How to Determine Electric Field Strength in a Thunderstorm?

[Ryo124]

A theoretical model of a thunderstorm cloud has a localized positive charge P = 40.0 C at 10 km altitude, a localized negative charge N = -40.0 C at 5 km altitude, and a localized positive charge p = 5.0 C at 2 km altitude.

The three charges are assumed to have equal horizontal coordinates. Determine the electric field strength at the same horizontal coordinates and height 6.8 km.


I know that I am supposed to calculate the individual forces but I really need someone to "walk" me through this problem step-by-step please.

 

[Mindscrape]

I can't walk you through because that will give you the answer, but I will give you the steps. You should use the eqn E = kq/r^2

1) You want the E field at 6.8km, so put your origin there
2) What the Efield contribution from the 10km charge? How will you find the distance between from the origin at 6.8km (or 0 in your new system) and the one at 10km (or what in your new system)? Don't think too hard, it's merely those translation guys that you cover in high school. What direction is it in?
3) From the 5km charge? Direction?
4) How about the 2km one?
5) What does superposition say you can do?
6) Did you get the answer? Does it make sense?

 

[Ryo124]

Having trouble with the directions of the E-fields. Is the hypothetical "point charge" a positive or negative value?

So, you calculate the separate Efields acting on 6.8km and add directions? I just am having trouble with the directions.

 

[Mindscrape]

The point charge is positive. So the 10km should give you a direction down, or -j if you use vector notation. What do you have for that one?

 

[Ryo124]

Got it. Thanks.