Quick question on sign convention for Work in Columbs

AI Thread Summary
The discussion revolves around understanding the sign convention for work in the context of electric forces and potential energy. Participants express confusion over the signs of work and forces when dealing with like charges, particularly in problems involving Coulomb's law and capacitors. It is clarified that when calculating work, the variable force must be considered, especially as charges approach each other, indicating an energy perspective is necessary. Additionally, the relationship between voltage and charge in capacitors is highlighted, emphasizing the need to understand the underlying principles rather than just applying formulas. The final point addresses the correct application of the right-hand rule, concluding that the charge in question must be negative based on the direction of the force.
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Homework Statement



http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam1/fa06

Homework Equations



W =F*D

The Attempt at a Solution



I have it solved, now i thought - would be if i put work into the thing to move it but looking at it since both of them are postive charge they are gona wana repel each other to begin with so why is it not +
 
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http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam1/sp06

Why am i stuck on 14 and 15

The equation that i would use would be F = Kq1Q2 / R^2 correct? and all it is is just plug and chug but i am not getting the answer
 
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Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam1/sp06

Why am i stuck on 14 and 15

The equation that i would use would be F = Kq1Q2 / R^2 correct? and all it is is just plug and chug but i am not getting the answer

#14 has a variable force acting the whole time Q1 is approaching Q2. This is not a simple matter of finding one force. Fortunately, the problem has been stated in terms of energy, which is a big hint about how it should be approached. What happens to the potential energy of Q1 as it approaches Q2?

For #15 you need to think about the relationship between voltage across a capacitor and the electrical quantity that causes that voltage. What is the same for the three capacitors?
 
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All right, can u explain this
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa06

Question 15


I assumed it would be positve, so if i do right hand rule, i have force is up so Electric field would have to point down CorrecT? so why is it negative
 
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Alt+F4 said:
All right, can u explain this
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa06

Question 15


I assumed it would be positve, so if i do right hand rule, i have force is up so Electric field would have to point down CorrecT? so why is it negative

There is no electric field in the problem. The charge q does produce an electric field, but that field has nothng to do with the forces acting on q. You used the right hand rule correctly and found that the force on a positve charge would be upward at the position shown in the diagram. Clearly the force at that point must be downward, so the charge cannot be postive and must be negative.
 
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