Question
question 105
A circuit contains a resistor in series with a capacitor, the series combination being connected across the terminals of a battery, as in Figure 20.37a. The time constant for charging the capacitor is 1.5 s when the resistor has a resistance of 2.0 Times 10^4 Ohm. What would the time constant be if the resistance had a value of 5.2 Times 10^4 Ohm? The circuit in the drawing contains two resistors and two capacitors that are connected to a battery via a switch. When the switch is closed, the capacitors begin to charge up. What is the time constant for the charging process?
Explanation / Answer
First combine the parallel resistance
Req = 4*2/(2+4) = 1.33 kohm
combine the parallel capacitors
Ceq = 3 + 6 = 9 uF
now Time constant = R*C
Time constant = Req*Ceq = 1.33 kohm * 9 uF = 12*10^(-3) = 12 ms