2015 USA Physics Olympiad Exam Part A 10 Of relevance to this question is the pressure/temperature phase plot for water,showing the re- gions where water exists in liquid form or gaseous form.The curve shows the coexistence condition, where water can exist simultaneously as gas or liquid. 240 200 160 ed) 120 Liquid Region 80 40 Gas Region 0 280 300 320 340 360 380 400 Temperature(K) The following graphs should be drawn on the answer sheet provided. a.Sketch a PT diagram for this cycle on the answer sheet.The coexistence curve for the liquid/gas state is shown.Clearly and accurately label the locations of points B through D and W through Z on this cycle. b.Sketch a PV diagram for this cycle on the answer sheet.You should estimate a reasonable value for Vmax,note the scale is logarithmic.Clearly and accurately label the locations of points B through D on this cycle.Provide reasonable approximate locations for points W through Z on this cycle. Solution We are using the Magnus form to approximate the coexistence curve: P=(610.94Pa)e17.625r+aao Note that T is measured in centigrade! We should compare this to the approximation that is valid for ideal gases at low temperatures compared to the critical temperature and constant latent heats: P=Pe ( Copyright C2015 American Association of Physics Teachers2015 USA Physics Olympiad Exam Part A 10 Of relevance to this question is the pressure/temperature phase plot for water, showing the re￾gions where water exists in liquid form or gaseous form. The curve shows the coexistence condition, where water can exist simultaneously as gas or liquid. 280 300 320 340 360 380 400 0 40 80 120 160 200 240 Temperature (K) Pressure (kPa) Liquid Region Gas Region The following graphs should be drawn on the answer sheet provided. a. Sketch a PT diagram for this cycle on the answer sheet. The coexistence curve for the liquid/gas state is shown. Clearly and accurately label the locations of points B through D and W through Z on this cycle. b. Sketch a PV diagram for this cycle on the answer sheet. You should estimate a reasonable value for Vmax, note the scale is logarithmic. Clearly and accurately label the locations of points B through D on this cycle. Provide reasonable approximate locations for points W through Z on this cycle. Solution We are using the Magnus form to approximate the coexistence curve: P = (610.94 Pa)e 17.625 T T +(243.04 ◦C) Note that T is measured in centigrade! We should compare this to the approximation that is valid for ideal gases at low temperatures compared to the critical temperature and constant latent heats: P = P0e L R  T −T0 T T0  Copyright c 2015 American Association of Physics Teachers