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Unformatted text preview: Solution for Homework 19 RL Circuits Solution to Homework Problem 19.1(Current in an LR Circuit) Problem: After the switch in the circuit in the figure is closed Select One of the Following: (a) The current in the circuit is zero at one time constant later. (b) The current is zero for a long time after the the switch is closed but eventually reaches I max . (c) The current is I max immediately after the switch is closed. (dAnswer) The current I max occurs a long time af ter the switch is closed. Solution The current in an L R circuit increases in time reaching its maximum current a long time after the circuit is completed. Thus, choice (d) is correct. Total Points for Problem: 3 Points Solution to Homework Problem 19.2(Direction of Induced Current Solenoid) Problem: The figure shows a solenoid with a constant current. What is the direction of the current induced in the loop? Select One of the Following: (a) The current induced in the loop is clockwise as viewed from the right. (b) The current induced in the loop is counterclock wise as viewed from the right. (cAnswer) There is zero current induced in the loop. Solution 1 (a) The current in the solenoid is constant. Because the current in the solenoid is constant, the magnetic field due to the solenoid is constant. (b) If the magnetic field due to the solenoid is unchanging and the loop is stationary, the magnetic flux through the loop is unchanging. (c) No current will be induced in the loop. Total Points for Problem: 3 Points Solution to Homework Problem 19.3(Long Time Behavior of RL Circuit) Problem: A battery with potential difference V is connected to an inductor with inductance L and a resistor with resistance R . What is the current flowing in the circuit a long time after the battery is connected? Select One of the Following: (a) (b) V/L (cAnswer) V/R (d) L/R (e) LR Solution After a long time, the current in an RL circuit is no longer changing and therefore the potential difference across the inductor is zero. The loop equation for the circuit becomes V IR 0 = 0 , and therefore the current in the circuit is I = V R . Total Points for Problem: 2 Points Solution to Homework Problem 19.4(Decaying RL Circuit) Problem: You measure the voltage curve below across a 1 resistor through which an inductor is discharging. What is the inductance of the inductor? Select One of the Following: (a) 4H (b) 12H (cAnswer) 2H (d) 1H (e) 6H 2 1 2 3 4 5 t(sec) 1 2 3 4 5 6 7 8 9 V(Volts) Solution Discharging, so V ( t ) = V exp t/ Substituting in values from the graph, V ( t ) = 6V = 10V exp( 1s / ) . Solving yields = 1 . 96s , and since = L/R, L = 2H Total Points for Problem: 3 Points Solution to Homework Problem 19.5(Time Delay RL Circuit) Problem: You wish to build a time delay circuit using a 75H inductor. You want the voltage across the inductor to fall to 3 . 0V from an initial voltage of 12V in a period of...
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 Spring '08
 Stewart
 Physics, Current, Work

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