What is the potential energy of the pair?

[dwx1]

Can someone check this for me?

Two point charges, Q₁ = + 5.00 nC and Q₂ = -3.00 nC, are separated by 35.0 cm. (a) What is the potential energy of the pair? What is the significance of the algebraic sign of your answer? (b) What is the electric potential at a point midway between the charges?

(a)
U = k_e * (5*10^-9) * (-3*10^-9) / .35
U = -3.85*10^-7 Nm
I have no idea what the significance of the algebraic sign is.

(b)
V₁ + V₂ = k_e/.175 * (5-3)*10^-9
V₁ + V₂ = 102.611

 

[mukundpa]

Seems correct- ve sign shows that the electrostatic potential energy of the system is negative so the interaction force is that of attraction and to make energy i.e. to separate the two charges to very large distance we have to supply the energy or we have to do work.