Quantum Mechanics Assessed Problems 2
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Quantum Mechanics Assessed Problems 2
Angular momentum
The spherical harmomics Ylm(θ,ϕ), l = 0, 1, 2, 3, . . . and m = -l, -l + 1,...,l - 1,l, are an orthonormal basis of functions satisfying:
The Ylm(θ,ϕ) are simultaneous eigenfunctions of L(ˆ)2 and L(ˆ)3 :
The inner product between two functions of polar coordinates r,θ,ϕ is
A function f(r,θ,ϕ) is normalised if < f, f >= 1.
1. This question is about a single particle in 3D, in potential V (x).
(a) The time derivative of the expectation value of an observable A is:
where the notation〈B(ˆ))is shorthand for〈ψ, B(ˆ)ψ) for any operator B(ˆ) .
The commutators of the Hamiltonian with the position and momentum operators are [see Chapter 2 Exercise 2.8]:
i. Use the formulae above to show that the time derivative of the expectation value of the angular momentum is given by
for i = 1, 2, 3.
[NOTE: Equation (5) has 3 components. You only need to show it holds for i = 1. The other two calculations for i = 2 and i = 3 are similar. Alternatively, If you are happy with index notation you can use the Levi-Civita epsilon tensor to derive the equation.]
ii. From (5), explain in one sentence why
when the potential V is a central potential V = V (r) where r = | x | .
(b) The particle has normalised wavefunction [5 marks]
where f(r) satisfies 10∞ |f(r)| 2r2 dr = 1, and a,b,c,d are complex numbers.
i. What condition do the complex coefficients a,b,c and d satisfy for ψ to be nor- malised?
ii. Find the expectation value of L2 in state ψ? What are the possible outcomes of a measurement of L2 and what are their probabilities?
iii. What are the possible outcomes of a measurement of L3 and what are their prob- abilities? [5 marks]
(c) i. State the reason that L2 and L3 can be measured simultaneously.
ii. L2 and L3 are measured simultaneously in state ψ . What is the probability for obtaining value 22 for L2 and 0 for L3?
iii. L2 is measured in the state ψ. The wavefunction collapses. Immediately after this, L3 is measured. Show that the probability of obtaining value 2 2 for L2 and then 0 for L3 in this sequence of measurements, one immediately after the other, is the same as the probability in the previous part for obtaining value 2 2 for L2 and 0 for L3 when they are measured simultaneously.
[Hint: the wavefunction must be normalised after it collapses.] [5 marks]
2024-01-31