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University Quantum Mechanics II · unit UQMII-03

Spin and Charge in Electromagnetic Fields

Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.

Extra challenging questions

A selection from this pathway. Open a topic below for its complete practice and challenge set.

Extra challenging · 01

The Heisenberg picture, conservation and Ehrenfest's theorem: infer an input with uncertainty

In the Heisenberg picture the state stays at |ψ(0)⟩ and each observable carries the time dependence, A_H(t) = U†(t)AU(t), evolving by dA_H/dt = (i/ħ)[H_H, A_H] + (∂A/∂t)_H, with every prediction the same as in the Schrödinger picture. Use the supplied topic model Y_out = -(x0+b*(x0^3+3*x0*s^2)), where Y_out is MEAN FORCE ⟨F⟩ = d⟨p⟩/dt (kℓ). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. s is Packet width σ, measured in ℓ. Keep x0 (Packet centre ⟨x⟩) = 0.8 ℓ; b (Anharmonicity β = 4λℓ²/k) = 0.6 fixed. For this exercise use only 0.153125 ℓ ≤ s ≤ 0.546875 ℓ on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = -1.3299604 kℓ, with a bounded reading uncertainty ±0.028349 kℓ. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown s by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.000039375 ℓ, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

MEAN FORCE ⟨F⟩ = d⟨p⟩/dt against s

Predictions from the supplied The Heisenberg picture, conservation and Ehrenfest's theorem model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): MEAN FORCE ⟨F⟩ = d⟨p⟩/dt / kℓ

MEAN FORCE ⟨F⟩ = d⟨p⟩/dt against sPredictions from the supplied The Heisenberg picture, conservation and Ehrenfest's theorem model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: s: Packet width σ / ℓ. Vertical axis: MEAN FORCE ⟨F⟩ = d⟨p⟩/dt / kℓ. Exact values are available in the data table below.-1.554-1.447-1.339-1.232-1.1250.13740.24370.350.45630.5626xy

Horizontal axis (x): s: Packet width σ / ℓ

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1s: Packet width σ / ℓ: 0.1531MEAN FORCE ⟨F⟩ = d⟨p⟩/dt / kℓ: -1.141
Read the exact data as a table
MEAN FORCE ⟨F⟩ = d⟨p⟩/dt against s — supplied values
s: Packet width σ / ℓMEAN FORCE ⟨F⟩ = d⟨p⟩/dt / kℓ
0.153125-1.1409640625000002
0.2515625-1.198328515625
0.35-1.2836
0.4484375-1.396778515625
0.546875-1.5378640625000002

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(s) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0.153125) = -1.14096 kℓ and Y_out(0.546875) = -1.53786 kℓ. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives s_line = 0.153125 + (-1.32996 − -1.14096)(0.546875 − 0.153125)/(-1.53786 − -1.14096) = 0.340621 ℓ. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives s = 0.393312 ℓ; substituting back gives Y_out = -1.3299604 kℓ.
  4. The interpolation error in the input is |s_line − s| = 0.0526911 ℓ. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = -1.3583094 kℓ and -1.3016114 kℓ. Sort the roots even if the response decreases: 0.367434 ℓ ≤ s ≤ 0.41759 ℓ.
  6. The lower and upper input deviations are 0.0258783 ℓ and 0.0242776 ℓ. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer s ≈ 0.393312 ℓ; compatible interval [0.367434, 0.41759] ℓ. Endpoint interpolation gives 0.340621 ℓ.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 02

The spin magnetic moment and the Hamiltonian H = −μ⋅B: infer an input with uncertainty

The spin magnetic moment is the vector operator μ = γS through which a particle's spin couples to a magnetic field, H = −μ⋅B; for the electron γ = −g_e e/2mₑ with g_e = 2.0023, so μ points opposite to S. Use the supplied topic model Y_out = 9.2848*field*sind(theta), where Y_out is TORQUE |⟨μ⟩ × B| (× 10⁻²⁴ N m). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sind/cosd/tand use degrees. theta is Tilt of ⟨S⟩ from B, θ, measured in °. Keep field (Field B at the spin) = 1 T; grad (Gradient ∂B_z/∂z) = 0 T m⁻¹ fixed. For this exercise use only 19.125 ° ≤ theta ≤ 85.275 ° on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 7.998121 × 10⁻²⁴ N m, with a bounded reading uncertainty ±0.18827 × 10⁻²⁴ N m. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown theta by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.006615 °, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

TORQUE |⟨μ⟩ × B| against theta

Predictions from the supplied The spin magnetic moment and the Hamiltonian H = −μ⋅B model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): TORQUE |⟨μ⟩ × B| / × 10⁻²⁴ N m

TORQUE |⟨μ⟩ × B| against thetaPredictions from the supplied The spin magnetic moment and the Hamiltonian H = −μ⋅B model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: theta: Tilt of ⟨S⟩ from B, θ / °. Vertical axis: TORQUE |⟨μ⟩ × B| / × 10⁻²⁴ N m. Exact values are available in the data table below.2.7944.4716.1487.8259.50216.4834.3452.270.0687.92xy

Horizontal axis (x): theta: Tilt of ⟨S⟩ from B, θ / °

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1theta: Tilt of ⟨S⟩ from B, θ / °: 19.13TORQUE |⟨μ⟩ × B| / × 10⁻²⁴ N m: 3.042
Read the exact data as a table
TORQUE |⟨μ⟩ × B| against theta — supplied values
theta: Tilt of ⟨S⟩ from B, θ / °TORQUE |⟨μ⟩ × B| / × 10⁻²⁴ N m
19.1253.041980691194412
35.66255.413127338944617
52.27.336431258905811
68.737500000000018.652772293736442
85.2759.25324597420337

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(theta) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(19.125) = 3.04198 × 10⁻²⁴ N m and Y_out(85.275) = 9.25325 × 10⁻²⁴ N m. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives theta_line = 19.125 + (7.99812 − 3.04198)(85.275 − 19.125)/(9.25325 − 3.04198) = 71.9079 °. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives theta = 59.4765 °; substituting back gives Y_out = 7.998121 × 10⁻²⁴ N m.
  4. The interpolation error in the input is |theta_line − theta| = 12.4314 °. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 7.809851 × 10⁻²⁴ N m and 8.186391 × 10⁻²⁴ N m. Sort the roots even if the response decreases: 57.2611 ° ≤ theta ≤ 61.8479 °.
  6. The lower and upper input deviations are 2.21541 ° and 2.37139 °. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer theta ≈ 59.4765 °; compatible interval [57.2611, 61.8479] °. Endpoint interpolation gives 71.9079 °.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 03

Larmor precession and the electron and proton rates: infer an input with uncertainty

Larmor precession is the rotation of a spin's expectation value ⟨S⟩ about a uniform magnetic field at the angular frequency ω₀ = |γ|B₀, with its component along the field, and so its tilt from the field, held fixed. Use the supplied topic model Y_out = -0.042577*bp*t, where Y_out is PROTON TURNS (+ = CCW) (turns). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. bp is Field on the proton, measured in T. Keep be (Field on the electron) = 1 mT; t (Time elapsed t) = 5 ns fixed. For this exercise use only 0.2125 T ≤ bp ≤ 1.7875 T on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = -0.24976733 turns, with a bounded reading uncertainty ±0.019615 turns. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown bp by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0001575 T, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

PROTON TURNS (+ = CCW) against bp

Predictions from the supplied Larmor precession and the electron and proton rates model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): PROTON TURNS (+ = CCW) / turns

PROTON TURNS (+ = CCW) against bpPredictions from the supplied Larmor precession and the electron and proton rates model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: bp: Field on the proton / T. Vertical axis: PROTON TURNS (+ = CCW) / turns. Exact values are available in the data table below.-0.3939-0.3034-0.2129-0.1224-0.031830.14950.574711.4251.85xy

Horizontal axis (x): bp: Field on the proton / T

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1bp: Field on the proton / T: 0.2125PROTON TURNS (+ = CCW) / turns: -0.04524
Read the exact data as a table
PROTON TURNS (+ = CCW) against bp — supplied values
bp: Field on the proton / TPROTON TURNS (+ = CCW) / turns
0.2125-0.045238062499999995
0.60625-0.12906153124999997
1-0.212885
1.3937499999999998-0.29670846874999995
1.7874999999999999-0.3805319374999999

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(bp) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0.2125) = -0.0452381 turns and Y_out(1.7875) = -0.380532 turns. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives bp_line = 0.2125 + (-0.249767 − -0.0452381)(1.7875 − 0.2125)/(-0.380532 − -0.0452381) = 1.17325 T. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives bp = 1.17325 T; substituting back gives Y_out = -0.24976733 turns.
  4. The interpolation error in the input is |bp_line − bp| = 2.22045e-16 T. This is within the requested numerical tolerance.
  5. Invert both limits Y_out = -0.26938233 turns and -0.23015233 turns. Sort the roots even if the response decreases: 1.08111 T ≤ bp ≤ 1.26539 T.
  6. The lower and upper input deviations are 0.0921389 T and 0.0921389 T. They agree on this branch; this does not establish linearity for changes in other inputs or outside the interval. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer bp ≈ 1.17325 T; compatible interval [1.08111, 1.26539] T. Endpoint interpolation gives 1.17325 T.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 04

Magnetic resonance solved exactly in the rotating frame: infer an input with uncertainty

Magnetic resonance is the flipping of a spin in a static field B₀ by a weak field B₁ rotating about B₀ near the Larmor frequency; in the frame that turns with B₁ the Hamiltonian is static, so the flip probability follows exactly. Use the supplied topic model Y_out = (0.042577*b)^2/((0.042577*b)^2+d^2), where Y_out is FLIP CEILING (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. d is Detuning f − f₀, measured in kHz. Keep b (Rotating field B₁) = 50 μT; t (Pulse length t) = 0.23 ms fixed. For this exercise use only 0.525 kHz ≤ d ≤ 7.875 kHz on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.15301992, with a bounded reading uncertainty ±0.012738. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown d by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.000735 kHz, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

FLIP CEILING against d

Predictions from the supplied Magnetic resonance solved exactly in the rotating frame model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): FLIP CEILING

FLIP CEILING against dPredictions from the supplied Magnetic resonance solved exactly in the rotating frame model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: d: Detuning f − f₀ / kHz. Vertical axis: FLIP CEILING. Exact values are available in the data table below.0.033120.26930.50540.74150.97770.2312.2164.26.1858.169xy

Horizontal axis (x): d: Detuning f − f₀ / kHz

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1d: Detuning f − f₀ / kHz: 0.525FLIP CEILING: 0.9427
Read the exact data as a table
FLIP CEILING against d — supplied values
d: Detuning f − f₀ / kHzFLIP CEILING
0.52500000000000040.9426692250645016
2.36250000000000030.44811818785045915
4.20.20440203174166882
6.03750.11058139086901103
7.8750.06810163644959454

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(d) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0.525) = 0.942669 and Y_out(7.875) = 0.0681016. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives d_line = 0.525 + (0.15302 − 0.942669)(7.875 − 0.525)/(0.0681016 − 0.942669) = 7.16133 kHz. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives d = 5.0085 kHz; substituting back gives Y_out = 0.15301992.
  4. The interpolation error in the input is |d_line − d| = 2.15283 kHz. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.14028192 and 0.16575792. Sort the roots even if the response decreases: 4.77589 kHz ≤ d ≤ 5.27014 kHz.
  6. The lower and upper input deviations are 0.232614 kHz and 0.261641 kHz. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer d ≈ 5.0085 kHz; compatible interval [4.77589, 5.27014] kHz. Endpoint interpolation gives 7.16133 kHz.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

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