☰ Contents · Physics

Thermal equilibrium and determining specific heat capacity

Lessons 20–21 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
20

Practical work: studying thermal equilibrium of bodies

Textbook: p. 63
GoalObserve how thermal equilibrium is reached; draw a temperature–time graph and draw conclusions.
New words
thermal equilibrium · issiqlik muvozanatiequilibrium temperature · muvozanat temperaturasitemperature–time graph · temperatura–vaqt grafigiheat exchange · issiqlik almashinuvi
Explanation

The aim is to see how heat exchange between hot and cold water proceeds and when the temperatures equalise. Equipment: a large vessel, two thermometers, a clock, a polyethylene bag, a stand and string. Procedure: the teacher prepares warm-hot water (about 50–60 °C) in the vessel; cold water is put in the bag with the second thermometer and the bag is immersed in the hot water. Every minute both thermometer readings are recorded in a table, and observation continues for several minutes after they have equalised. The hot water cools (its internal energy falls), the cold water warms (its internal energy rises); once the temperatures are equal, net heat exchange between them stops. On the graph two curves approach each other and reach one value. Students do not work with boiling water; only the teacher pours hot water.

Worked examples
Sample table (approximate): 0 min: hot 60 °C, cold 20 °C; 3 min: 48 and 36; 6 min: 43 and 41; 9 min: 42 and 42. The equilibrium temperature ≈ 42 °C was reached at about 9 min; afterwards both cool slowly toward room temperature.
Theoretical check: 200 g of water at 60 °C and 100 g at 30 °C. Balance: t = (200·60 + 100·30)/(200 + 100) = 50 °C. If the experimental value is slightly lower, the cause is heat lost to the bag, vessel and air.
Class activity

Groups complete the table and graph: draw the two lines in two colours, mark the equilibrium point and read off the equilibrium time. Compare results with another group. Keep the thermometer away from the vessel wall and bag; if a thermometer breaks, tell the teacher.

Practice
1
How do the internal energies of the hot and cold water change during heat exchange?
2
If 200 g of water at 60 °C and 100 g at 30 °C are mixed, what is the final temperature in °C?
3
What does equilibrium look like on the temperature–time graph?
4
After equilibrium both thermometers still fall slowly. Why?