FE Electrical & Computer Practice Question Solution included after investigation
CASE E-020 Β· FE

The Capacitor That Drew Too Much Current

The steady-state current is zero. At switch-on, the circuit violates its 5 mA startup limit.

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Case E-020 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

Follow the investigation process used in the field β€” in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A 10 V DC source is connected through a 1 kΞ© resistor to an initially uncharged 1000 Β΅F capacitor. The design review says the capacitor is an open circuit on DC, so the branch current should be zero. In testing, the startup current reaches 10 mA, exceeding a 5 mA design requirement. Diagnose the missing initial-condition check, calculate the time constant, and choose a resistor that satisfies the startup-current limit.

Linear Systems Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-020
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DEBRIEF (LOCKED)

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Find the root cause, confirm the fix, and see how this connects to the exam.

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CASE CLOSED Β· ENGINEERING VERDICT

What actually failed β€” and what should change.

What violated the limit

The initially uncharged capacitor made the 1 kΞ© resistor carry 10 mA at t = 0+.

What was missed

The open-circuit capacitor rule is a DC steady-state result, not the switching-instant model.

Corrected model

Use capacitor-voltage continuity first, then calculate i(0+) and the RC time constant.

QUICK ANSWER

An initially uncharged capacitor has 0 V across it at t = 0+, so a series resistor initially sees the full source voltage. With 10 V and 1 kΞ©, the initial current is 10 mA; it then decays exponentially with time constant RC.

ENGINEERING TAKEAWAY

Use the capacitor's initial voltage at t = 0+ before applying the steady-state model. For an initially uncharged RC circuit, i(0+) = V/R and the current then decays exponentially with time constant RC.

CASE QUESTIONS

Questions this case should settle.

Why does an uncharged capacitor draw current when first connected to DC?

Its voltage cannot change instantaneously, so it begins at its initial voltage and the series resistor initially carries current set by the source voltage and resistance.

What is the initial current in a 10 V, 1 kΞ© RC circuit with an uncharged capacitor?

i(0+) = 10 V / 1 kΞ© = 10 mA.

What is the time constant for 1 kΞ© and 1000 Β΅F?

Ο„ = RC = 1 second.

NEXT FE PRACTICE QUESTION

The Transfer Function That Settled at 5, Not 20

Linear Systems Β· 5–10 min

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WHAT YOU’LL LEARN

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Each case is designed to build the judgment, analysis, and confidence you need for engineering exams β€” and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
Do I need prior knowledge to use these cases?

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What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.