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University Physics II · unit UPII-05

Capacitance and Dielectrics

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

Capacitance and parallel plates: infer an input with uncertainty

Capacitance is the charge on either conductor of an oppositely charged pair divided by the potential difference between them: a constant set by their size, shape and spacing, not by how much charge they happen to be carrying. Use the supplied topic model Y_out = d/max(w,1), where Y_out is GAP RATIO d/w (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; max(a,b) selects the larger value. w is Plate side w, measured in mm. Keep d (Gap d) = 2 mm; v (Source voltage v) = 9 V fixed. For this exercise use only 39.125 mm ≤ w ≤ 180.875 mm 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 = 0.015924518, with a bounded reading uncertainty ±0.00073007. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown w by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.014175 mm, 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

GAP RATIO d/w against w

Predictions from the supplied Capacitance and parallel plates 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): GAP RATIO d/w

GAP RATIO d/w against wPredictions from the supplied Capacitance and parallel plates 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: w: Plate side w / mm. Vertical axis: GAP RATIO d/w. Exact values are available in the data table below.0.0094550.020270.031090.04190.0527233.4571.73110148.3186.5xy

Horizontal axis (x): w: Plate side w / mm

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

Reading 1w: Plate side w / mm: 39.13GAP RATIO d/w: 0.05112
Read the exact data as a table
GAP RATIO d/w against w — supplied values
w: Plate side w / mmGAP RATIO d/w
39.1250.051118210862619806
74.56250.026823134953897737
1100.01818181818181818
145.43750.013751611516974646
180.8750.0110573600552868

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(w) = 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(39.125) = 0.0511182 and Y_out(180.875) = 0.0110574. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives w_line = 39.125 + (0.0159245 − 0.0511182)(180.875 − 39.125)/(0.0110574 − 0.0511182) = 163.653 mm. 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 w = 125.592 mm; substituting back gives Y_out = 0.015924518.
  4. The interpolation error in the input is |w_line − w| = 38.0607 mm. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.015194448 and 0.016654588. Sort the roots even if the response decreases: 120.087 mm ≤ w ≤ 131.627 mm.
  6. The lower and upper input deviations are 5.50547 mm and 6.03453 mm. 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 w ≈ 125.592 mm; compatible interval [120.087, 131.627] mm. Endpoint interpolation gives 163.653 mm.

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

Extra challenging · 02

Capacitors in Series & Parallel: infer an input with uncertainty

The equivalent capacitance of a network is the single capacitance that would draw the same charge from the same source: C_eq = Q/ΔV. Parallel members share ΔV and their capacitances add; series members share Q and their reciprocals add. Use the supplied topic model Y_out = c1*c2/(c1+c2), where Y_out is SERIES Cₛ (μF). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. c1 is Capacitance C₁, measured in μF. Keep c2 (Capacitance C₂) = 4 μF; v (Applied ΔV) = 24 V fixed. For this exercise use only 2.38125 μF ≤ c1 ≤ 12.6187 μF 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 = 2.7327862 μF, with a bounded reading uncertainty ±0.045667 μF. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown c1 by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00102375 μF, 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

SERIES Cₛ against c1

Predictions from the supplied Capacitors in Series & Parallel 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): SERIES Cₛ / μF

SERIES Cₛ against c1Predictions from the supplied Capacitors in Series & Parallel 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: c1: Capacitance C₁ / μF. Vertical axis: SERIES Cₛ / μF. Exact values are available in the data table below.1.4311.8482.2652.6823.0991.9724.7367.510.2613.03xy

Horizontal axis (x): c1: Capacitance C₁ / μF

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

Reading 1c1: Capacitance C₁ / μF: 2.381SERIES Cₛ / μF: 1.493
Read the exact data as a table
SERIES Cₛ against c1 — supplied values
c1: Capacitance C₁ / μFSERIES Cₛ / μF
2.38124999999999961.492654260528893
4.9406249999999992.2104159384830475
7.4999999999999992.6086956521739126
10.0593752.861969326517004
12.6187499999999993.037232042121098

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(c1) = 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(2.38125) = 1.49265 μF and Y_out(12.6187) = 3.03723 μF. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives c1_line = 2.38125 + (2.73279 − 1.49265)(12.6187 − 2.38125)/(3.03723 − 1.49265) = 10.6009 μF. 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 c1 = 8.62613 μF; substituting back gives Y_out = 2.7327862 μF.
  4. The interpolation error in the input is |c1_line − c1| = 1.97475 μF. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 2.6871192 μF and 2.7784532 μF. Sort the roots even if the response decreases: 8.18694 μF ≤ c1 ≤ 9.09815 μF.
  6. The lower and upper input deviations are 0.439185 μF and 0.472022 μF. 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 c1 ≈ 8.62613 μF; compatible interval [8.18694, 9.09815] μF. Endpoint interpolation gives 10.6009 μF.

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

Extra challenging · 03

Energy Stored in Capacitors: infer an input with uncertainty

The energy stored in a capacitor is the work done against its own rising potential difference while the charge is put on: U = Q²/(2C) = ½QΔV = ½C(ΔV)², the three forms agreeing because Q = CΔV. Use the supplied topic model Y_out = 0.5*C*V*V, where Y_out is STORED U = ½C(ΔV)² (μJ). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. V is Final p.d. ΔV, measured in V. Keep C (Capacitance C) = 10 μF fixed. For this exercise use only 7.9125 V ≤ V ≤ 22.0875 V 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 = 1371.0438 μJ, with a bounded reading uncertainty ±158.7 μJ. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown V by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0014175 V, 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

STORED U = ½C(ΔV)² against V

Predictions from the supplied Energy Stored in Capacitors 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): STORED U = ½C(ΔV)² / μJ

STORED U = ½C(ΔV)² against VPredictions from the supplied Energy Stored in Capacitors 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: V: Final p.d. ΔV / V. Vertical axis: STORED U = ½C(ΔV)² / μJ. Exact values are available in the data table below.228802.11376195025247.34511.171518.8322.65xy

Horizontal axis (x): V: Final p.d. ΔV / V

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

Reading 1V: Final p.d. ΔV / V: 7.912STORED U = ½C(ΔV)² / μJ: 313
Read the exact data as a table
STORED U = ½C(ΔV)² against V — supplied values
V: Final p.d. ΔV / VSTORED U = ½C(ΔV)² / μJ
7.9125313.03828125
11.456249999999999656.2283203124998
151125
18.543751719.3533203125
22.08752439.2882812499997

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(V) = 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(7.9125) = 313.038 μJ and Y_out(22.0875) = 2439.29 μJ. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives V_line = 7.9125 + (1371.04 − 313.038)(22.0875 − 7.9125)/(2439.29 − 313.038) = 14.9659 V. 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 V = 16.5592 V; substituting back gives Y_out = 1371.0438 μJ.
  4. The interpolation error in the input is |V_line − V| = 1.59338 V. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 1212.3438 μJ and 1529.7438 μJ. Sort the roots even if the response decreases: 15.5714 V ≤ V ≤ 17.4914 V.
  6. The lower and upper input deviations are 0.987842 V and 0.932141 V. 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 V ≈ 16.5592 V; compatible interval [15.5714, 17.4914] V. Endpoint interpolation gives 14.9659 V.

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

Extra challenging · 04

Dielectric Polarization: infer an input with uncertainty

Polarization is the displacement or alignment of a material's bound charge under an applied field, measured by P, the dipole moment per unit volume in C m⁻². Its surfaces then carry bound charge that partly cancels the applied field. Use the supplied topic model Y_out = (sf/8.85)/(1+xe), where Y_out is FIELD IN SLAB E (MV m⁻¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. xe is Susceptibility χ_e, dimensionless. Keep sf (Free charge density σ_f) = 8.85 μC m⁻² fixed. For this exercise use only 0.95625 ≤ xe ≤ 8.04375 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 = 0.15924518 MV m⁻¹, with a bounded reading uncertainty ±0.0073007 MV 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 xe by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00070875, 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 IN SLAB E against xe

Predictions from the supplied Dielectric Polarization 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 IN SLAB E / MV m⁻¹

FIELD IN SLAB E against xePredictions from the supplied Dielectric Polarization 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: xe: Susceptibility χ_e. Vertical axis: FIELD IN SLAB E / MV m⁻¹. Exact values are available in the data table below.0.094550.20270.31090.4190.52720.67272.5864.56.4148.327xy

Horizontal axis (x): xe: Susceptibility χ_e

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

Reading 1xe: Susceptibility χ_e: 0.9562FIELD IN SLAB E / MV m⁻¹: 0.5112
Read the exact data as a table
FIELD IN SLAB E against xe — supplied values
xe: Susceptibility χ_eFIELD IN SLAB E / MV m⁻¹
0.95624999999999990.5111821086261982
2.7281250.26823134953897737
4.50.18181818181818182
6.2718750.13751611516974646
8.043750.11057360055286801

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(xe) = 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(0.95625) = 0.511182 MV m⁻¹ and Y_out(8.04375) = 0.110574 MV m⁻¹. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives xe_line = 0.95625 + (0.159245 − 0.511182)(8.04375 − 0.95625)/(0.110574 − 0.511182) = 7.18266. 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 xe = 5.27962; substituting back gives Y_out = 0.15924518 MV m⁻¹.
  4. The interpolation error in the input is |xe_line − xe| = 1.90304. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.15194448 MV m⁻¹ and 0.16654588 MV m⁻¹. Sort the roots even if the response decreases: 5.00435 ≤ xe ≤ 5.58135.
  6. The lower and upper input deviations are 0.275273 and 0.301726. 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 xe ≈ 5.27962; compatible interval [5.00435, 5.58135]. Endpoint interpolation gives 7.18266.

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

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