

for part a
I took the area of each face and multiplied it by the net E field forces
(-647 - 647)
and i got that right
apparently density can be negative?
but
mostly what i don't understand is how to go about solving part b
I have no idea what it's asking...

 

[collinsmark]

apparently density can be negative?

Yes, when the net charge (within the volume) is negative.

mostly what i don't understand is how to go about solving part b
I have no idea what it's asking...

elementary charge, e = 1.60217646 × 10^-19 Coulombs (use positive or negative as appropriate, and significant figures as appropriate for your coursework.)