Copyright Goodheart-Willcox Co., Inc. Chapter Review and Assessment 105 Summary • The magnet was fi rst discovered hundreds of years ago in Turkey. • The early uses of the magnet were for navigation. • Magnets have poles that are termed “north” and “south.” • Like poles repel, while unlike poles attract. • Fields of magnetic force surround magnets. • Electromagnets are magnets created by electrical current fl ow. • Relays are magnetically operated switches, usually to control high voltages and currents. • Solenoids use electricity to create a magnetic fi eld that is then used to provide for mechanical motion. • Magnetic fi elds cannot be stopped. One can only infl uence their presence by using highly permeable materials to divert their fi elds. Review Questions Answer the following questions using the information in this chapter. 1. A substance that has the ability to attract certain kinds of ferrous (containing iron) metals is known as a(n) _____. 2. The magnetic fi eld, consisting of many lines of magnetic force, running from the north pole to the south pole of a magnet is called the magnetic _____. 3. Draw two sketches showing the position of the molecules in an un-magnetized and a magnetized piece of steel. 4. If a magnetized bar of steel were broken into three pieces, what effect would it have on the magnetism of pieces of the steel bar? 5. The property of a substance to conduct a magnetic fi eld is called _____. 6. The resistance a material has to conducting a magnetic fi eld is called _____. 7. Like poles _____ each other. 8. Draw a sketch of two bar magnets with the N pole of one near the S pole of the other. Draw lines showing the magnetic fi elds. 9. Draw another sketch of two bar magnets with the N pole of one near the N pole of the other. Draw lines showing magnetic fi elds. 10. The strength of the magnetic force of a coil depends on what two factors? 11. A coil of 500 turns has a current of 2 amperes fl owing through it. What is its fi eld strength? 12. Why will a solenoid coil increase its magnetic strength when an iron core is inserted? 13. Describe how a control circuit in a relay coil is used both when establishing and removing a magnetic fi eld. 14. Why will a magnet pick up a nail? 15. What is the purpose of magnetic shielding? Critical Thinking Questions and Activities 1. Obtain a compass that is used for camping and hiking activities. Experiment with it to determine north and south directions from your location. Now, take the compass to functioning electrical equipment to see if it is affected by the equipment. Be prepared to share your results with the class. 2. Search the Internet for super-magnets. Determine how these types of magnets are made and what goes into them. Write a report on your fi ndings. 3. Water valves found in clothes washing machines and dishwashing machines make use of electromechanical switches called solenoids. Obtain used solenoids and disassemble them to better understand their operation.