Inferring charge via electric field properties

[phil ess]

Homework Statement

You are 1.5m from a charge distribution whose size is much less than 1m. You measure an electric field strength of 282 N/C. You move to a distance of 2m, and the field strength becomes 119 N/C. What is the net charge of the distribution? HINT: Don't try to calculate the charge. Determine instead how the field decreases with distance, and from that infer the charge.

Homework Equations

E = kq/r²

The Attempt at a Solution

Well the electric field strength doesn't fall off as 1/r², because if we calculate q for 1.5m using simple substitution, then use that q at 2m, I don't get the electric field given.

How do I figure out the field? If I am thinking graphically, I have two points (1.5,282) (2,119) and I have to find a curve that crosses those two points, a curve that describes how the electric field falls off.

I think the problem I am having here is that I am rusty on the math. I have a feeling that it would help to differentiate E = kq/r² to perhaps be able to describe how the field is changing:

E' = -2kq/r³ But I am not sure how to use the given points to help...

E₂/E₁ = 0.422
r₂/r₁ = 4/3

Any hints would be appreciated!

 

[kuruman]

Read on. Classmates of yours?

https://www.physicsforums.com/showthread.php?t=342853
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