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Which one of the following pairs of charges separated by the same distance r will experience a maximum force?

  • A.0.3 C and 0.7 C
  • B.0.1 C and 0.9 C
  • C.0.2 C and 0.8 C
  • D.0.5 C and 0.5 C
  • E.0.4 C and 0.6 C

Answer: D

The maximum force is between 0.5 C and 0.5 C.

Every pair adds up to 1 C, and at the same separation Coulomb's force is proportional to the product \(q_{1}q_{2}\). For a fixed sum, the product is largest when the two parts are equal.

\[ \begin{aligned} 0.5 \times 0.5 &= 0.25 \\ 0.4 \times 0.6 = 0.24,\ 0.3 \times 0.7 &= 0.21 \\ 0.2 \times 0.8 = 0.16,\ 0.1 \times 0.9 &= 0.09 \end{aligned} \]

So the equal split gives the largest product and hence the largest force; the more unequal the split, the smaller the force.

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