The force when the distance is reduced

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Homework Help Overview

The discussion revolves around the forces between two charges and how the force changes when the distance between them is altered. The original poster presents a scenario where two charges attract each other with a force of 1.5 N and questions how this force will change if the distance is reduced to one-ninth of its original value, referencing the equation for electrostatic force.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between force and distance, questioning how to set up the problem correctly. Some suggest using ratios to compare forces at different distances, while others propose considering how force changes with doubling or halving distance.

Discussion Status

The discussion is active, with participants offering various perspectives on the problem setup and the mathematical relationships involved. Some guidance has been provided regarding the nature of the force's dependence on distance, although there is no explicit consensus on the correct approach yet.

Contextual Notes

Participants are grappling with the implications of the inverse square law in the context of electrostatic forces, and there are references to similar problems that may provide additional context. There is also mention of confusion regarding the mathematical representation of distance in the force equation.

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Homework Statement



Two charges attract each other with a force of 1.5 N. What will be the force if the distance between them is reduced to one-ninth of its original value?

F=1.5 N

Homework Equations



F=k|q1||q2|/r^2

The Attempt at a Solution



1.5 N = k |q1||q2|/(1/9)r^2

I don't think I am setting this up right, but would I multiply 1.5N by 1/9 and by the constant k?

Thanks in advance!
 
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aChordate said:
I don't think I am setting this up right, but would I multiply 1.5N by 1/9 and by the constant k?
Set it up so that 1.5 N is the force when the distance = R.

Then see what the force is when the distance = R/9.

(Set up a ratio.)
 
Another way to approach this is to just ask yourself what happens to the force if the distance doubles? If the distance is cut in half? That should point you in the right direction.
 
Try answering the questions I asked in post #3.
 
Doc Al said:
Another way to approach this is to just ask yourself what happens to the force if the distance doubles? If the distance is cut in half? That should point you in the right direction.

If the distance doubles, the force increases x4 (exponentially?). If the distance is halved, the force decreases.
 
aChordate said:
If the distance doubles, the force increases x4 (exponentially?).
Careful. The force is inversely proportional to the distance squared. So if the distance doubles, the force decreases by a factor of 4. (Multiplied by (1/2)2 = 1/4.)

If the distance is halved, the force decreases.
No, the force would increase by a factor of 4.

F1 = (const)*(1/R)2

If you reduce the distance by 2, replace R with R/2:

F2 = (const)*(1/(R/2))2 = 4*(const)*(1/R)2 = 4*F1

(Now do the same thing for the distance being reduced by 1/9.)
 
F2 = (const)*(1/(R/9))2 = 81*(const)*(1/R)2 = 81*1.5 N = 121.5 N

Wow, I really need to work on more of these.

Thanks so much for your help!
 
Hi Doc Al,

I am trying to understand the answer that you posted above.

F2 = (const)*(1/(R/2))2 = 4*(const)*(1/R)2 = 4*F1

Could you explain why the squared sign is outside the last bracket. Should it not be inside the bracket as it is the distance squared.

F2 = (const)*(1/(R/2)2)

Also, how did you simplify it to be 4*(const), how did you calculate the 4?

Many thanks,
Cathal
 
  • #10
cathal84 said:
Could you explain why the squared sign is outside the last bracket. Should it not be inside the bracket as it is the distance squared.
Realize that ##(\frac{1}{R})^2 = \frac{1}{R^2}##

cathal84 said:
Also, how did you simplify it to be 4*(const), how did you calculate the 4?
##(\frac{1}{R/2})^2 = (\frac{2}{R})^2 = 4(\frac{1}{R})^2##
 

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