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The electrical force between two point charges is 200 N. If one charge is increased by 10% and the other is decreased by 10%, the electrical force between them at the same distance becomes

Asked in GUJCET 2008 · Coulomb's law and superposition

• (1) 200 N
• (2) 99 N
• (3) 198 N
• (4) 100 N

Step-by-step solution

Why the other options are wrong

• (1) Assumes the +10% and the -10% cancel. They multiply: 1.1×0.9=0.99, not 1.
• (2) The factor 0.99 is right, but it has been applied to 100 N instead of 200 N: 0.99×100=99 N.
• (4) No single slip gives 100 N; the force barely changes. Sanity check: 200×1.1×0.9=198 N.

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