☰ Contents · Physics

Quantity of heat

Lessons 18–19 · 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
18

Quantity of heat

Textbook: pp. 55–59
GoalHeat transfer and quantity of heat; specific heat capacity; the formula Q = cmΔt; the heat-balance equation.
New words
quantity of heat · issiqlik miqdorispecific heat capacity · solishtirma issiqlik sig‘imiheat-balance equation · issiqlik balansi tenglamasicalorimeter · kalorimetr
Explanation

Transfer of energy from one body to another without doing work is called heat transfer; the internal energy gained or lost is the quantity of heat Q (unit joule, 1 cal ≈ 4.19 J). To change the temperature of a body of mass m from t₁ to t₂ needs Q = cm(t₂ – t₁), where c is the specific heat capacity of the substance: the heat needed to change the temperature of 1 kg of it by 1 K, unit J/(kg·K). For water c = 4200 J/(kg·K), aluminium 890, steel 460, copper 390, lead 130. Q > 0 on heating, Q < 0 on cooling. In a closed system the sum of the heat given up by the hotter bodies equals the sum of the heat received by the cooler ones (the heat-balance equation); this is the law of conservation of energy. A calorimeter is a vessel that measures quantities of heat and reduces heat exchange with the surroundings.

Worked examples
To heat 2 kg of water from 20 °C to 70 °C: Q = cmΔt = 4200·2·50 = 420 000 J = 420 kJ.
100 g of water at 80 °C is added to 300 g of water at 20 °C. Balance: c·0.3·(t – 20) = c·0.1·(80 – t) → 3(t – 20) = 80 – t → 4t = 140 → t = 35 °C. (The vessel and surroundings are neglected.)
Class activity

At home or school with an adult’s supervision: leave equal masses of cold water and cooking oil for the same time in sunlight or a warm room and compare with a thermometer which warms more. Do not use boiling water, a stove or fire; link the result to c.

Practice
1
What is specific heat capacity? What does “c of water = 4200 J/(kg·K)” mean?
2
0.5 kg of aluminium (c = 890 J/(kg·K)) is heated from 20 °C to 40 °C. How many joules is Q?
3
A 2 kg body cools from 100 °C to 60 °C and gives off 36 800 J. Find c (J/(kg·K)).
4
Why is the climate near the sea milder than inland?