RC charging — time to reach 95%: In a first-order RC circuit, how many time constants (tau = RC) are required for a capacitor’s voltage to charge to approximately 95% of its final value?

Introduction / Context:
Charging in a first-order RC circuit follows an exponential law. Designers and technicians often use simple “time-constant rules of thumb” to estimate how fast a capacitor reaches a certain percentage of its final (steady-state) voltage. This question asks for the number of time constants needed to reach about 95% of the final value.

Given Data / Assumptions:

• First-order RC network with time constant tau = R * C.
• Ideal source and lumped components.
• Target: capacitor voltage Vc reaching 95% of its final value Vf.

Concept / Approach:
The charging law is Vc(t) = Vf * (1 - exp(-t / tau)). To find when Vc/Vf = 0.95, solve 0.95 = 1 - exp(-t / tau), which implies exp(-t / tau) = 0.05 and t / tau = -ln(0.05) ≈ 2.996. Thus, it takes about 3 time constants to hit 95% in an ideal RC charge.

Step-by-Step Solution:

Verification / Alternative check:
Rule-of-thumb milestones: 63.2% at 1 tau, 86.5% at 2 tau, 95% at ~3 tau, 98.2% at 4 tau, and 99.3% at 5 tau. These standard checkpoints confirm the 95% ≈ 3 tau result.

Why Other Options Are Wrong:

• 2: Only reaches ~86.5%, not enough for 95%.
• 4: Reaches ~98.2%, higher than 95% (not the first time constant count meeting 95%).
• 5: Reaches ~99.3%, more conservative than necessary.

Common Pitfalls:
Confusing the 5-tau “essentially fully charged” rule with the 95% mark; mixing discharge and charge curves; forgetting that the time constant is RC and not dependent on source magnitude for the percentage-of-final calculation.

Final Answer:
3 time constants are required to reach approximately 95%.