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Two spheres start with charges of +4 C and +8 C. They interact, and afterward the blue sphere carries +10 C. Using conservation of charge, what is the final charge on the red sphere?
A-2 C
B+2 C
C+4 C
D+12 C
Answer & Solution
Correct answer: B. +2 C
1. Conservation of charge requires the total charge before and after to be equal.
2. The initial total charge is +4 C + 8 C = +12 C.
3. After the interaction the total must still be +12 C, and the blue sphere alone holds +10 C.
4. The red sphere therefore holds +12 C minus +10 C, which is +2 C.
_Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section Conservation of Charge_
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