12. a) (2) Wherein does blue light differ from red light ? 



b) (2) If an object colored a pure red (such as a red rose) is illuminated by- 



pure blue light only, what is the effect on its appearance ? 



c) (2) Explain the cause of this effect. 



d) (2) What are infra-red rays; ultra-violet rays? 



e) (2) How may each be detected ? 



Group V 



13. a) (3) Make a sketch of an electromagnet, showing clearly the direction of 



the current through the wire, and the resulting polarity of each 

 end of the magnet. 



b) (2) Mention two important uses of the electromagnet. 



c) (2) What are its advantages over a permanent steel magnet ? 



d) (3) Suppose you were presented with two steel bars exactly alike except 



that one was magnetized. How could you decide which was the 

 magnet ? 



14. a) (4) A certain incandescent lamp of constant resistance takes 50 watts from 



a 100-volt circuit. How much power will it take from a 200-volt 

 circuit ? 



b) (4) How much power will two such lamps in series take from the 200-volt 



line? 



c) (2) What is the current flowing in each of the above three cases ? 



15. a) (6) A J horse-power motor, a 750-watt electric sto\-e, and an electric flat- 



iron using 5 amperes are connected in parallel on a 110-volt circuit. 

 What is tlie total current ? 

 b) (4) What would be the cost of using this current for half an hour at 3 

 cents per kilowatt-hour? 



i-27 



