Q4. Is a cup of hot tea cooling on a table in thermal equilibrium with the room? When does it get there?
No. Its temperature is still changing and heat is still flowing out of it. It reaches thermal equilibrium only when it has cooled to the room’s temperature and stops changing.
Q5. Why must a thermometer be left in contact with a body for a while before it is read?
The thermometer and the body must reach thermal equilibrium. Only then are their temperatures equal, so the thermometer’s reading is the body’s temperature.
Q6. Two rigid containers each hold 1 mol of helium, one at 400 K and 3.0 atm, the other at 200 K. They are joined by a diathermic wall and insulated from outside. Find the final temperature and the final pressure of the first container.
Equal amounts of the same gas, so the final temperature is (400 + 200)/2 = 300 K. The first container’s volume is fixed, so its pressure falls in proportion: 3.0 × 300/400 = 2.25 atm.