Equivalent Electri Force, Find the charge.

pmd28

1. The problem statement, all variables and given/known data
Two objects are identical and small enough that their sizes can be ignored relative to the distance between them, which is 0.3 m. In a vacuum, each object carries a different charge, and they attract each other with a force of 2.3 N. The objects are brought into contact, so the net charge is shared equally, and then they are returned to their initial positions. Now it is found that the objects repel one another with a force whose magnitude is equal to that of the initial attractive force. What is the initial charge on each object? (Note: there are two possible pairs of answers, but assume q1 to be the larger number.)


2. Relevant equations
F=kq1q2/r²


3. The attempt at a solution
So I know that since force is equal Fr²/k is a constant. Also since the charge in the 2nd case is equally shared I can re-write the equation for force as F=kq²/r². Solving for q gets me 4.8e-6.


From here since the charge was shared equally between q1 and q2, q=q1+q2/2. Using substitution to solve for q2, I came up with the following quadratic:

-q1²+9.6e-6(q1)-9.6e-6. This results in an invalid quadratic equation because b₂-4ac gives me a negative number. I think I made math errors or I'm missing a step, but the underlying concept makes sense.

SammyS

Be careful with parentheses. That should be written,

q = (q₁+q₂)/2

It's likely that the signs of the charges is what's causing your problems.

The magnitude of the force is the same for both situations, so

|q₁q₂| = q²

but because q₁q₂ is negative,

|q₁q₂| = -q₁q₂ .

pmd28

ok now I just got a completely wrong answer. My new quadratic equation is q2²-9.59e-6+4.8e-6. I got answer of like .00348

SammyS

Coulombs?

What equations are you putting together to get your answers ?