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.
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.
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
Horizontal axis (x): s: Packet width σ / ℓ
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| s: Packet width σ / ℓ | MEAN 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
- 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.
- 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.
- 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.
- The interpolation error in the input is |s_line − s| = 0.0526911 ℓ. This is larger than the requested numerical tolerance.
- 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.
- 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.
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.
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
Horizontal axis (x): theta: Tilt of ⟨S⟩ from B, θ / °
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| theta: Tilt of ⟨S⟩ from B, θ / ° | TORQUE |⟨μ⟩ × B| / × 10⁻²⁴ N m |
|---|---|
| 19.125 | 3.041980691194412 |
| 35.6625 | 5.413127338944617 |
| 52.2 | 7.336431258905811 |
| 68.73750000000001 | 8.652772293736442 |
| 85.275 | 9.25324597420337 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |theta_line − theta| = 12.4314 °. This is larger than the requested numerical tolerance.
- 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 °.
- 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.
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.
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
Horizontal axis (x): bp: Field on the proton / T
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| bp: Field on the proton / T | PROTON 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
- 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.
- 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.
- 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.
- The interpolation error in the input is |bp_line − bp| = 2.22045e-16 T. This is within the requested numerical tolerance.
- 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.
- 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.
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.
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
Horizontal axis (x): d: Detuning f − f₀ / kHz
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| d: Detuning f − f₀ / kHz | FLIP CEILING |
|---|---|
| 0.5250000000000004 | 0.9426692250645016 |
| 2.3625000000000003 | 0.44811818785045915 |
| 4.2 | 0.20440203174166882 |
| 6.0375 | 0.11058139086901103 |
| 7.875 | 0.06810163644959454 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |d_line − d| = 2.15283 kHz. This is larger than the requested numerical tolerance.
- 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.
- 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.