A cup of water at 5°C is placed in a microwave oven and begins to boil
Q. 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?
A. \( q = +ve,\; w = -ve,\; \Delta U = +ve \)
B. \( q = +ve,\; w = 0,\; \Delta U = -ve \)
C. \( q = +ve,\; w = -ve,\; \Delta U = -ve \)
D. \( q = -ve,\; w = -ve,\; \Delta U = -ve \)
Correct Answer: A

Explanation

The system is water kept inside a microwave oven. When the oven is switched on, energy is supplied to the system, hence heat absorbed by the system is positive.

Therefore,

$$ q = +ve $$

As the water starts boiling, it expands against atmospheric pressure and does work on the surroundings. Hence, work done by the system is negative.

$$ w = -ve $$

According to the first law of thermodynamics,

$$ \Delta U = q + w $$

Since the magnitude of heat supplied is greater than the work done during heating and boiling, the internal energy of the system increases.

$$ \Delta U = +ve $$

Thus, the correct option is:

$$ \boxed{q = +ve,\; w = -ve,\; \Delta U = +ve} $$

Related JEE Main Questions

'x' is the product which is obtained from propanenitrile and stannous chloride in the presence of hydrochloric acid followed by hydrolysis. 'y' is the product which is obtained from the but-2-ene by the ozonolysis followed by hydrolysis.
Match List - I with List - II. List - I Functional group (detection) Unsaturation (Baeyer's test), Alcoholic group (Ceric ammonium nitrate test), Aldehyde group (Tollen's reagent), Phenolic group (FeCl3 test).
In the given electrochemical cell, Ag(s)|AgCl(s)|FeCl2(aq), FeCl3(aq) | Pt at 298 K, the cell potential (Ecell) will increase when :
Consider the general reaction given below at 400 K x A(g) ⇌ y B(g). The values of Kp and Kc are studied under the same condition of temperature but variation in x and y.
In two separate Young's double-slit experimental set-ups and two monochromatic light sources of different wavelengths are used to get fringes of equal width.
Home / Study Material

Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.

Scroll to Top