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I 1 ! V 1 supplied by the source is equal to the power I 2 ! V 2 in the secondary circuit. That is, I 1 ! V 1 " I 2 ! V 2 (33.42) The value of the load resistance R L determines the value of the secondary current because I 2 " ! V 2 /R L . Furthermore, the current in the primary is I 1 " ! V 1 /R eq , where (33.43) is the equivalent resistance of the load resistance when viewed from the primary We can now also understand why transformers are useful for transmitting power over long distances. Because the generator voltage is stepped up, the current in the 2 R losses are reduced. In practice, the voltage is stepped up to around 230 000 V at the generating station, stepped down to around There is a practical upper limit to the voltages that can be used in transmission lines. Excessive voltages could ionize the air surrounding the transmission lines, Many common household electronic devices require low voltages to operate properly. A small transformer that plugs directly into the wall, like the one illustrated R eq " " N
1 N
2 # 2 R L S E C T I O N 3 3 . 8 • The Transformer and Power Transmission 1053 Nikola Tesla American Physicist (1856–1943) Tesla was born in Croatia but spent most of his professional life as an inventor in the United States. He was a key figure in the development of alternating current electricity, high-voltage transformers, and the transport of electrical power using AC transmission lines. Tesla’s viewpoint was at odds with the ideas of Thomas Edison, who committed himself to the use of direct current in power transmission. Tesla’s AC approach won out. (UPI/CORBIS) Figure 33.23 The primary winding in this transformer is directly attached to the prongs of the plug. The secondary winding is connected to the power cord on the right, which runs to an electronic device. Many of these power-supply transformers also convert alternating current to direct current. This transformer is smaller than the one in the opening photograph for this chapter. In addition, it is a step-down transformer. It drops the voltage from 4 000 V to 240 V for delivery to a group of residences. George Semple George Semple |