Is Electrostatic Potential Energy Always Equal to Work Done by Discrete Charges?

AI Thread Summary
The discussion revolves around the relationship between electrostatic potential energy and work done by discrete charges. Participants debate the truth of several statements regarding potential energy for different charge configurations. It is clarified that the formula for potential energy differs from that of electric potential, impacting the validity of the statements. Specifically, the confusion lies in the interpretation of work and potential energy in systems of charges. Understanding the correct formulas is crucial for accurately assessing the statements presented.
jaejoon89
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I'm trying to figure out the following question

Which of the following is false?

1) The total work for a a collection of discreet charges is the system's electrostatic potential energy
2) The potential energy for a pair of bodies with positive charges is positive
3) The potential energy for a pair of bodies with opposite charges is positive
4) The potential energy for a pair of bodies with opposite charges is positive
5) The potential energy for a pair of bodies with negative charges is negative

---

I know Vtot = V1 + V2 + ... and V = kq/r
so 2) and 5) seem to be true
For 1) W = - delta U so it's not exactly wrong (since the magnitudes are the same) but I don't think it can be considered right either
I'm not sure about 2) or 3)...
 
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Hi jaejoon89,

jaejoon89 said:
I'm trying to figure out the following question

Which of the following is false?

1) The total work for a a collection of discreet charges is the system's electrostatic potential energy
2) The potential energy for a pair of bodies with positive charges is positive
3) The potential energy for a pair of bodies with opposite charges is positive
4) The potential energy for a pair of bodies with opposite charges is positive
5) The potential energy for a pair of bodies with negative charges is negative

---

I know Vtot = V1 + V2 + ... and V = kq/r

That is the formula for potential, not potential energy. What is the formula for the potential energy of two point charges? Do you see how that will change your answers?
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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