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CBSE Class 12 Physics 2014 Outside Delhi Set 1 Paper

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Question : 3 of 30
Marks: +1, -0
What is the force between two small charges of 2×10−7C2 \times 10^{-7} \mathrm{C} and 3×10−7C3 \times 10^{-7} \mathrm{C} placed 30cm30 \mathrm{cm} apart in air?
Solution:  
F  =  kq1q2r2F \;=\; \frac{k q_1 q_2}{r^2}
  =  9×109×2×10−7×3×10−7(30×10−2)2\;=\; \frac{9 \times 10^9 \times 2 \times 10^{-7} \times 3 \times 10^{-7}}{(30 \times 10^{-2})^2}
  =6×10−3N\;= 6 \times 10^{-3} \mathrm{N}
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