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University Quantum Mechanics I

University Quantum Mechanics I · Angular Momentum · 12.03

Commutation relations: [Lₓ, Ly] = i ℏ Lz

Expanding the components against [xᵢ, pⱼ] = i ℏ δᵢⱼ gives [Lₓ, Ly] = i ℏ Lz and its two cyclic partners, so no complete basis of common eigenstates of two components exists and no state with l > 0 has a definite direction for L.

Course-map guide · not a complete lesson or simulation

Scope & orientation

What this subsection covers

Expanding the components against [xᵢ, pⱼ] = i ℏ δᵢⱼ gives [Lₓ, Ly] = i ℏ Lz and its two cyclic partners, so no complete basis of common eigenstates of two components exists and no state with l > 0 has a definite direction for L.

A strong response uses polar plots of |yₗm(θ, φ)|² and states where the model stops being reliable.

Reasoning checklist

Evidence, assumptions and limits

01

Assumptions to state

State the system, observable, approximation, and conditions held fixed before using a model.

02

Evidence to collect

Do matrices built only from the Lₚₘ matrix elements reproduce [Lₓ, Ly] = i ℏ Lz, and does L² come out as ℏ² l(l+1) times the identity for each l?

Read the complete note

Do matrices built only from the Lₚₘ matrix elements reproduce [Lₓ, Ly] = i ℏ Lz, and does L² come out as ℏ² l(l+1) times the identity for each l? Useful evidence includes matrices for l = 1, 2 and 3 built from the Lₚₘ elements, commutator residuals near machine ε, L² equal to ℏ² l(l+1) times I, Lₚₗᵤₛ annihilating m = l, and the algebra fixing the spectrum, not which l occurs..

03

Limits to state

This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus.

Read the complete note

This GioPhysics course map is an adaptable learning sequence, not academic credit, accreditation, or a universal university syllabus. It is a first dedicated quantum-mechanics course, distinct from University Physics V, which covers quantum mechanics alongside atomic, nuclear and particle physics in twenty units: this course is narrower, slower, and teaches the linear algebra it needs rather than assuming it. Hydrogen appears here as an introduction, with the full radial derivation belonging to a second course. A midterm examination is assumed around week 8. Follow your institution's published scope, notation, laboratory programme, and assessment rules. A result should be checked against units, signs, limiting cases, and the conditions under which its model was derived.

Diagram & examples

Work the claim before choosing an equation

Interactive concept map

Follow the model from claim to evidence.

01 · Physical claimCommutation relations: [Lₓ, Ly] = i ℏ Lz

Expanding the components against [xᵢ, pⱼ] = i ℏ δᵢⱼ gives [Lₓ, Ly] = i ℏ Lz and its two cyclic partners, so no complete basis of common eigenstates of two components exists and no state with l > 0 has a definite direction for L.

Read the complete note

Expanding the components against [xᵢ, pⱼ] = i ℏ δᵢⱼ gives [Lₓ, Ly] = i ℏ Lz and its two cyclic partners, so no complete basis of common eigenstates of two components exists and no state with l > 0 has a definite direction for L. The canonical commutator behind it is a postulate.

02 · RepresentationThe subsection's claim

Model, evidence, and boundary

03 · TestPrediction before measurement

What is the strongest test of a claim about Commutation relations: [Lₓ, Ly] = i ℏ Lz?

04 · Evidence & boundaryDecide, then qualify

Do matrices built only from the Lₚₘ matrix elements reproduce [Lₓ, Ly] = i ℏ Lz, and does L² come out as ℏ² l(l+1) times the identity for each l?

Read the complete note

Do matrices built only from the Lₚₘ matrix elements reproduce [Lₓ, Ly] = i ℏ Lz, and does L² come out as ℏ² l(l+1) times the identity for each l? Useful evidence includes matrices for l = 1, 2 and 3 built from the Lₚₘ elements, commutator residuals near machine ε, L² equal to ℏ² l(l+1) times I, Lₚₗᵤₛ annihilating m = l, and the algebra fixing the spectrum, not which l occurs..

Interactive diagram for Commutation relations: [Lₓ, Ly] = i ℏ Lz: follow the physical claim through its representation, proposed test, evidence, and model boundary.

modelModel, evidence, and boundary turns the stated idea into a representation that can make a prediction.

Example questions

Try the reasoning before revealing the structure.

Diagram check

What is the strongest test of a claim about Commutation relations: [Lₓ, Ly] = i ℏ Lz?

Quick check

Test the reasoning, not recall

What is the strongest test of a claim about Commutation relations: [Lₓ, Ly] = i ℏ Lz?

Choose an answer to test the model.