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Asked in GUJCET 2009 · Energy of a system of charges
Given: q₁=q₂=-2 μC=-2×10⁻⁶ C, r=1 m, k=9×10⁹ SI units.
Idea: U=(kq₁q₂)/r, keeping the signs of the charges.
q₁q₂=(-2×10⁻⁶)(-2×10⁻⁶)=+4×10⁻¹² C² — two negatives give a positive product.
U=(9×10⁹×4×10⁻¹²)/1=36×10⁻³ J.
U=3.6×10⁻² J, positive because like charges repel.
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