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Question #43772

Difficulty: 8/10 | 🟢 Free

The Question

In a precision photoelectric experiment, a metallic emitter (work function Φ\Phi) is maintained at temperature TT and illuminated by monochromatic light of photon energy hν>Φh\nu > \Phi. Classically, the photocurrent I(V)I(V) exhibits a sharp cutoff, dropping to zero at the stopping potential V0=(hν−Φ)/eV_0 = (h\nu - \Phi)/e. However, due to thermal energy partitioning, the initial energies of electrons inside the metal follow the Fermi-Dirac distribution f(E)=[exp⁡(E/kBT)+1]−1f(E) = \left[ \exp(E/k_B T) + 1 \right]^{-1}, where EE is the energy measured relative to the Fermi level. Assuming the density of states is constant and the emission probability is uniform for all electrons energetically capable of reaching the collector against a retarding potential VV, the saturation curve blurs into a continuous tail. Evaluate the ratio I(V1)I(V0)\frac{I(V_1)}{I(V_0)}, where V1=V0+kBTeV_1 = V_0 + \frac{k_B T}{e}.

Options

  • A.

    ln⁡(1+e−1)ln⁡2\frac{\ln(1 + e^{-1})}{\ln 2}

  • B.

    e−1e^{-1}

  • C.

    1eln⁡2\frac{1}{e \ln 2}

  • D.

    ln⁡(1+e)ln⁡2\frac{\ln(1 + e)}{\ln 2}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Intuition_Breaker

Train your brain to resist "obvious" answers that are designed to deceive your intuition.

Concepts Behind This Question

Heisenberg's Limitation|Photoelectric Saturation|Energy Partitioning

Why This Question Is Tricky

Common Mistakes

This question targets the misconception that the photoelectric saturation curve has a sharp, absolute cutoff at eV0=hν−ΦeV_0 = h\nu - \Phi.

Students often fail to account for the thermal energy partitioning of electrons in the metal, which follows the Fermi-Dirac distribution.

A common trap is to either assume the current is strictly zero for V>V0V > V_0, or to incorrectly apply the classical Maxwell-Boltzmann approximation (f(E)≈e−E/kBTf(E) \approx e^{-E/k_B T}) near the Fermi level, which would erroneously yield a ratio of e−1e^{-1} (Option B).

By forcing the evaluation of the exact Fermi-Dirac integral at the classical boundary, the problem tests the student's ability to reconcile quantum statistical mechanics with the semi-classical photoelectric equation.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterDual Nature of Radiation and Matter - Class 12
ConceptHeisenberg's Limitation
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree