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.
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.
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
Horizontal axis (x): w: Plate side w / mm
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| w: Plate side w / mm | GAP RATIO d/w |
|---|---|
| 39.125 | 0.051118210862619806 |
| 74.5625 | 0.026823134953897737 |
| 110 | 0.01818181818181818 |
| 145.4375 | 0.013751611516974646 |
| 180.875 | 0.0110573600552868 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |w_line − w| = 38.0607 mm. This is larger than the requested numerical tolerance.
- 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.
- 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.
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.
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
Horizontal axis (x): c1: Capacitance C₁ / μF
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| c1: Capacitance C₁ / μF | SERIES Cₛ / μF |
|---|---|
| 2.3812499999999996 | 1.492654260528893 |
| 4.940624999999999 | 2.2104159384830475 |
| 7.499999999999999 | 2.6086956521739126 |
| 10.059375 | 2.861969326517004 |
| 12.618749999999999 | 3.037232042121098 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |c1_line − c1| = 1.97475 μF. This is larger than the requested numerical tolerance.
- 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.
- 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.
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.
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
Horizontal axis (x): V: Final p.d. ΔV / V
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| V: Final p.d. ΔV / V | STORED U = ½C(ΔV)² / μJ |
|---|---|
| 7.9125 | 313.03828125 |
| 11.456249999999999 | 656.2283203124998 |
| 15 | 1125 |
| 18.54375 | 1719.3533203125 |
| 22.0875 | 2439.2882812499997 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |V_line − V| = 1.59338 V. This is larger than the requested numerical tolerance.
- 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.
- 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.
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.
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
Horizontal axis (x): xe: Susceptibility χ_e
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| xe: Susceptibility χ_e | FIELD IN SLAB E / MV m⁻¹ |
|---|---|
| 0.9562499999999999 | 0.5111821086261982 |
| 2.728125 | 0.26823134953897737 |
| 4.5 | 0.18181818181818182 |
| 6.271875 | 0.13751611516974646 |
| 8.04375 | 0.11057360055286801 |
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
- 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.
- 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.
- 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.
- The interpolation error in the input is |xe_line − xe| = 1.90304. This is larger than the requested numerical tolerance.
- 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.
- 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.