Physics Department Thermodynamic Laboratory Vaporization Heat of Liquid N 2 1. Goal Determination of the latent heat of vaporization of liquid Nitrogen. 2. Foundations Warning: Liquid nitrogen can cause terrible burns (death of living tissue caused by the extreme cold). Latent heat, l, is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. The more common forms of latent heat are latent heat of fusion (melting) and latent heat of vaporization (boiling). We will start pouring liquid nitrogen on a polystyrene vessel. Liquid nitrogen is colorless and odorless and its boiling temperature is 77ºK. Since at room temperature (25ºC or 298ºK) we are well above the boiling temperature, liquid nitrogen will be evaporating, Fig. 1. Figure 1 Liquid nitrogen gets heat from its environment, i.e. container and atmosphere, and uses it for a phase transition from liquid to gas. Since, by definition, the latent heat of vaporization is the amount of heat necessary to evaporate a unit mass, the heat exchanged with the surroundings per unit time, A, can be used to evaporate dm dt A l dm [1] dt 1
To determinate the latent heat of vaporization, l, we will measure the lost mass rate, dm/dt, using an analytical bascule and a chronograph. But, how to measure A? The answer is: introducing an additional known heat source. The one that will be used is based on the Joule effect, the heat delivered per unit time by an electrical resistance E IV. The two heat sources: the environment, contributing with A, and the electrical resistance E, will be used together to evaporate the liquid nitrogen: where A B l [1] and [2] and the Joule effect: dm [2] dt I dm dt 1 is the amount liquid nitrogen evaporated per time unit. Combining equations l dm IV l dt dm [3] dt I thus giving the following expression for the latent heat of vaporization: 3. Learn more... IV l dm dm dt I dt [4] Paul A. Tipler, Gene Mosca Physics for Scientists and Engineers, Vol. 1, 6 th Edition. Ed. W. H. Freeman; ISBN-1: 716789647, ISBN-13: 978-716789642 (27) 4. Equipment 1. Polystyrene vessel. 2. Analytical balance. 3. Electrical resistance. 4. Chronograph. 5. Voltmeter and ammeter 6. Regulated power supply. 7. Liquid nitrogen. 2
5. Experimental setup. Figure 2 5.1 Description of the measurement system. Place the polystyrene vessel on the analytical balance and fill it with liquid nitrogen. Make the electrical connections according to Fig. 3. Don t turn on the power supply yet. Insert the electrical resistance in the nitrogen. We will assume that the current, I, and potential difference, V, will be those indicated by the power supply. Figure 3. 3
5.2 Measurements. To estimate the latent heat of vaporization with equation [4] we need to measure: i) the evaporation rate of nitrogen without connecting the electrical circuit v, ii) the current flowing in the resistance I, iii) the potential difference V, and finally, iv) the evaporation rate of nitrogen with the electrical circuit on, v I dm dt I. Since the heat exchange with the surroundings changes with time several estimations of it are needed. 5.2.1 First measurement of V. Fill the 5.2.1 part of the form Setup the equipment according to Fig. 3. Don t connect the power supply yet. Estimate v. 5.2.2 First measurement of V I. Fill the 5.2.2 part of the form Turn on the power supply and adjust the current intensity to 2 A. Record the current intensity and the potential difference. Estimate v I 2 dm dt I 2 5.2.3 Second measurement of V. Fill the 5.2.3 part of the form Turn off the power supply. For the next 4 minutes, record the nitrogen-vessel set weight m(t) every 2 seconds. Estimate v 5.2.4 Second measurement of V I. Fill the 5.2.4 part of the form Turn on the power supply and adjust the current intensity to 3 A. Record the current intensity and the potential difference. Estimate v I 3 dm dt I 3 5.2.5 Third measurement of V. Fill the 5.2.5 part of the form Turn off the power supply. For the next 4 minutes, record the nitrogen-vessel set weight m(t) every 2 seconds. Estimate v. 4
5.3 The latent heat of vaporization of liquid N itrogen We will obtain two estimates of the latent heat of vaporization of liquid nitrogen, one for I 2 A and the other for I 3 A. 5.3.1 I = 2 A. Estimate the evaporation rate, v, of liquid nitrogen with I A averaging the results of sections 5.2.1 and 5.2.3. Estimate the latent heat of vaporization l I 2 inserting the values of the evaporation rate v, the evaporation rate v I 2 (5.2.2), the current intensity and potential difference in equation [4]. 5.3.2 I = 3A. Estimate the evaporation rate, v, of liquid nitrogen with I A averaging the results of sections 5.2.3 and 5.2.5. Estimate the latent heat of vaporization l I 3 inserting the values of the evaporation rate v, the evaporation rate v I 3 (5.2.4), the current intensity and potential difference in equation [4]. 5.3.3 Compare results. Compare and discuss the results obtained in the previous two sections for the latent heat of vaporization of nitrogen. 5