The work done in placing a charge of 8 × 10⁻¹⁸ coulomb on a condenser of capacity 100 micro-farad is (a) 16 × 10⁻³² joule (b) 3.1 × 10⁻²⁶ joule (c) 4 × 10⁻¹⁰ joule (d) 32 × 10⁻³² joule

Answer: (d). W = q²/2C W = (8 × 10⁻¹⁸)²2 × 100 × 10⁻⁶ = 64 × 10⁻³⁶2 × 10⁻⁴ W = 32 × 10⁻³² J So the answer is (d) 32 × 10⁻³² joule.

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Energy Stored & Force between Plates

The work done in placing a charge of $8 \times 10^{-18}$ coulomb on a condenser of capacity $100$ micro-farad is

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Answer: (d)

$W = \frac{q^2}{2C}$
$W = \frac{(8 \times 10^{-18})^2}{2 \times 100 \times 10^{-6}} = \frac{64 \times 10^{-36}}{2 \times 10^{-4}}$
$W = 32 \times 10^{-32}\text{ J}$
So the answer is (d) $32 \times 10^{-32}$ joule.