FE Electrical & Computer Practice Question Solution included after investigation
CASE E-031 · FE

The 8 Ohm Load That Became 128 Ohms

The secondary load was 8 Ω. The primary source did not see 8 Ω—or even 32 Ω.

▶ INTERACTIVE CASE FILE • 5–10 MIN • CLICK TO INVESTIGATE

Inspect, commit, prove, fix, and sign off.

Case E-031 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

Follow the investigation process used in the field — 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

An ideal transformer has Np:Ns = 4:1 and an 8 Ω resistive load on the secondary. A review note says the primary sees 32 Ω. With 120 V rms applied to the primary, measured primary current is about 0.94 A, corresponding to 128 Ω. Diagnose the turns-ratio error and verify voltage, current, and reflected impedance.

Power Systems Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-031
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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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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🔒Exam takeawayComplete the case
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

The primary impedance was predicted as 32 Ω instead of 128 Ω.

What was assumed

Impedance was scaled linearly with turns ratio.

Corrected model

Zref = a² Zload.

QUICK ANSWER

For an ideal transformer, impedance referred to the primary is Zp = (Np/Ns)² Zs. With a 4:1 turns ratio and an 8 Ω load, the primary sees 16×8 = 128 Ω.

ENGINEERING TAKEAWAY

Voltage scales as a, current scales inversely as 1/a, so impedance V/I scales as a².

CASE QUESTIONS

Questions this case should settle.

How do you reflect impedance through an ideal transformer?

Multiply by the square of the turns ratio in the direction of reflection.

What does an 8 Ω load look like through a 4:1 transformer from the primary side?

It appears as 128 Ω.

NEXT FE PRACTICE QUESTION

The Feeder That Delivered 228 V Instead of 240 V

Power Systems · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — 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?

Basic subject familiarity helps, but every case is designed to teach through the investigation itself.

How long does a case take?

Most cases are designed for a focused 5–10 minute investigation.

Are the cases aligned with engineering exams?

Each case is mapped to a verified exam, subject, topic, and misconception before publication.

What happens after I complete a case?

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