4. Gas laws. Charles' law After Gay-Lussac, another 19th-centur y French physicist, Jacques Charles, studied the variations in the volume of a gas when the temperature changed but the pressure remained constant. We can perform similar experiments, following the scientific method . Data analysis ⇒ The higher the temperature, the higher the volume, and vice versa. ⇒ In each row, the quotient of V / T is constant. Its value depends on the pressure. ⇒ The volume–temperature graph is a straight line that passes through the point (0, 0). ⇒ The gradient of the line depends on the pressure. 3 Conclusion ⇒ Absolute temperature and volume are directly proportional quantities. 4 Experiment ⇒ Choose the value for pressure, pA, and keep it constant by moving the piston. • Put the container in a thermostatic bath. Write down the temperature T1 (in K). • Move the piston up or down so that the manometer reads pA. Write down the volume V1. • Write down the values of T1 and V1 in a row in a table. • Heat the thermostatic bath until T2. Write down the value of V2 that keeps the pA value constant on the manometer. • Repeat until there are several rows of data. ⇒ Repeat the experiment with a different pressure on the manometer, pB. Obtain another set of data. 1 pA T1 V1 pA T2 V2 Data collection 2 Table Graph dependent variable independent variable 2 6 4 200 400 600 V (L) T (K) 0 0 Experiment A (constant pA) Experiment B (constant pB) T (K) V (L) T (K) V (L) 100 2 200 2 150 3 300 3 200 4 400 4 250 5 500 5 300 6 600 6 11 Think like a scientist: observe a phenomenon, search for information, propose hypotheses, etc. Why does a plastic bottle with some water in it get crushed in the fridge? Charles' law When a gas experiences transformations at a constant pressure, the quotient of the volume of the gas and its absolute temperature remains constant. V T V T V T = = constant ; 1 1 2 2 pA < pB CHALLENGE independent variable dependent variable A B 16
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