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 options is true?
- q = +ve, w = -ve, U = +ve
- q = +ve, w = 0, U = -ve
- q = +ve, w = -ve, U = -ve
- q = -ve, w = -ve, U = -ve
Answer
(1) q = +ve, w = -ve, U = +ve
Solution
The water (system) absorbs energy from the microwaves and its temperature rises to the boiling point q = +ve
On heating and boiling, the water expands and vapour forms against the atmosphere, i.e. the system does work on the surroundings w = -ve
The heat absorbed is much larger than the small expansion work, and the temperature rises:
U = q + w > 0 U = +ve
Answer: (1)