#set document(title: "8.3 Energy Stored in a Capacitor", author: "OpenStax / XYZ Homework") #set page(width: 8.5in, height: auto, margin: 1in) #import "@preview/cetz:0.5.2" #set text(font: ("STIX Two Text", "Libertinus Serif", "New Computer Modern"), size: 10.5pt, lang: "en") #show math.equation: set text(font: ("STIX Two Math", "New Computer Modern Math")) #set par(justify: true, leading: 0.62em, spacing: 0.9em) #set enum(spacing: 1.1em) // room between list items so tall inline fractions don't collide #set list(spacing: 1.1em) #set table(stroke: 0.5pt + rgb("#c7ccd3")) #let BLUE = rgb("#183B6F") // brand navy — section bars + example/solution labels (white on navy 11.09:1) #let ORANGE = rgb("#A94509") // brand primary-700 — AA-safe deep orange for TEXT (5.93:1 on white; raw brand #F37021 is 2.94:1 and must never carry text) #let RED = rgb("#DC2626") // brand error-600 #let GREEN = rgb("#059669") // brand success-600 (decoration only; small green text uses green-text #007942) #show heading.where(level: 1): it => block(width: 100%, above: 0pt, below: 16pt, fill: gradient.linear(BLUE, rgb("#2C5AA0")), inset: (x: 14pt, y: 12pt), radius: 3pt, text(fill: white, weight: "bold", size: 19pt, it.body)) #show heading.where(level: 2): it => block(width: 100%, above: 18pt, below: 10pt, fill: BLUE, inset: (x: 10pt, y: 6pt), radius: 2pt, text(fill: white, weight: "bold", size: 12pt, it.body)) #show heading.where(level: 3): it => text(fill: ORANGE, weight: "bold", size: 12.5pt, it.body) #show heading.where(level: 4): it => text(fill: BLUE, weight: "bold", size: 10.5pt, it.body) #let examplebox(label, title, body) = block(width: 100%, breakable: true, fill: rgb("#EFF1F5"), stroke: 0.5pt + rgb("#CFDDF0"), radius: 4pt, inset: 10pt, above: 12pt, below: 12pt)[ #block(below: 6pt)[#box(fill: BLUE, inset: (x: 6pt, y: 2pt), radius: 2pt, text(fill: white, weight: "bold", size: 8.5pt, label)) #h(0.4em) #strong[#title]] #body] // rail = decorative left rule (raw brand token); labelcolor = AA-safe label text shade #let notebox(label, rail, labelcolor, tint, body) = block(width: 100%, breakable: true, fill: tint, stroke: (left: 3pt + rail), inset: (left: 10pt, rest: 8pt), radius: (right: 4pt), above: 11pt, below: 11pt)[ #text(fill: labelcolor, weight: "bold", size: 7.5pt, tracking: 0.5pt)[#upper(label)] #linebreak() #body] #let solutionbox(body) = block(above: 4pt, below: 8pt)[ #text(fill: BLUE, weight: "bold", size: 8.5pt)[Solution] #linebreak() #body] #let figph(msg) = block(width: 100%, height: 60pt, fill: rgb("#f6f7f9"), stroke: (paint: rgb("#c7ccd3"), dash: "dashed"), radius: 4pt, inset: 10pt)[ #align(center + horizon, text(fill: rgb("#889"), style: "italic", size: 9pt, msg))] // Standardize inlined figure sizes: measure the natural CeTZ canvas, then scale to a // consistent envelope (aspect-aware; see build_typst.py FIG_* constants). Unlike the // print preamble, dimensions are FLOORED: in an editor a user can trim a figure to a // degenerate 1-D shape (a bare line), and w/h or tw/w would then divide by zero. #let _STD_W = 3.5 #let _WIDE_W = 5.6 #let _MAX_H = 3.4 #let _ASPECT_WIDE = 2.2 #let _UPSCALE_MAX = 1.15 #let stdfig(body) = context { let m = measure(body) let w = calc.max(m.width / 1in, 0.01) let h = calc.max(m.height / 1in, 0.01) let tw = if w / h > _ASPECT_WIDE { _WIDE_W } else { _STD_W } let s = calc.min(tw / w, _MAX_H / h, _UPSCALE_MAX) align(center, box(scale(x: s * 100%, y: s * 100%, reflow: true, body))) } #show figure: set block(breakable: false) #set figure(gap: 8pt) #show figure.caption: set text(size: 8.5pt, fill: rgb("#555")) == 8.3#h(0.6em)Energy Stored in a Capacitor Most of us have seen dramatizations of medical personnel using a defibrillator to pass an electrical current through a patient’s heart to get it to beat normally. Often realistic in detail, the person applying the shock directs another person to “make it 400 joules this time.” The energy delivered by the defibrillator is stored in a capacitor and can be adjusted to fit the situation. SI units of joules are often employed. Less dramatic is the use of capacitors in microelectronics to supply energy when batteries are charged . Capacitors are also used to supply energy for flash lamps on cameras. #figure(figph[This is a photograph of a PCB with an IC and various other components on it. The PCB is attached to a USB connector. Labels for all components are printed on the board.], alt: "This is a photograph of a PCB with an IC and various other components on it. The PCB is attached to a USB connector. Labels for all components are printed on the board.", caption: [The capacitors on the circuit board for an electronic device follow a labeling convention that identifies each one with a code that begins with the letter “C.”]) The energy #math.equation(block: false, alt: "U sub C")[$U_(C)$] stored in a capacitor is electrostatic potential energy and is thus related to the charge #emph[Q] and voltage #emph[V] between the capacitor plates. A charged capacitor stores energy in the electrical field between its plates. As the capacitor is being charged, the electrical field builds up. When a charged capacitor is disconnected from a battery, its energy remains in the field in the space between its plates. To gain insight into how this energy may be expressed (in terms of #emph[Q] and #emph[V]), consider a charged, empty, parallel-plate capacitor; that is, a capacitor without a dielectric but with a vacuum between its plates. The space between its plates has a volume #emph[Ad], and it is filled with a uniform electrostatic field #emph[E]. The total energy #math.equation(block: false, alt: "U sub C")[$U_(C)$] of the capacitor is contained within this space. The #strong[energy density] #math.equation(block: false, alt: "u sub E")[$u_(E)$] in this space is simply #math.equation(block: false, alt: "U sub C")[$U_(C)$] divided by the volume #emph[Ad]. If we know the energy density, the energy can be found as #math.equation(block: false, alt: "U sub C equals u sub E open parenthesis A d close parenthesis")[$U_(C) = u_(E) ( A d )$]. We will learn in Electromagnetic Waves (after completing the study of Maxwell’s equations) that the energy density #math.equation(block: false, alt: "u sub E")[$u_(E)$] in a region of free space occupied by an electrical field #emph[E] depends only on the magnitude of the field and is #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "u sub E equals the fraction 1 over 2 ε sub 0 E squared .")[$u_(E) = frac(1, 2) ε_(0) E^(2) .$] ] If we multiply the energy density by the volume between the plates, we obtain the amount of energy stored between the plates of a parallel-plate capacitor:#math.equation(block: false, alt: "U sub C equals u sub E open parenthesis A d close parenthesis equals the fraction 1 over 2 ε sub 0 E squared A d equals the fraction 1 over 2 ε sub 0 the fraction V squared over d squared A d equals the fraction 1 over 2 V squared ε sub 0 the fraction A over d equals the fraction 1 over 2 V squared C")[$U_(C) = u_(E) ( A d ) = frac(1, 2) ε_(0) E^(2) A d = frac(1, 2) ε_(0) frac(V^(2), d^(2)) A d = frac(1, 2) V^(2) ε_(0) frac(A, d) = frac(1, 2) V^(2) C$]. In this derivation, we used the fact that the electrical field between the plates is uniform so that #math.equation(block: false, alt: "E equals V / d")[$E = V "/" d$] and #math.equation(block: false, alt: "C equals ε sub 0 A / d .")[$C = ε_(0) A "/" d .$] Because #math.equation(block: false, alt: "C equals Q / V")[$C = Q "/" V$], we can express this result in other equivalent forms: #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "U sub C equals the fraction 1 over 2 V squared C equals the fraction 1 over 2 the fraction Q squared over C equals the fraction 1 over 2 Q V .")[$U_(C) = frac(1, 2) V^(2) C = frac(1, 2) #h(0.2em) frac(Q^(2), C) = frac(1, 2) Q V .$] ] The expression in for the energy stored in a parallel-plate capacitor is generally valid for all types of capacitors. To see this, consider any uncharged capacitor (not necessarily a parallel-plate type). At some instant, we connect it across a battery, giving it a potential difference #math.equation(block: false, alt: "V equals q / C")[$V = q "/" C$] between its plates. Initially, the charge on the plates is #math.equation(block: false, alt: "Q equals 0 .")[$Q = 0 .$] As the capacitor is being charged, the charge gradually builds up on its plates, and after some time, it reaches the value #emph[Q]. To move an infinitesimal charge #emph[dq] from the negative plate to the positive plate (from a lower to a higher potential), the amount of work #emph[dW] that must be done on #emph[dq] is #math.equation(block: false, alt: "d W equals V d q equals the fraction q over C d q")[$d W = V d q = frac(q, C) d q$]. This work becomes the energy stored in the electrical field of the capacitor. In order to charge the capacitor to a charge #emph[Q], the total work required is #math.equation(block: true, alt: "W equals ∫ 0 W open parenthesis Q close parenthesis d W equals ∫ 0 Q the fraction q over C d q equals the fraction 1 over 2 the fraction Q squared over C .")[$W = display(∫_(0)^(W ( Q )) d W =) display(∫_(0)^(Q) frac(q, C) d q) = frac(1, 2) #h(0.2em) frac(Q^(2), C) .$] Since the geometry of the capacitor has not been specified, this equation holds for any type of capacitor. The total work #emph[W] needed to charge a capacitor is the electrical potential energy #math.equation(block: false, alt: "U sub C")[$U_(C)$] stored in it, or #math.equation(block: false, alt: "U sub C equals W")[$U_(C) = W$]. When the charge is expressed in coulombs, potential is expressed in volts, and the capacitance is expressed in farads, this relation gives the energy in joules. Knowing that the energy stored in a capacitor is #math.equation(block: false, alt: "U sub C equals Q squared / open parenthesis 2 C close parenthesis")[$U_(C) = Q^(2) "/" ( 2 C )$], we can now find the energy density #math.equation(block: false, alt: "u sub E")[$u_(E)$] stored in a vacuum between the plates of a charged parallel-plate capacitor. We just have to divide #math.equation(block: false, alt: "U sub C")[$U_(C)$] by the volume #emph[Ad] of space between its plates and take into account that for a parallel-plate capacitor, we have #math.equation(block: false, alt: "E equals σ / ε sub 0")[$E = σ "/" ε_(0)$] and #math.equation(block: false, alt: "C equals ε sub 0 A / d")[$C = ε_(0) A "/" d$]. Therefore, we obtain #math.equation(block: true, alt: "u sub E equals the fraction U sub C over A d equals the fraction 1 over 2 the fraction Q squared over C the fraction 1 over A d equals the fraction 1 over 2 the fraction Q squared over ε sub 0 A / d the fraction 1 over A d equals the fraction 1 over 2 the fraction 1 over ε sub 0 open parenthesis the fraction Q over A close parenthesis squared equals the fraction σ squared over 2 ε sub 0 equals the fraction open parenthesis E ε sub 0 close parenthesis squared over 2 ε sub 0 equals the fraction ε sub 0 over 2 E squared .")[$u_(E) = frac(U_(C), A d) = frac(1, 2) #h(0.2em) frac(Q^(2), C) #h(0.2em) frac(1, A d) = frac(1, 2) #h(0.2em) frac(Q^(2), ε_(0) A "/" d) #h(0.2em) frac(1, A d) = frac(1, 2) #h(0.2em) frac(1, ε_(0)) attach(( frac(Q, A) ), t: 2) = frac(σ^(2), 2 ε_(0)) = frac(attach(( E ε_(0) ), t: 2), 2 ε_(0)) = frac(ε_(0), 2) E^(2) .$] We see that this expression for the density of energy stored in a parallel-plate capacitor is in accordance with the general relation expressed in . We could repeat this calculation for either a spherical capacitor or a cylindrical capacitor—or other capacitors—and in all cases, we would end up with the general relation given by . #examplebox("Example 1")[Energy Stored in a Capacitor][ Calculate the energy stored in the capacitor network in (a) when the capacitors are fully charged and when the capacitances are #math.equation(block: false, alt: "C sub 1 equals 12.0 μ F , C sub 2 equals 2.0 μ F ,")[$C_(1) = 12.0 #h(0.2em) μ "F" , C_(2) = 2.0 #h(0.2em) μ "F" ,$] and #math.equation(block: false, alt: "C sub 3 equals 4.0 μ F,")[$C_(3) = 4.0 #h(0.2em) μ "F,"$] respectively. Strategy We use to find the energy #math.equation(block: false, alt: "U sub 1")[$U_(1)$], #math.equation(block: false, alt: "U sub 2")[$U_(2)$], and #math.equation(block: false, alt: "U sub 3")[$U_(3)$] stored in capacitors 1, 2, and 3, respectively. The total energy is the sum of all these energies. Solution We identify #math.equation(block: false, alt: "C sub 1 equals 12.0 μ F")[$C_(1) = 12.0 #h(0.2em) μ "F"$] and #math.equation(block: false, alt: "V sub 1 equals 4.0 V")[$V_(1) = 4.0 #h(0.2em) "V"$], #math.equation(block: false, alt: "C sub 2 equals 2.0 μ F")[$C_(2) = 2.0 #h(0.2em) μ "F"$] and #math.equation(block: false, alt: "V sub 2 equals 8.0 V")[$V_(2) = 8.0 #h(0.2em) "V"$], #math.equation(block: false, alt: "C sub 3 equals 4.0 μ F")[$C_(3) = 4.0 #h(0.2em) μ "F"$] and #math.equation(block: false, alt: "V sub 3 equals 8.0 V .")[$V_(3) = 8.0 #h(0.2em) "V" "."$] The energies stored in these capacitors are #math.equation(block: true, alt: "U sub 1, equals, the fraction 1 over 2 C sub 1 V 1 2 equals the fraction 1 over 2 open parenthesis 12.0 μ F close parenthesis open parenthesis 4.0 V close parenthesis squared equals 96 μ J,; U sub 2, equals, the fraction 1 over 2 C sub 2 V 2 2 equals the fraction 1 over 2 open parenthesis 2.0 μ F close parenthesis open parenthesis 8.0 V close parenthesis squared equals 64 μ J,; U sub 3, equals, the fraction 1 over 2 C sub 3 V 3 2 equals the fraction 1 over 2 open parenthesis 4.0 μ F close parenthesis open parenthesis 8.0 V close parenthesis squared equals 130 μ J .")[$U_(1) & = & frac(1, 2) C_(1) V_(1)^(2) = frac(1, 2) ( 12.0 #h(0.2em) μ "F" ) attach(( 4.0 #h(0.2em) "V" ), t: 2) = 96 #h(0.2em) μ "J," \ U_(2) & = & frac(1, 2) C_(2) V_(2)^(2) = frac(1, 2) ( 2.0 #h(0.2em) μ "F" ) attach(( 8.0 #h(0.2em) "V" ), t: 2) = 64 #h(0.2em) μ "J," \ U_(3) & = & frac(1, 2) C_(3) V_(3)^(2) = frac(1, 2) ( 4.0 #h(0.2em) μ "F" ) attach(( 8.0 #h(0.2em) "V" ), t: 2) = 130 #h(0.2em) μ "J" "."$] The total energy stored in this network is #math.equation(block: true, alt: "U sub C equals U sub 1 plus U sub 2 plus U sub 3 equals 96 μ J plus 64 μ J plus 130 μ J equals 0.29 mJ .")[$U_(C) = U_(1) + U_(2) + U_(3) = 96 #h(0.2em) μ "J" + 64 #h(0.2em) μ "J" + 130 #h(0.2em) μ "J" = 0.29 #h(0.2em) "mJ" .$] Significance We can verify this result by calculating the energy stored in the single #math.equation(block: false, alt: "4 .0- μ F")[$4 ".0-" "μ" "F"$] capacitor, which is found to be equivalent to the entire network. The voltage across the network is 12.0 V. The total energy obtained in this way agrees with our previously obtained result,#math.equation(block: false, alt: "U sub C equals the fraction 1 over 2 C V squared equals the fraction 1 over 2 open parenthesis 4.0 μ F close parenthesis open parenthesis 12.0 V close parenthesis squared equals 0.29 mJ")[$U_(C) = frac(1, 2) C V^(2) = frac(1, 2) ( 4.0 #h(0.2em) μ "F" ) attach(( 12.0 #h(0.2em) "V" ), t: 2) = 0.29 #h(0.2em) "mJ"$]. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ The potential difference across a 5.0-pF capacitor is 0.40 V. (a) What is the energy stored in this capacitor? (b) The potential difference is now increased to 1.20 V. By what factor is the stored energy increased? #solutionbox[ a.#math.equation(block: false, alt: "4.0 times 10 to the power −13 J")[$4.0 #h(0.2em) × #h(0.2em) 10^(−13) "J"$]; b. 9 times ] ] In a cardiac emergency, a portable electronic device known as an automated external defibrillator (AED) can be a lifesaver. A #strong[defibrillator] delivers a large charge in a short burst, or a shock, to a person’s heart to correct abnormal heart rhythm (an arrhythmia). A heart attack can arise from the onset of fast, irregular beating of the heart—called cardiac or ventricular fibrillation. Applying a large shock of electrical energy can terminate the arrhythmia and allow the body’s natural pacemaker to resume its normal rhythm. Today, it is common for ambulances to carry AEDs. AEDs are also found in many public places. These are designed to be used by lay persons. The device automatically diagnoses the patient’s heart rhythm and then applies the shock with appropriate energy and waveform. CPR (cardiopulmonary resuscitation) is recommended in many cases before using a defibrillator. #figure(figph[Photograph of an automated external defibrillator.], alt: "Photograph of an automated external defibrillator.", caption: [Automated external defibrillators are found in many public places. These portable units provide verbal instructions for use in the important first few minutes for a person suffering a cardiac attack.]) #examplebox("Example 2")[Capacitance of a Heart Defibrillator][ A heart defibrillator delivers #math.equation(block: false, alt: "4.00 times 10 squared J")[$4.00 #h(0.2em) × #h(0.2em) 10^(2) "J"$] of energy by discharging a capacitor initially at #math.equation(block: false, alt: "1.00 times 10 to the power 4 V .")[$1.00 #h(0.2em) × #h(0.2em) 10^(4) #h(0.2em) "V" "."$] What is its capacitance? Strategy We are given #math.equation(block: false, alt: "U sub C")[$U_(C)$] and #emph[V], and we are asked to find the capacitance #emph[C]. We solve for #emph[C] and substitute. Solution Solving this expression for #emph[C] and entering the given values yields #math.equation(block: false, alt: "C equals 2 the fraction U sub C over V squared equals 2 the fraction 4.00 times 10 squared J over open parenthesis 1.00 times 10 to the power 4 V close parenthesis squared equals 8.00 μ F .")[$C = 2 frac(U_(C), V^(2)) = 2 frac(4.00 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "J", attach(( 1.00 #h(0.2em) × #h(0.2em) 10^(4) #h(0.1em) "V" ), t: 2)) = 8.00 #h(0.2em) μ "F" "."$] ] === Summary - Capacitors are used to supply energy to a variety of devices, including defibrillators, microelectronics such as calculators, and flash lamps. - The energy stored in a capacitor is the work required to charge the capacitor, beginning with no charge on its plates. The energy is stored in the electrical field in the space between the capacitor plates. It depends on the amount of electrical charge on the plates and on the potential difference between the plates. - The energy stored in a capacitor network is the sum of the energies stored on individual capacitors in the network. It can be computed as the energy stored in the equivalent capacitor of the network. === Conceptual Questions If you wish to store a large amount of energy in a capacitor bank, would you connect capacitors in series or parallel? Explain. === Problems How much energy is stored in an #math.equation(block: false, alt: "8.00 - μ F")[$8.00 "-" "μ" "F"$] capacitor whose plates are at a potential difference of 6.00 V? A capacitor has a charge of #math.equation(block: false, alt: "2.5 μ C")[$2.5 #h(0.2em) "μ" "C"$] when connected to a 6.0-V battery. How much energy is stored in this capacitor? #solutionbox[ #math.equation(block: true, alt: "7.5 μ J")[$7.5 #h(0.2em) "μ" "J"$] ] How much energy is stored in the electrical field of a metal sphere of radius 2.0 m that is kept at a 10.0-V potential? (a) What is the energy stored in the #math.equation(block: false, alt: "10.0 - μ F")[$10.0 "-" μ "F"$] capacitor of a heart defibrillator charged to #math.equation(block: false, alt: "9.00 times 10 cubed V")[$9.00 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V"$] ? (b) Find the amount of the stored charge. #solutionbox[ a. 405 J; b. 90.0 mC ] In open-heart surgery, a much smaller amount of energy will defibrillate the heart. (a) What voltage is applied to the #math.equation(block: false, alt: "8.00 - μ F")[$8.00 "-" μ "F"$] capacitor of a heart defibrillator that stores 40.0 J of energy? (b) Find the amount of the stored charge. A #math.equation(block: false, alt: "165 - μ F")[$165 "-" μ "F"$] capacitor is used in conjunction with a dc motor. How much energy is stored in it when 119 V is applied? #solutionbox[ 1.17 J ] Suppose you have a 9.00-V battery, a #math.equation(block: false, alt: "2.00 - μ F")[$2.00 "-" μ "F"$] capacitor, and a #math.equation(block: false, alt: "7.40 - μ F")[$7.40 "-" μ "F"$] capacitor. (a) Find the charge and energy stored if the capacitors are connected to the battery in series. (b) Do the same for a parallel connection. An anxious physicist worries that the two metal shelves of a wood frame bookcase might obtain a high voltage if charged by static electricity, perhaps produced by friction. (a) What is the capacitance of the empty shelves if they have area #math.equation(block: false, alt: "1.00 times 10 squared m squared")[$1.00 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "m"^(2)$] and are 0.200 m apart? (b) What is the voltage between them if opposite charges of magnitude 2.00 nC are placed on them? (c) To show that this voltage poses a small hazard, calculate the energy stored. (d) The actual shelves have an area 100 times smaller than these hypothetical shelves with a connection to the same voltage. Are his fears justified? #solutionbox[ a. #math.equation(block: false, alt: "4.43 times 10 to the power − 9 F")[$4.43 #h(0.2em) × #h(0.2em) 10^("−" 9) #h(0.2em) "F"$]; b. 0.453 V; c. #math.equation(block: false, alt: "4.53 times 10 to the power − 10 J")[$4.53 #h(0.2em) × #h(0.2em) 10^("−" 10) #h(0.2em) "J"$]; d. no ] A parallel-plate capacitor is made of two square plates 25 cm on a side and 1.0 mm apart. The capacitor is connected to a 50.0-V battery. With the battery still connected, the plates are pulled apart to a separation of 2.00 mm. What are the energies stored in the capacitor before and after the plates are pulled farther apart? Why does the energy decrease even though work is done in separating the plates? Suppose that the capacitance of a variable capacitor can be manually changed from 100 pF to 800 pF by turning a dial, connected to one set of plates by a shaft, from #math.equation(block: false, alt: "0 °")[$0 "°"$] to #math.equation(block: false, alt: "180 °")[$180 "°"$]. With the dial set at #math.equation(block: false, alt: "180 °")[$180 "°"$] (corresponding to #math.equation(block: false, alt: "C equals 800 pF")[$C = 800 #h(0.2em) "pF"$]), the capacitor is connected to a 500-V source. After charging, the capacitor is disconnected from the source, and the dial is turned to #math.equation(block: false, alt: "0 °")[$0 "°"$]. If friction is negligible, how much work is required to turn the dial from #math.equation(block: false, alt: "180 °")[$180 "°"$] to #math.equation(block: false, alt: "0 °")[$0 "°"$] ? #solutionbox[ 0.7 mJ ]