The time constant formula for an RC circuit.

• A. t = RC
• B. t = CR
• C. t = R/C
• D. t = RC^2

Answer: (A)

The time constant tells you the speed of the circuit’s exponential response. For an RC circuit, it is defined as the product of the resistance and the capacitance: t = RC. Since ohms times farads give seconds, this quantity is a time measure. After one time constant, charging brings the capacitor to about 63% of its final voltage, while discharging leaves about 37% of the initial voltage. Forms like R/C or RC^2 don’t have the correct dimensional meaning or behavior, whereas the product RC (or CR, which is the same value) correctly represents the circuit’s time scale.