358 PROBLEMS 



V-3. A drop of water is 2 X 10 -4 cm in diameter. To what pressure is the water 

 subjected by its surface energy? 



V-4. What force, due to surface energy, is required to remove a clean ring of 

 2-cm internal and 2.1-cm external diameter through a clean water surface? 



V-5. What will happen when two soap bubbles of unequal size are connected with 

 a glass tube? 



V-6. Why does a linen towel absorb moisture faster than a cotton one? 



V-7. What is the gas pressure inside a 8-cm-diameter soap bubble if the soap 

 solution-air surface has a surface energy of 35 ergs/cm 2 at 20° C? 



V-8. Calculate the osmotic pressure of a 1 per cent solution of cane sugar when 

 dialyzed against pure water through a membrane impermeable to the sugar mole- 

 cules. 



Ans. 494 mm Hg. 



V-9. In one of his experiments Pfeffer showed that a certain concentration of 

 sugar produced a pressure of 53.5 cm Hg at 20° C. His product pv was 1.82 X 10 6 

 dynes X cm. What was the concentration in grams per liter? 



Ans. 10.0 grams /liter. 



V-10. If a 0.001 N solution of KC1 is completely dissociated, what concentra- 

 tion of KC1 in water will be required to give the same osmotic pressure as that ob- 

 tained from a 10-grams-per-liter sugar solution? 



V-ll. The freezing point of a solution was found to be -0.93° C. What is its 

 osmotic pressure? 



Ans. 11.2 atmospheres. 



V-12. Mammalian blood serum gives A = 0.56° C. What per cent NaCl solu- 

 tion gives the same osmotic pressure? What per cent solution of NaCl is isotonic or 

 isosmotic with mammalian serum? 



Ans. 0.95 per cent. 



VI-1. The following voltages were applied to tissue in order just to produce 

 excitation. Stimulus measured in volts; time measured in milliseconds. Stimulus: 

 70, 60, 50, 40, 38. Time: 0.10, 0.17, 0.28, 0.49, 1.20. Draw a graph and interpolate 

 the value of the chronaxie. 



VI-2. Two idealized axon spikes start in phase. The first has a velocity of 

 16 meters per second and the second a velocity of 15 meters per second. The first 

 rises to 300 microvolt crest potential in 0.2 millisecond and the second to 200 micro- 

 volts in 0.4 millisecond. Draw the resultant compound action potential observed 

 at a point 10 cm from their common origin. 



VI-3. Plot the curve y = a sin 2vt/T when the amplitude is 10 cm and the 

 period 4 seconds. 



VI-4. A particle is acted on simultaneously by two simple harmonic motions 

 given by y = 4 sin 2irt/% and y = 6 sin 2^/4. Plot a graph showing the resultant 

 motion. 



VI-5. Between two points A and B, along whose line a train of simple waves 

 is traveling, there are 8| cycles. What is the difference in phase between the points 

 A and B? 



VI-6. Compound three trains of waves in the ratio 1, |, and 5 and of amplitude 

 4, 3, and 2 starting in the same phase. 



VI-7. The data for a strength-duration curve showed that when the logarithm 

 of V/(V—R) was plotted against time a linear relation was obtained. Interpret 

 your results if V is the excitation potential, R the rheobase, and t the time. Interpret 

 the slope of the graph. 



