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8. A stove burning 150 grams of alcohol changes 4,500 grams of ice at 0° C. into 



water at 100° C. in 10 minutes. If the heat of combustion of alcohol is 

 7,400 calorics per gram, what per cent of the heat produced is utilized ? 



9. a) What is meant by the absolute zero ? 



b) From what point on the Centigrade scale are absolute temperatures 



measured ? 



c) Two liters of air at atmospheric pressure and at 20° C. are heated until 



the volume and the pressure are both doubled. What is the tempera- 

 ture of the air ? 



Group IV 

 (Omit one question from this group.) 



10. a) Distinguish carefully between refraction and dispersion. 



b) Why does a very thin layer of oil on water produce colors ? 



c) What is total reflection ? 



d) What is the critical angle ? 



11. Two shadows of an upright rod are cast side by side upon a screen, one by a 



standard candle and the other by a Welsbach gas burner. The standard 

 candle is placed 2 feet from the screen. When the gas burner is moved 

 until the two shadows are of the same density the burner is found to be 

 14 feet from the screen. What is the candle-power of the gas burner ? 



12. An incandescent lamp is placed 8 feet from a concave spherical mirror. If 



the image of the lamp is seen 1 foot in front of the mirror : 

 c) What is the radius of curvature of the rriirror ? 



b) Is the image real or virtual ? 



c) What are the relative sizes of the image and of the object ? 



Group V 

 (Omit one question from this group.) 



13. a) Explain how an electroscope might be given either a positive or a negative 



charge by means of a positively charged body. 

 b) Describe the construction and explain the operation of a condenser as 

 illustrated by a Leyden jar. 



14. a) A motor takes 8 amperes from a 500- volt circuit and delivers 5 horse-power. 



What is its efficiency ? 

 b) An ammeter in circuit with a small motor indicates 7 amperes when the 

 motor is starting and 3.5 amperes when the motor is running at full 

 speed. Explain. 



15. A resistance coil marked 10 ohms is found by careful measurement to have a 



resistance of 10.8 ohms. How much wire whose resistance is 1.35 ohms 

 per foot would be needed and how should it be connected with the coil in 

 order that the resistance of the combination should be 10 ohms ? 



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