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Unformatted text preview: Physics 21 Fall 2007 Solution to HW-23 Transformers Consider the transformer ideal unless otherwise noted. (A) The primary coil of a transformer contains 100 turns; the secondary has 200 turns. The primary coil is connected to a size AA battery that supplies a constant voltage of 1.5 volts. What vol- tage would be measured across the secondary coil? ( multiple choice: the correct answer is zero since the AA battery is dc ) (B) A transformer is intended to decrease the rms value of the alterna- ting voltage from 500 volts to 25 volts. The primary coil contains 200 turns. Find the necessary number of turns N _2 in the secon- dary coil. 500/25 = N _P/ N _S = 200/ N _S => N _S = 10 (C) A trans- former is intended to decrease the rms value of the alternating current from 500 amperes to 25 amperes. The primary coil con- tains 200 turns. Find the necessary number of turns N _2 in the secondary coil. Power = I V is constant. Current decreases by 500A/25A = 20, so N must go up by 20. 20 x 200 = N _2 = 4000 (D) In a transformer, the primary coil contains 400 turns, and the secondary coil contains 80 turns. If the primary current is 2.5 amperes, what is the secondary current I_2? amperes, what is the secondary current I_2?...
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