Finding electric force between two particles

AI Thread Summary
To find the electric force between two charges Q and -3Q separated by 2.5 m, Coulomb's law is applied using the equation F = k * q1 * q2 / r^2, where k = 9.0 * 10^9. The initial calculation incorrectly used (2.5/2)^2 for the distance instead of 2.5^2, leading to an incorrect force value. Additionally, the signs of the charges must be considered, as they affect the direction of the force. Correcting these mistakes is essential to arrive at the accurate electric force between the particles.
theskyisgreen
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Homework Statement


Two particles with electric charges Q and −3Q are separated by a distance of 2.5 m.
If Q = 4.9 C, what is the electric force between the two particles?


Homework Equations



Coloumb's equation: F=k*q1*q2/r^2
k=9.0*10^9

The Attempt at a Solution


I just tried to plug in the variables into the above equation.
so I got F= (9e9)*(4.9)*(3*4.9)/((2.5/2)^2)
F= 4.15e11
Apparently, that is not the correct answer, but I don't know what I did wrong!
I also tried to divide that number by 2 to get F= 2.07e11 which was also not right.

What am I doing wrong?
 
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theskyisgreen said:
Coloumb's equation: F=k*q1*q2/r^2
k=9.0*10^9

The Attempt at a Solution


I just tried to plug in the variables into the above equation.
so I got F= (9e9)*(4.9)*(3*4.9)/((2.5/2)^2)
F= 4.15e11
Apparently, that is not the correct answer, but I don't know what I did wrong!
I also tried to divide that number by 2 to get F= 2.07e11 which was also not right.
What am I doing wrong?
If the separation between the charges is r = 2.5 m, then r^2 = 2.5^2, not (2.5/2)^2. I am not sure what all those divisions by 2 are meant to accomplish.
 
Why are you dividing the distance by 2? Consider your signs, too.

[edit: beaten by kuruman. not happy]
 

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