Calculating Electrostatic Force on a Charged Particle Near a Flat Surface

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Homework Help Overview

The problem involves calculating the electrostatic force on a charged particle located at different distances from a charged flat surface. The original poster presents a scenario where a particle of charge +q experiences a force F when at a distance r from the surface and seeks to determine the force at a distance 2r.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the applicability of Coulomb's law and Gauss's law in determining the electric field and force. There are questions regarding the assumptions about the charged surface, such as whether it is infinite and the nature of the charge distribution.

Discussion Status

Some participants have offered guidance on using Gauss's law to find the electric field and have raised questions about the assumptions made in the problem. There is an acknowledgment of ambiguity in the problem statement, particularly regarding the size and charge distribution of the surface.

Contextual Notes

Participants note the lack of information about the charge on the surface and the dimensions of the charged plate, which may affect the calculations. There is also mention of the need for clarity on whether the plate is infinite or finite in size.

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Homework Statement


A particle of charge +q is a distance r away from a charged flat surface and experiences a force of magnitude F pulling it toward the surface. What is the magnitude of the force exerted on a particle of charge +q that is a distance 2r from the surface?


Homework Equations



F=KQ1Q2/r^2
E=F/q
E=Kq/r^2

The Attempt at a Solution



The coulomb's formula seems to fit in i.e F=KQ1Q2/r^2
but I stopped solving it further since Q2 or the value of any other charge isn't mentioned in a question. Anyone please help me do this question?

Thanks.
 
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Well the question seems rather ambiguous as it doesn't say whether the plate is infinite in area, and if it isn't, just how large is it compared to the distance r. If you assume it's the former, and also assume the charge distribution on it is homogenous, you should start by finding the electric field due to the plate at an arbitrary distance from it using Gauss's law, and then find the force using the equation \vec{F}=q\vec{E} (you can also find the field using Coulomb's law, but the procedure is a bit more complicated).
 
You mean by applying the formula ; E= charge density/permittivity?
 
The answer should be F/4, isn't it?
 
huzzi.123 said:
You mean by applying the formula ; E= charge density/permittivity?

I agree with kontejnjer: If you assume an infinite plate with uniform charge distribution, then you can find the E-field it produces using Gauss' Law. It is not quite the result you stated above.
 
That's what I got after working out my intuition but unfortunately this isn't a right answer.
 

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