MAGNETS AND ELECTROMAGNETS 



245 



TOPIC 2. SECURING ELECTRICAL ENERGY FROM MAGNETS 

 AND ELECTROMAGNETS 



SUGGESTED PROBLEMS AND QUESTIONS 



1. Why do magnets pull some things to them? 



2. Why does a compass point northward? 



3. What are electromagnets, and for what are 

 they used? 



SUGGESTIONS AND HELPS FOR STUDY 



1. As magnets and electromagnets are used in a 

 great many common devices such as telephone re- 

 ceivers, electric bells, and telegraphs, they should be 

 thoroughly understood. 



2. The following new words and phrases may be 

 found in this study: 



electromagnet a coil of wire surrounding an iron core 

 which is magnetized only when a current of electricity 

 flows through the coil. 



magnetic field the space about a magnet which is in- 

 fluenced by the magnet. 



magnetic lines of force the invisible lines of magnetic 

 energy which make up the magnetic field; they may 

 be mapped with iron filings. 



magnetic pole the place on a magnet where the mag- 

 netism seems to be concentrated. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 149. What things will magnets pick up? 



Assemble a horseshoe or bar magnet and several small 

 articles made of the following substances : copper, iron, glass, 

 rubber, nickel, paper, silver, zinc, carbon, lead, brass, steel, 

 and aluminum. By bringing the magnet near each one of 

 these in turn, classify them in two groups, magnetic sub- 

 stances and non-magnetic substances. Record your results. 



Experiment 150. Why do magnets draw some things 

 to them? 



Carefully study a bar magnet and observe any markings 

 on it. Suspend a bar magnet horizontally by a string at- 

 tached at its center. Bring the end marked N of another 

 magnet near the TV end of the original magnet. Bring the 

 end marked 5" near the end marked S of the suspended 

 magnet. Record your results. Bring the ./V end of the mag- 

 net near the end marked 5" on the suspended magnet. Re- 

 peat, using the other ends of the magnets. 



Place a bar magnet on the table and cover with a clean 

 sheet of paper. Sprinkle iron filings over the paper and 

 tap lightly with your pencil. In your notebook draw the 

 results of your experiment. In a similar way map two N 

 poles placed near each other; map two unlike poles placed 

 near each other. Draw the results in your notebook and 

 complete the following statements. 



The letter appears at one end of a bar magnet while 



1 See workbook, p. 87. 



appears at the other end. These letters refer to the 



pole and the pole. Magnetic poles which are 



seem to push apart or repel, while poles which are 



seem to attract. 



Experiment 151. How does a compass work? 



Bring a magnet near a compass needle and observe what 

 happens. Bring the north pole of the magnet, and then the 

 south pole, near the compass. Place the bar magnet on the 

 table with its ./V pole pointing north. Hold the compass about 

 four inches from the magnet above its center. Which way 

 is the ./V of the compass pointing? Now reverse the magnet 

 and again hold the compass above the magnet. What change 

 is noted? In your notebook write the notes of your experi- 

 ment and complete the following statements. 



The compass needle is a and has the same as 



other magnets have. When the north pole of a magnet is 



brought near the needle, the end of the compass is 



attracted; the end is attracted by the south end of the 



magnet. A compass seems to have its magnetic pole 



attracted toward the north and its magnetic pole 



toward the south. This would seem to show that the mag- 

 netic attraction from the north is of a polarity, and 



that from the south is of a polarity. 



Experiment 152. What are electromagnets and how do 

 they work? 



Secure a large nail or bolt, about four feet of bell wire, 

 two dry cells, a compass, and a box of iron tacks or small 

 brads. Wind about three feet of wire on the nail, leaving 

 some at each end. Bring the coil and nail near the com- 

 pass ; also try to pick up some of the tacks. Attach the ends 

 of the coil to one dry cell. Bring each end of the nail near 

 the compass. Change the wires at the dry cell and repeat 

 with the compass. Record the results in your notebook. 

 Bring the magnet near the tacks. Break the connection to 

 the cell while the tacks are pulled to the magnet. Results? 

 Count the number of tacks which you can pick up. Wind 

 another foot of wire on the nail and see how many tacks 

 can be picked up. Add another cell and see how many tacks 

 can be picked up. 



In your notebook complete the following statements. 



A coil of wire wound on an iron core and attached to 

 a dry cell is a magnet when . When the dry cell is dis- 

 connected the electromagnet its magnetism. One end 



of the magnet is a pole and the other a pole. 



When the direction of the current through the coil is re- 

 versed by changing the wires at the dry cell, the poles of 



the magnet are An electromagnet can be made stronger 



by or by 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



Why do magnets pull some things to them? Mag- 

 nets have been known for many centuries. Long ago, 

 even before the birth of Christ, it was known that a 

 certain kind of iron ore existed in Asia Minor which 

 would attract small bits of iron to it. This ore came 

 to be known as magnetic ore. Even before that time 



