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University Physics II · unit UPII-03

Gauss's Law

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

Electric Flux: infer an input with uncertainty

The electric flux through a surface is the field component perpendicular to it, summed over the whole surface: Φ_E = ∫ E⋅dA, a signed scalar measured in N m² C⁻¹ that counts how much field crosses. Use the supplied topic model Y_out = s*s, where Y_out is PANEL AREA A (m²). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. s is Panel side, measured in m. Keep t (Angle θ from normal to field) = 55 °; ef (Field magnitude E) = 300 N C⁻¹ fixed. For this exercise use only 0.2425 m ≤ s ≤ 0.5575 m 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 = 0.18892062 m², with a bounded reading uncertainty ±0.018283 m². 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 0.0000315 m, 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

PANEL AREA A against s

Predictions from the supplied Electric Flux 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): PANEL AREA A / m²

PANEL AREA A against sPredictions from the supplied Electric Flux 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: Panel side / m. Vertical axis: PANEL AREA A / m². Exact values are available in the data table below.0.048730.11680.18480.25280.32090.22990.31490.40.4850.5701xy

Horizontal axis (x): s: Panel side / m

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

Reading 1s: Panel side / m: 0.2425PANEL AREA A / m²: 0.05881
Read the exact data as a table
PANEL AREA A against s — supplied values
s: Panel side / mPANEL AREA A / m²
0.24250.05880625
0.321250.10320156249999998
0.39999999999999990.15999999999999992
0.47874999999999990.2292015624999999
0.55749999999999990.3108062499999999

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(0.2425) = 0.0588062 m² and Y_out(0.5575) = 0.310806 m². The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives s_line = 0.2425 + (0.188921 − 0.0588062)(0.5575 − 0.2425)/(0.310806 − 0.0588062) = 0.405143 m. 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 = 0.43465 m; substituting back gives Y_out = 0.18892062 m².
  4. The interpolation error in the input is |s_line − s| = 0.029507 m. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.17063762 m² and 0.20720362 m². Sort the roots even if the response decreases: 0.413083 m ≤ s ≤ 0.455196 m.
  6. The lower and upper input deviations are 0.0215669 m and 0.0205462 m. 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 ≈ 0.43465 m; compatible interval [0.413083, 0.455196] m. Endpoint interpolation gives 0.405143 m.

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

Extra challenging · 02

Gauss's law in integral form: infer an input with uncertainty

Gauss's law says the net electric flux out of any closed surface equals the net charge enclosed by that surface divided by ε₀, whatever the surface's shape and whatever charges sit outside it. Use the supplied topic model Y_out = 89.88*(qin/(rad_*rad_)+qout/((11+rad_)*(11+rad_))), where Y_out is FAR-SIDE E_r (kN C⁻¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. rad_ is Gaussian radius, measured in cm. Keep qin (Enclosed charge q_in) = 3 nC; qout (Outside charge q_out) = 4 nC fixed. For this exercise use only 2.925 cm ≤ rad_ ≤ 6.075 cm 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 = 12.911344 kN C⁻¹, with a bounded reading uncertainty ±0.6558 kN C⁻¹. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown rad_ by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.000315 cm, 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

FAR-SIDE E_r against rad_

Predictions from the supplied Gauss's law in integral form 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): FAR-SIDE E_r / kN C⁻¹

FAR-SIDE E_r against rad_Predictions from the supplied Gauss's law in integral form 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: rad_: Gaussian radius / cm. Vertical axis: FAR-SIDE E_r / kN C⁻¹. Exact values are available in the data table below.7.54614.2520.9527.6634.362.7993.6494.55.3516.201xy

Horizontal axis (x): rad_: Gaussian radius / cm

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

Reading 1rad_: Gaussian radius / cm: 2.925FAR-SIDE E_r / kN C⁻¹: 33.37
Read the exact data as a table
FAR-SIDE E_r against rad_ — supplied values
rad_: Gaussian radius / cmFAR-SIDE E_r / kN C⁻¹
2.92533.370205661343526
3.712499999999999521.224643298843247
4.514.811996762631518
5.287510.999818998354925
6.0749999999999998.53931278505241

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(rad_) = 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(2.925) = 33.3702 kN C⁻¹ and Y_out(6.075) = 8.53931 kN C⁻¹. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives rad__line = 2.925 + (12.9113 − 33.3702)(6.075 − 2.925)/(8.53931 − 33.3702) = 5.52037 cm. 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 rad_ = 4.8465 cm; substituting back gives Y_out = 12.911344 kN C⁻¹.
  4. The interpolation error in the input is |rad__line − rad_| = 0.673872 cm. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 12.255544 kN C⁻¹ and 13.567144 kN C⁻¹. Sort the roots even if the response decreases: 4.71829 cm ≤ rad_ ≤ 4.98546 cm.
  6. The lower and upper input deviations are 0.12821 cm and 0.138958 cm. 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 rad_ ≈ 4.8465 cm; compatible interval [4.71829, 4.98546] cm. Endpoint interpolation gives 5.52037 cm.

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

Extra challenging · 03

Choosing a Gaussian surface: infer an input with uncertainty

A Gaussian surface is an imaginary closed surface chosen so that symmetry makes ∮E⋅dA trivial: on every piece, either E is constant in magnitude and perpendicular to the surface, or the flux through that piece is zero. Use the supplied topic model Y_out = 898.8*q - 89880*p/(d-10)^2, where Y_out is E⊥ NEAR SIDE (N C⁻¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. d is q₂ distance from centre, measured in cm. Keep q (Enclosed charge q₁) = 3 nC; p (Outside charge q₂) = 2 nC fixed. For this exercise use only 20.8687 cm ≤ d ≤ 42.1312 cm 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 = 2380.0837 N C⁻¹, with a bounded reading uncertainty ±21.33 N C⁻¹. 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 0.00212625 cm, 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

E⊥ NEAR SIDE against d

Predictions from the supplied Choosing a Gaussian surface 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): E⊥ NEAR SIDE / N C⁻¹

E⊥ NEAR SIDE against dPredictions from the supplied Choosing a Gaussian surface 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: q₂ distance from centre / cm. Vertical axis: E⊥ NEAR SIDE / N C⁻¹. Exact values are available in the data table below.1121148518482212257620.0225.7631.537.2442.98xy

Horizontal axis (x): d: q₂ distance from centre / cm

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

Reading 1d: q₂ distance from centre / cm: 20.87E⊥ NEAR SIDE / N C⁻¹: 1175
Read the exact data as a table
E⊥ NEAR SIDE against d — supplied values
d: q₂ distance from centre / cmE⊥ NEAR SIDE / N C⁻¹
20.868751174.6831110263106
26.1843749999999962010.120234787501
31.4999999999999962307.519524067063
36.8156252446.413122413025
42.1312499999999942522.2843465848364

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(20.8687) = 1174.68 N C⁻¹ and Y_out(42.1312) = 2522.28 N C⁻¹. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives d_line = 20.8687 + (2380.08 − 1174.68)(42.1312 − 20.8687)/(2522.28 − 1174.68) = 39.8876 cm. 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 = 33.8389 cm; substituting back gives Y_out = 2380.0837 N C⁻¹.
  4. The interpolation error in the input is |d_line − d| = 6.04873 cm. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 2358.7537 N C⁻¹ and 2401.4137 N C⁻¹. Sort the roots even if the response decreases: 33.0736 cm ≤ d ≤ 34.6857 cm.
  6. The lower and upper input deviations are 0.765265 cm and 0.846835 cm. 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 ≈ 33.8389 cm; compatible interval [33.0736, 34.6857] cm. Endpoint interpolation gives 39.8876 cm.

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

Extra challenging · 04

Spherical charge distributions: infer an input with uncertainty

A spherically symmetric charge distribution is one whose charge density depends only on the distance r from a single centre; its field is radial everywhere, and at radius r it is fixed entirely by the charge enclosed within r. Use the supplied topic model Y_out = (539.28/rs^2)*clamp(((rp/rs)^3-c^3)/(1-c^3),0,1)/(rp/rs)^2, where Y_out is FIELD AT PROBE (kV m⁻¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; clamp(x,a,b) confines x to [a,b]. rp is Probe radius r, measured in cm. Keep rs (Outer radius R) = 6 cm; c (Empty core fraction a/R) = 0 fixed. For this exercise use only 6.66687 cm ≤ rp ≤ 17.9281 cm 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 = 2.9431906 kV m⁻¹, with a bounded reading uncertainty ±0.18981 kV m⁻¹. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown rp by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00112612 cm, 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

FIELD AT PROBE against rp

Predictions from the supplied Spherical charge distributions 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): FIELD AT PROBE / kV m⁻¹

FIELD AT PROBE against rpPredictions from the supplied Spherical charge distributions 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: rp: Probe radius r / cm. Vertical axis: FIELD AT PROBE / kV m⁻¹. Exact values are available in the data table below.1.264.0836.9059.72812.556.2169.25712.315.3418.38xy

Horizontal axis (x): rp: Probe radius r / cm

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

Reading 1rp: Probe radius r / cm: 6.667FIELD AT PROBE / kV m⁻¹: 12.13
Read the exact data as a table
FIELD AT PROBE against rp — supplied values
rp: Probe radius r / cmFIELD AT PROBE / kV m⁻¹
6.66687499999999912.13304167304676
9.4821874999999985.9978725980215915
12.29753.5659943576279294
15.1128124999999992.3611508030536323
17.9281249999999981.6778169248910673

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(rp) = 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(6.66687) = 12.133 kV m⁻¹ and Y_out(17.9281) = 1.67782 kV m⁻¹. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives rp_line = 6.66687 + (2.94319 − 12.133)(17.9281 − 6.66687)/(1.67782 − 12.133) = 16.5652 cm. 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 rp = 13.5362 cm; substituting back gives Y_out = 2.9431906 kV m⁻¹.
  4. The interpolation error in the input is |rp_line − rp| = 3.02896 cm. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 2.7533806 kV m⁻¹ and 3.1330006 kV m⁻¹. Sort the roots even if the response decreases: 13.1198 cm ≤ rp ≤ 13.995 cm.
  6. The lower and upper input deviations are 0.416446 cm and 0.458799 cm. 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 rp ≈ 13.5362 cm; compatible interval [13.1198, 13.995] cm. Endpoint interpolation gives 16.5652 cm.

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

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