An uncharged,series RC circuit is connected across a battery.For which situation will the time for the capacitor to reach 90% of its final charge remain unchanged?
A)The RC time constant is doubled.
B)The battery voltage is doubled.
C)A second resistor is added in series with the original resistor.
D)All of these situations result in the time for the capacitor to reach 90% of its final charge to change.

Question

An uncharged,series RC circuit is connected across a battery.For which situation will the time for the capacitor to reach 90% of its final charge remain unchanged?​
A)​The RC time constant is doubled.
B)​The battery voltage is doubled.
C)​A second resistor is added in series with the original resistor.
D)​All of these situations result in the time for the capacitor to reach 90% of its final charge to change.

Quizplus · Accepted Answer

The time constant of a circuit is given by the product of resistance and capacitance (RC). Doubling the RC time constant would result in a longer charging time for the capacitor, causing the time for the capacitor to reach 90% of its final charge to change. Adding a second resistor in series would alter the resistance in the circuit and change the time constant, also altering the charging time for the capacitor. However, if the battery voltage is doubled, the charging current would also double, resulting in the capacitor reaching 90% of its final charge in the same amount of time as before. Therefore, the time for the capacitor to reach 90% of its final charge would remain unchanged.

An Uncharged,series RC Circuit Is Connected Across a Battery