Homework Help: PHYSICS

Posted by Tom on Monday, October 10, 2011 at 7:59pm.

In a particular region of Earth's atmosphere, the electric field above Earth's surface has been measured to be 151 N/C downward at an altitude of 280 m and 167 N/C downward at an altitude of 430 m. Calculate the volume charge density of the atmosphere, assuming it to be uniform between 280 and 430 m. (Hint: You may neglect the curvature of Earth. Why?)

I tried using E=pz/ε, where E was 151+167 and z was (430-280)/2, half the distance between the ends, and got, 3.75E-11 C/m^3, but that was wrong as the online software told me. What should I do?

PHYSICS - drwls, Monday, October 10, 2011 at 8:47pm
Treat the atmosphere as planar and use Gauss' Law. The net flux from an volume of cross sectional area A and thickness t is:

(167 - 151) *A = 46 Volt/C * A

That equals the charge inside the volume, divided by ε.

46 V/C*A = (charge density)*A*(thickness)/ε

charge density = ε * 46 /t

You should have subtracted the two field strengths, not added them, since E enters the volume on one side and leaves ion the other. The flux is in opposite directions.
PHYSICS - Granger, Sunday, February 12, 2012 at 12:00am
^^^^drwls

How did you get 167 - 151 = 46???

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