# Homework Help: Flux Problem

1. Oct 4, 2014

### absolutezer0es

1. The problem statement, all variables and given/known data

A 1x1x1 cm box with edges aligned in the xyz-axes is in the electric field E = (350x + 150)i N/C, where x is in meters. What is the net electric flux through the box?

2. Relevant equations

flux = ExAx + EyAy + EzAz

3. The attempt at a solution

I substituted into the formula above, including 0.01m for x. I get:

flux = [(350*0.01 + 150)(0.01)^2] + 0 + 0 = 0.01535 Nm^2/C

I then multiply by two because the field is coming through the back and front of the box. I'm not getting the answer right though. What am I missing?

2. Oct 4, 2014

### Staff: Mentor

You cannot just multiply by 2. Calculate the flux through the front and back sides. (They aren't the same.) Then add to get the net flux, being careful with signs.

3. Oct 4, 2014

### absolutezer0es

Why aren't they the same? Isn't it a cube? Maybe I don't understand the expression (350x + 150)i like I think I do.

Does the expression mean 350x on the way in and 150 on the way out?

4. Oct 4, 2014

### Staff: Mentor

What's the field at x = 0? (Where one side is.) At x = 0.01? (Where the other side is.)

5. Oct 4, 2014

### absolutezer0es

I'm still lost. The field at x=0? There are an infinite number of points on x=0, depending on y and z, no? Same at x=0.01.

I mean, the answer is 3.5x10^-4, according to my text. I substituted 0.01m into the expression and multiplied by (0.01)^2. Don't have a clue why though. What happened to the 150?

6. Oct 4, 2014

### Staff: Mentor

The field only depends on x. So all points with the same x value have the same field.

7. Oct 4, 2014

### absolutezer0es

So what does the 150 mean? Does it have anything to do with the problem?

8. Oct 4, 2014

### Staff: Mentor

Of course it does! 350x + 150 is a function that gives you the magnitude of the electric field at any point. You can't leave out the 150.