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JEE Main202623 January 2026Morning ShiftChemistryThermodynamics (C)Actual

A cup of water at 5^ C (system) is placed in a microwave oven and the oven is turned on for one minute during which the water begins to boil. Which of the following option is true ?

Options

  1. Aq =+ ve , w =0, U =- ve
  2. Bq =+ ve , w =- ve , U =- ve
  3. Cq =- ve , w =- ve , U =- ve
  4. Dq =+ ve , w =- ve , U =+ ve

Correct answer

D. q =+ ve , w =- ve , U =+ ve

Step-by-step solution

For water heating in a microwave: Heat (q): The microwave radiates electromagnetic energy that is absorbed by water molecules, so q = +ve Work (w): As water heats and boils, it expands against atmospheric pressure. The system does work on the surroundings, so w = -ve Internal Energy (ΔU): From the first law, ΔU = q + w. Since heat absorbed (+ve) is much larger in magnitude than work done against atmosphere (-ve), the net change is ΔU = +ve (confirmed by temperature increase from 5°C to boiling) Therefore: q = +ve,

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