The circuit shown in Fig. P7.89 is used to close the switch between a and b for a

Chapter 23, Problem 89

(choose chapter or problem)

The circuit shown in Fig. P7.89 is used to close the switch between a and b for a predetermined length of time. The electric relay holds its contact arms down as long as the voltage across the relay coil exceeds 5 V. When the coil voltage equals 5 V, the relay contacts return to their initial position by a mechanical spring action. The switch between a and b is initially closed by momentarily pressing the push button. Assume that the capacitor is fully charged when the push button is first pushed down. The resistance of the relay coil is and the inductance of the coil is negligible. a) How long will the switch between a and b remain closed? b) Write the numerical expression for i from the time the relay contacts first open to the time the capacitor is completely charged. c) How many milliseconds (after the circuit between a and b is interrupted) does it take the capacitor to reach 85% of its final value?