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The work done in placing a charge of 8×10⁻¹⁸ coulomb on a condenser of capacity 100 μ F is

Asked in JEE Main 2005 · Energy stored and energy density

• (1) 16×10⁻³² joule
• (2) 3.1×10⁻²⁶ joule
• (3) 4×10⁻¹⁰ joule
• (4) 32×10⁻³² joule

Step-by-step solution

Why the other options are wrong

• (1) Divides by 4C instead of 2C, i.e. halves the correct answer once more.
• (2) Not reachable from the printed data: 3.1×10⁻²⁶ J corresponds to a capacitance of about 1 nF, while 100 μ F gives 3.2×10⁻³¹ J and reading C as 100 F gives 3.2×10⁻³⁷ J, so this is a printed filler.
• (3) Not reachable from the printed data: 4×10⁻¹⁰ J is twenty-one orders of magnitude above the correct 3.2×10⁻³¹ J and would need C≈8×10⁻²⁶ F, while dropping the 1/2 only doubles the answer to 6.4×10⁻³¹ J.

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