Determining the magnitude of the charge (Coulombs Law)

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Homework Help Overview

The discussion revolves around determining the magnitude of electric charges using Coulomb's Law. Participants are examining the relationship between the force exerted by two equal charges and their separation distance, with specific numerical values provided.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants attempt to rearrange Coulomb's Law to find the charge values, questioning their calculations and the significance of their results. Some express uncertainty about the accuracy of their numerical outcomes and the implications of significant figures.

Discussion Status

There is ongoing exploration of the calculations involved, with participants sharing their results and questioning discrepancies. Some suggest re-evaluating the calculations and the use of calculators, while others express confusion about the mathematical operations involved.

Contextual Notes

Participants note potential issues with calculator settings and the importance of significant figures in their calculations. There is also mention of the need to clarify the handling of exponents in division.

meeklobraca
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Homework Statement



Two equal charges exert an electric force on each other of 0.0385N when positioned 0.15 m apart. Determine the magnitude of the charge on each point.



Homework Equations



Fe=Kq1q2/R^2

The Attempt at a Solution



I narrowed it down to

q1q2=FeR^2/K

q1q2=96357.0634C^2

I squared this and I get q1q2=310.4143415C

This is not the answer to the question. I am close as far as the numbers go but I am not sure what I am missing here.
 
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meeklobraca said:

Homework Statement



Two equal charges exert an electric force on each other of 0.0385N when positioned 0.15 m apart. Determine the magnitude of the charge on each point.

Homework Equations



Fe=Kq1q2/R^2

The Attempt at a Solution



I narrowed it down to

q1q2=FeR^2/K

q1q2=96357.0634C^2

I squared this and I get q1q2=310.4143415C

This is not the answer to the question. I am close as far as the numbers go but I am not sure what I am missing here.

Could be the number of significant digits.
Could be that the charges could either both be plus or minus, but they do only ask for magnitude.
 
The answer given is 3.1 x 10-7 C

The answer I got was 3.1 x 10^2C right? So I figure the numbers are correct, just something missing, or maybe even the answer in the book is wrong.
 
meeklobraca said:
The answer given is 3.1 x 10-7 C

The answer I got was 3.1 x 10^2C right? So I figure the numbers are correct, just something missing, or maybe even the answer in the book is wrong.

Recalculate it again then. (K is 9*109 And you are dividing by it.)
 
yeah, that's been done.
 
meeklobraca said:
yeah, that's been done.

If you divided by 109 and you got 102 ... that's a lot of 10's to be off.

Why don't you type out your whole calculation?

On the face of it you have 10-2*10-1*10-1/109

102 seems like an unlikely result.
 
I have here

qq=0.0385N x (0.15)(0.15) / 8.99x10^9

So I get

8.66x10-4 / 8.99x10^9
 
Last edited:
meeklobraca said:
I have here

qq=0.0385N x (0.15)(0.15) / 8.99x10^9

So I get

8.66x10-4 / 8.99x10^9

And that gives 9.62 x 10-14

And taking the square root ...
 
"And that gives 9.62 x 10-14"

I wish I could copy and paste what my calculator is saying because I just don't get that answer.
 
  • #10
I tried it with an oline calculator and I got what you got, so I am not sure if the settings on my calculator are just messed up.
 
  • #11
meeklobraca said:
I tried it with an oline calculator and I got what you got, so I am not sure if the settings on my calculator are just messed up.

Nothing like pencil and paper.

If in doubt, work it out.
 
  • #12
Its been a while since I've done that. How do you divide exponents? Can you give me a quick crash course?
 
  • #13
meeklobraca said:
Its been a while since I've done that. How do you divide exponents? Can you give me a quick crash course?

Just look at post 6:
https://www.physicsforums.com/showpost.php?p=1924414&postcount=6

Dividing by 10 makes it 10-1

Dividing by 109 makes it 10-9

Dividing by exponents subtracts. Multiplying adds.

Don't let your teacher know you didn't know.
 
  • #14
OK see there we go.

I have 8.66x10-4 / 8.99x10^9 right?

Im taking 8.66x10^+4 and dividing it by 8.99x10^+9

But if I am dividing an exponent I am supposed to make the +9 a negative 9?
 
  • #15
meeklobraca said:
OK see there we go.

I have 8.66x10-4 / 8.99x10^9 right?

Im taking 8.66x10^+4 and dividing it by 8.99x10^+9

But if I am dividing an exponent I am supposed to make the +9 a negative 9?

That should be -4 and dividing by 109 makes that 10-13 and the numerator being less than denominator makes it 9.xx * 10-14.
 
  • #16
That was a typo on my part.

That makes perfect sense, I still don't get though why my calculator spits out 96329.25473

This is on a TI83 sci. calulator

8.6610^(-4)/8.9910^(9)

96329.25473
 
  • #17
meeklobraca said:
That was a typo on my part.

That makes perfect sense, I still don't get though why my calculator spits out 96329.25473

This is on a TI83 sci. calulator

8.6610^(-4)/8.9910^(9)

96329.25473

I'd recommend not using that calculator to figure orbital trajectories for a Mars landing then.

Maybe use it for grocery shopping?
 
  • #18
its going to be used as a stool for my radio here if it doesn't smarten up.
 

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