Physics Electronics PMDC Conceptual Practice
PMDC Verified Question 43 of 494
A change of \(0.1\text{ V}\) in the forward voltage across a \(\text{p-n}\) junction diode causes the forward current to increase by \(5.0\text{ mA}\). What is the dynamic forward resistance of the diode?
A
\(50.0\,\Omega\)
B
\(20.0\,\Omega\)
C
\(0.5\,\Omega\)
D
\(2.0\,\Omega\)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \(20.0\,\Omega\)
Concept:

Dynamic resistance (\(r_d\)) is defined as the ratio of an incremental change in forward voltage (\(\Delta V_f\)) to the resulting incremental change in forward current (\(\Delta I_f\)).

Formula:

$$r_d = \frac{\Delta V_f}{\Delta I_f}$$

Solution:

  • Given values: \(\Delta V_f = 0.1\text{ V}\), \(\Delta I_f = 5.0\text{ mA} = 5.0 \times 10^{-3}\text{ A}\).


  • $$r_d = \frac{0.1\text{ V}}{5.0 \times 10^{-3}\text{ A}} = \frac{100}{5} = 20.0\,\Omega$$


Why other options are incorrect:

  • Option A: \(50.0\,\Omega\) results from an incorrect arithmetic calculation: \(5.0 / 0.1\).
  • Option C: \(0.5\,\Omega\) is off by a factor of 40 due to unit conversion errors.
  • Option D: \(2.0\,\Omega\) is off by a decimal place.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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