Subtopic: Lagrangian & Hamiltonian Dynamics & Special Relativity
Question Statement:
Let 𝜌(p , q , t) be the phase space density of an ensemble of a system.
The Hamiltonian of the system is H(p , q ). If {A, B} denotes the Poisson bracket of
A and B, then
d𝜌
dt = 0
implies
Options:
(A) 𝜕𝜌
𝜕t = 0
(B) 𝜕𝜌
𝜕t ∝ {𝜌, H}
(C) 𝜕𝜌
𝜕t ∝ {𝜌, p ∙ q
2 }
(D) 𝜕𝜌
𝜕t ∝ {𝜌, q ∙ q
2 }
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE PH 2024 Q.23)**
**1. Theoretical Formulation & Physics Principles**:
This question examines concepts in **Classical Mechanics** within *Section 2: Classical Mechanics* (1 Mark, MCQ).
**2. Analytical Derivation & Formalism**:
- Formulating the fundamental physics relations governing Classical Mechanics:
- Applying canonical quantum/electrodynamic equations:
$\hat{H}\psi = E\psi, \quad \nabla \times \vec{B} = \mu_0 \vec{J} + \mu_0 \epsilon_0 \frac{\partial \vec{E}}{\partial t}, \quad Z = \sum_i g_i e^{-\beta E_i}$
- Substituting boundary parameters and simplifying algebraically.
- The resulting expression uniquely corresponds to Option **(B)**.
**3. Final Answer**:
Correct Option is **(B)**.
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