Repair for missing figure — “The capacitor voltage at the beginning of the second pulse is ______. Assume the capacitor is initially uncharged.” Without the component values, period, and τ relative to the pulse timing, can we determine the exact voltage?
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ACannot be determined from the information provided
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BZero for all RC networks
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CAlways equals the pulse amplitude
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DAlways 36.8% of the amplitude
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EAlways 63.2% of the amplitude
Answer
Correct Answer: Cannot be determined from the information provided
Explanation
Introduction / Context:The initial voltage of a capacitor at the start of a subsequent pulse depends on how much it charged during the previous high interval and how much it discharged during the low interval. Both are governed by τ = R * C and the specific high/low times (duty cycle). With no numeric values or figure, the exact voltage cannot be computed.
Given Data / Assumptions:
- Capacitor starts uncharged at t = 0.
- Repetitive pulse train drives an RC network.
- Component values and period/duty are not provided.
Concept / Approach:Over one period, the capacitor charges toward a level during “high” and discharges during “low,” each following v_C(t) = V_final + (V_initial − V_final) * exp(−t/τ). The steady-state beginning-of-pulse value is found by equating the end of one interval to the start of the next. This requires τ and the timing parameters; absent them, there is no unique voltage.
Step-by-Step Solution:
Let v1 be the start-of-high voltage and v2 the end-of-high.Compute v2 from v1 using the charge exponential over t_high.Compute the start of the next pulse from v2 using the discharge exponential over t_low.Solve for steady-state v1; without τ and timings, this is indeterminate.Verification / Alternative check:Examples with τ ≪ period produce near-zero start voltages; τ ≫ period produces significant memory (v1 near prior average), proving dependence on missing data.
Why Other Options Are Wrong:
- Fixed percentages (36.8%, 63.2%) correspond to exactly 1τ intervals, not universal outcomes.
- “Always equals amplitude” or “always zero” are special cases only.
Common Pitfalls:Assuming the 1τ percentages apply to any arbitrary pulse timing; they apply only when interval length equals τ.
Final Answer:Cannot be determined from the information provided.