Register to reply 
Equivalent Electri Force, Find the charge. 
Share this thread: 
#1
Feb1213, 05:04 PM

P: 61

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/r^{2} 3. The attempt at a solution So I know that since force is equal Fr^{2}/k is a constant. Also since the charge in the 2nd case is equally shared I can rewrite the equation for force as F=kq^{2}/r^{2}. Solving for q gets me 4.8e6. 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: q1^{2}+9.6e6(q1)9.6e6. This results in an invalid quadratic equation because b_{2}4ac gives me a negative number. I think I made math errors or I'm missing a step, but the underlying concept makes sense. 


#2
Feb1213, 05:41 PM

Emeritus
Sci Advisor
HW Helper
PF Gold
P: 7,801

q = (q_{1}+q_{2})/2 The magnitude of the force is the same for both situations, so q_{1}q_{2} = q^{2}but because q_{1}q_{2} is negative, q_{1}q_{2} = q_{1}q_{2} . 


#3
Feb1213, 06:02 PM

P: 61

ok now I just got a completely wrong answer. My new quadratic equation is q2^{2}9.59e6+4.8e6. I got answer of like .00348



#4
Feb1213, 11:07 PM

Emeritus
Sci Advisor
HW Helper
PF Gold
P: 7,801

Equivalent Electri Force, Find the charge.
What equations are you putting together to get your answers ? 


Register to reply 
Related Discussions  
Find the net force on charge  Introductory Physics Homework  3  
Charged cylinder with charge inside, find force  Advanced Physics Homework  0  
Find force on the +q charge  Advanced Physics Homework  2  
Find i j k force on charge  Introductory Physics Homework  7  
Find the Force and Direction of a charge  Advanced Physics Homework  1 