#set document(title: "7.3 Calculations of Electric Potential", 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")) == 7.3#h(0.6em)Calculations of Electric Potential Point charges, such as electrons, are among the fundamental building blocks of matter. Furthermore, spherical charge distributions (such as charge on a metal sphere) create external electric fields exactly like a point charge. The electric potential due to a point charge is, thus, a case we need to consider. We can use calculus to find the work needed to move a test charge #emph[q] from a large distance away to a distance of #emph[r] from a point charge #emph[q]. Noting the connection between work and potential #math.equation(block: false, alt: "W equals − q Δ V ,")[$W = "−" q "Δ" V ,$] as in the last section, we can obtain the following result. #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Electric Potential #emph[V] of a Point Charge] The electric potential #emph[V] of a point charge is given by #math.equation(block: true, alt: "V equals the fraction k sub e q over r open parenthesis point charge close parenthesis")[$V = frac(k_(e) q, r) ( "point charge" )$] where #emph[k] is a constant equal to #math.equation(block: false, alt: "8.99 times 10 to the power 9 N times m squared /C squared .")[$8.99 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/C"^(2) .$] ] The potential at infinity is chosen to be zero. Thus, #emph[V] for a point charge decreases with distance, whereas #math.equation(block: false, alt: "E →")[$arrow("E")$] for a point charge decreases with distance squared: #math.equation(block: true, alt: "E equals the fraction F over q sub t equals the fraction k sub e q over r squared .")[$E = frac(F, q_(t)) = frac(k_(e) q, r^(2)) .$] Recall that the electric potential difference #emph[V] is a scalar and has no direction, whereas the electric field #math.equation(block: false, alt: "E →")[$arrow("E")$] is a vector. To find the voltage due to a combination of point charges, given zero voltage at infinitely far away, you add the individual voltages as numbers. We will assume for the rest of this chapter that there is zero voltage measured infinitely far away. To find the total electric field, you must add the individual fields as vectors, taking magnitude and direction into account. This is consistent with the fact that #emph[V] is closely associated with energy, a scalar, whereas #math.equation(block: false, alt: "E →")[$arrow("E")$] is closely associated with force, a vector. #examplebox("Example 1")[What Voltage Is Produced by a Small Charge on a Metal Sphere?][ Charges in static electricity are typically in the nanocoulomb (nC) to microcoulomb #math.equation(block: false, alt: "open parenthesis μ C close parenthesis")[$( μ "C" )$] range. What is the voltage 5.00 cm away from the center of a 1-cm-diameter solid metal sphere that has a –3.00-nC static charge? Strategy As we discussed in Electric Charges and Fields, charge on a metal sphere spreads out uniformly and produces a field like that of a point charge located at its center. Thus, we can find the voltage using the equation #math.equation(block: false, alt: "V equals the fraction k sub e q over r .")[$V = frac(k_(e) q, r) .$] Solution Entering known values into the expression for the potential of a point charge, we obtain #math.equation(block: true, alt: "V equals k sub e the fraction q over r equals open parenthesis 8.99 times 10 to the power 9 N times m squared /C squared close parenthesis open parenthesis the fraction −3.00 times 10 to the power −9 C over 5.00 times 10 to the power −2 m close parenthesis equals −539 V .")[$V = k_(e) frac(q, r) = ( 8.99 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/C"^(2) ) ( frac(−3.00 #h(0.2em) × #h(0.2em) 10^(−9) #h(0.2em) "C", 5.00 #h(0.2em) × #h(0.2em) 10^(−2) #h(0.2em) "m") ) = −539 #h(0.2em) "V" "."$] Significance The negative value for voltage means a positive charge would be attracted from a larger distance, since the potential is lower (more negative) than at larger distances. Conversely, a negative charge would be repelled, as expected. ] #examplebox("Example 2")[What Is the Excess Charge on a Van de Graaff Generator?][ A demonstration #strong[Van de Graaff generator] has a 25.0-cm-diameter metal sphere that produces a voltage of 100 kV near its surface . What excess charge resides on the sphere? (Assume that each numerical value here is shown with three significant figures.) #figure(figph[The figure shows the parts of Van de Graaff generator.], alt: "The figure shows the parts of Van de Graaff generator.", caption: [The voltage of this demonstration Van de Graaff generator is measured between the charged sphere and ground. Earth’s potential is taken to be zero as a reference. The potential of the charged conducting sphere is the same as that of an equal point charge at its center.]) Strategy The potential on the surface is the same as that of a point charge at the center of the sphere, 12.5 cm away. (The radius of the sphere is 12.5 cm.) We can thus determine the excess charge using the equation #math.equation(block: true, alt: "V equals the fraction k sub e q over r .")[$V = frac(k_(e) q, r) .$] Solution Solving for #emph[q] and entering known values gives #math.equation(block: true, alt: "q equals the fraction r V over k sub e equals the fraction open parenthesis 0.125 m close parenthesis open parenthesis 100 times 10 cubed V close parenthesis over 8.99 times 10 to the power 9 N times m squared /C squared equals 1.39 times 10 to the power −6 C equals 1.39 μ C .")[$q = frac(r V, k_(e)) = frac(( 0.125 #h(0.2em) "m" ) ( 100 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V" ), 8.99 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/C"^(2)) = 1.39 #h(0.2em) × #h(0.2em) 10^(−6) #h(0.2em) "C" = 1.39 #h(0.2em) μ "C" .$] Significance This is a relatively small charge, but it produces a rather large voltage. We have another indication here that it is difficult to store isolated charges. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ What is the potential inside the metal sphere in ? #solutionbox[ #math.equation(block: false, alt: "V equals k the fraction q over r equals open parenthesis 8.99 times 10 to the power 9 N times m squared /C squared close parenthesis open parenthesis the fraction −3.00 times 10 to the power −9 C over 5.00 times 10 to the power −3 m close parenthesis equals −5390 V;")[$V = k frac(q, r) = ( 8.99 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/C"^(2) ) ( frac(−3.00 #h(0.2em) × #h(0.2em) 10^(−9) #h(0.2em) "C", 5.00 #h(0.2em) × #h(0.2em) 10^(−3) #h(0.2em) "m") ) = −5390 #h(0.2em) "V;"$] recall that the electric field inside a conductor is zero. Hence, any path from a point on the surface to any point in the interior will have an integrand of zero when calculating the change in potential, and thus the potential in the interior of the sphere is identical to that on the surface. ] ] The voltages in both of these examples could be measured with a meter that compares the measured potential with ground potential. Ground potential is often taken to be zero (instead of taking the potential at infinity to be zero). It is the potential difference between two points that is of importance, and very often there is a tacit assumption that some reference point, such as Earth or a very distant point, is at zero potential. As noted earlier, this is analogous to taking sea level as #math.equation(block: false, alt: "h equals 0")[$h = 0$] when considering gravitational potential energy #math.equation(block: false, alt: "U sub g equals m g h")[$U_(g) = m g h$]. === Systems of Multiple Point Charges Just as the electric field obeys a superposition principle, so does the electric potential. Consider a system consisting of #emph[N] charges #math.equation(block: false, alt: "q sub 1 , q sub 2 , … , q sub N .")[$q_(1) , q_(2) , "…" , q_(N) .$] What is the net electric potential #emph[V] at a space point #emph[P] from these charges? Each of these charges is a source charge that produces its own electric potential at point #emph[P], independent of whatever other changes may be doing. Let #math.equation(block: false, alt: "V sub 1 , V sub 2 , … , V sub N")[$V_(1) , V_(2) , "…" , V_(N)$] be the electric potentials at #emph[P] produced by the charges #math.equation(block: false, alt: "q sub 1 , q sub 2 , … , q sub N ,")[$q_(1) , q_(2) , "…" , q_(N) ,$] respectively. Then, the net electric potential #math.equation(block: false, alt: "V sub P")[$V_(P)$] at that point is equal to the sum of these individual electric potentials. You can easily show this by calculating the potential energy of a test charge when you bring the test charge from the reference point at infinity to point #emph[P]: #math.equation(block: true, alt: "V sub P equals V sub 1 plus V sub 2 plus ⋯ plus V sub N equals ∑ 1 N V sub i .")[$V_(P) = V_(1) + V_(2) + "⋯" + V_(N) = display(limits(∑)^(N)_(1) V_(i)) .$] Note that electric potential follows the same principle of superposition as electric field and electric potential energy. To show this more explicitly, note that a test charge #math.equation(block: false, alt: "q sub t")[$q_(t)$] at the point #emph[P] in space has distances of #math.equation(block: false, alt: "r sub 1 , r sub 2 , … , r sub N")[$r_(1) , r_(2) , "…" , r_(N)$] from the #emph[N] charges fixed in space above, as shown. Using our formula for the potential of a point charge for each of these (assumed to be point) charges, we find that #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "V sub P equals ∑ 1 N k sub e the fraction q sub i over r sub i equals k sub e ∑ 1 N the fraction q sub i over r sub i .")[$V_(P) = display(limits(∑)^(N)_(1) k_(e) frac(q_(i), r_(i))) = k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i))) .$] ] Therefore, the electric potential energy of the test charge is #math.equation(block: true, alt: "U sub P equals q sub t V sub P equals q sub t k sub e ∑ 1 N the fraction q sub i over r sub i ,")[$U_(P) = q_(t) V_(P) = q_(t) k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i))) ,$] which is the same as the work to bring the test charge into the system, as found in the first section of the chapter. #figure(figph[The figure shows N charges located at different distances from a fixed point P.], alt: "The figure shows N charges located at different distances from a fixed point P.", caption: [Notation for direct distances from charges to a space point #emph[P].]) === The Electric Dipole An #strong[electric dipole] is a system of two equal but opposite charges a fixed distance apart. This system is used to model many real-world systems, including atomic and molecular interactions. One of these systems is the water molecule, under certain circumstances. These circumstances are met inside a microwave oven, where electric fields with alternating directions make the water molecules change orientation. This vibration is the same as heat at the molecular level. The example Electric Potential of a Dipole, seen along the dipole's own axis: charges of q = 3.0 nC and −3.0 nC separated by d = 4.0 cm, with V = kΣqᵢ/rᵢ = 2.70(1/|z − d/2| − 1/|z + d/2|). The horizontal axis is z in centimetres and the vertical axis is the potential in kilovolts; the two dashed vertical lines sit on the charges and move with the slider. The example's own answers are points on this curve — at z = 1.0 cm it reads 1.8 kV and at z = −5.0 cm it reads −0.51 kV, both matching the book to the digits given. Between the charges the curve is steep and one-signed; outside them it collapses toward zero from both ends, and it crosses zero exactly at the midpoint, which is the Check Your Understanding answer that the potential vanishes on the plane bisecting the dipole. Now close the separation with the slider. The two spikes march together and the far field dies away faster and faster: at large z the two 1/r terms cancel to leading order and what is left is the dipole potential kp cos θ/r², falling as 1/z² rather than 1/z.{"functions":\[{"color":"\#1f4e79","domain":\[-10,10\],"expression":"2.70\*(1/abs(z - dd/2) - 1/abs(z + dd/2))","expression2":"","id":"dipole-potential","inequality":"lt","lineDash":\[\],"lineWidth":2.5,"mode":"cartesian","tMax":10,"tMin":-10,"variable":"z","visible":true},{"color":"\#b8442a","domain":null,"expression":"dd/2","expression2":"t","id":"positive-charge","inequality":"lt","lineDash":\[4,4\],"lineWidth":1.25,"mode":"parametric","tMax":6.2,"tMin":-6.2,"variable":"t","visible":true},{"color":"\#b8442a","domain":null,"expression":"-dd/2","expression2":"t","id":"negative-charge","inequality":"lt","lineDash":\[4,4\],"lineWidth":1.25,"mode":"parametric","tMax":6.2,"tMin":-6.2,"variable":"t","visible":true}\],"grid":{"axisColor":"\#333333","gridColor":"\#cccccc","majorSpacing":1,"minorGridColor":"\#eeeeee","showAxes":true,"showLabels":true},"parameters":\[{"id":"up2-7-3-dd","label":"Charge separation d (the worked example uses 4.0 cm)","max":8,"min":0.5,"name":"dd","step":0.1,"unit":"cm","value":4}\],"title":"Potential on the axis of a 3.0 nC dipole of separation dd","version":1,"viewport":{"centerX":0,"centerY":0,"scale":38}}#examplebox("Example 3")[Electric Potential of a Dipole][ Consider the dipole in with the charge magnitude of #math.equation(block: false, alt: "q equals 3.0 nC")[$q = 3.0 #h(0.2em) "nC"$] and separation distance #math.equation(block: false, alt: "d equals 4.0 cm .")[$d = 4.0 #h(0.2em) "cm" "."$] What is the potential at the following locations in space? (a) (0, 0, 1.0 cm); (b) (0, 0, –5.0 cm); (c) (3.0 cm, 0, 2.0 cm). #figure(figph[The figure shows an electric dipole with two charges (3.0nC and -3.0nC) located 4.0cm apart on the z axis. The center of the dipole is at the origin and three other points are marked at (0, 0, 1.0 cm), (0, 0, –5.0 cm) and (3.0 cm, 0, 2.0 cm).], alt: "The figure shows an electric dipole with two charges (3.0nC and -3.0nC) located 4.0cm apart on the z axis. The center of the dipole is at the origin and three other points are marked at (0, 0, 1.0 cm), (0, 0, –5.0 cm) and (3.0 cm, 0, 2.0 cm).", caption: [A general diagram of an electric dipole, and the notation for the distances from the individual charges to a point #emph[P] in space.]) Strategy Apply #math.equation(block: false, alt: "V sub P equals k sub e ∑ 1 N the fraction q sub i over r sub i")[$V_(P) = k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i)))$] to each of these three points. Solution + #math.equation(block: false, alt: "V sub P equals k sub e ∑ 1 N the fraction q sub i over r sub i equals open parenthesis 9.0 times 10 to the power 9 N times m squared / C squared close parenthesis open parenthesis the fraction 3.0 nC over 0.010 m minus the fraction 3.0 nC over 0.030 m close parenthesis equals 1.8 times 10 cubed V")[$V_(P) = k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i))) = ( 9.0 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/" "C"^(2) ) ( frac(3.0 #h(0.2em) "nC", 0.010 #h(0.2em) "m") − frac(3.0 #h(0.2em) "nC", 0.030 #h(0.2em) "m") ) = 1.8 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V"$] + #math.equation(block: false, alt: "V sub P equals k sub e ∑ 1 N the fraction q sub i over r sub i equals open parenthesis 9.0 times 10 to the power 9 N times m squared / C squared close parenthesis open parenthesis the fraction 3.0 nC over 0.070 m minus the fraction 3.0 nC over 0.030 m close parenthesis equals −5.1 times 10 squared V")[$V_(P) = k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i))) = ( 9.0 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/" "C"^(2) ) ( frac(3.0 #h(0.2em) "nC", 0.070 #h(0.2em) "m") − frac(3.0 #h(0.2em) "nC", 0.030 #h(0.2em) "m") ) = −5.1 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "V"$] + #math.equation(block: false, alt: "V sub P equals k ∑ 1 N the fraction q sub i over r sub i equals open parenthesis 9.0 times 10 to the power 9 N times m squared / C squared close parenthesis open parenthesis the fraction 3.0 nC over 0.030 m minus the fraction 3.0 nC over 0.050 m close parenthesis equals 3.6 times 10 squared V")[$V_(P) = k display(limits(∑)^(N)_(1) frac(q_(i), r_(i))) = ( 9.0 #h(0.2em) × #h(0.2em) 10^(9) #h(0.2em) "N" · "m"^(2) "/" "C"^(2) ) ( frac(3.0 #h(0.2em) "nC", 0.030 #h(0.2em) "m") − frac(3.0 #h(0.2em) "nC", 0.050 #h(0.2em) "m") ) = 3.6 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "V"$] Significance Note that evaluating potential is significantly simpler than electric field, due to potential being a scalar instead of a vector. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ What is the potential on the #emph[x]-axis? The #emph[z]-axis? #solutionbox[ The #emph[x]-axis the potential is zero, due to the equal and opposite charges the same distance from it. On the #emph[z]-axis, we may superimpose the two potentials; we will find that for #math.equation(block: false, alt: "z greater than greater than d")[$z #h(0.2em) > > #h(0.2em) d$], again the potential goes to zero due to cancellation. ] ] Now let us consider the special case when the distance of the point #emph[P] from the dipole is much greater than the distance between the charges in the dipole, #math.equation(block: false, alt: "r ≫ d ;")[$r ≫ d ;$] for example, when we are interested in the electric potential due to a polarized molecule such as a water molecule. This is not so far (infinity) that we can simply treat the potential as zero, but the distance is great enough that we can simplify our calculations relative to the previous example. We start by noting that in the potential is given by #math.equation(block: true, alt: "V sub P equals V sub + plus V sub − equals k sub e open parenthesis the fraction q over r sub + minus the fraction q over r sub − close parenthesis")[$V_(P) = V_("+") + V_("−") = k_(e) ( frac(q, r_("+")) − frac(q, r_("−")) )$] where #math.equation(block: true, alt: "r sub ± equals the square root of x squared plus open parenthesis z ∓ the fraction d over 2 close parenthesis squared .")[$r_("±") = sqrt(x^(2) + attach(( z ∓ frac(d, 2) ), t: 2)) .$] #figure(figph[The figure shows an electric dipole located on the z axis with center at the origin. Point P, located at (x, 0, z) is distance r away from the origin.], alt: "The figure shows an electric dipole located on the z axis with center at the origin. Point P, located at (x, 0, z) is distance r away from the origin.", caption: [A general diagram of an electric dipole, and the notation for the distances from the individual charges to a point #emph[P] in space.]) This is still the exact formula. To take advantage of the fact that #math.equation(block: false, alt: "r ≫ d ,")[$r ≫ d ,$] we rewrite the radii in terms of polar coordinates, with #math.equation(block: false, alt: "x equals r sin θ")[$x = r #h(0.2em) "sin" #h(0.2em) θ$] and #math.equation(block: false, alt: "z equals r cos θ")[$z = r #h(0.2em) "cos" #h(0.2em) θ$]. This gives us #math.equation(block: true, alt: "r sub ± equals the square root of r squared sin squared θ plus open parenthesis r cos θ ∓ the fraction d over 2 close parenthesis squared .")[$r_("±") = sqrt(r^(2) #h(0.2em) "sin"^(2) #h(0.2em) θ + attach(( r #h(0.2em) "cos" #h(0.2em) θ ∓ frac(d, 2) ), t: 2)) .$] We can simplify this expression by pulling #emph[r] out of the root, #math.equation(block: true, alt: "r sub ± equals r the square root of sin squared θ plus open parenthesis cos θ ∓ the fraction d over 2 r close parenthesis squared")[$r_("±") = r sqrt("sin"^(2) #h(0.2em) θ + attach(( "cos" #h(0.2em) θ ∓ frac(d, 2 r) ), t: 2))$] and then multiplying out the parentheses #math.equation(block: true, alt: "r sub ± equals r the square root of sin squared θ plus cos squared θ ∓ cos θ the fraction d over r plus open parenthesis the fraction d over 2 r close parenthesis squared equals r the square root of 1 ∓ cos θ the fraction d over r plus open parenthesis the fraction d over 2 r close parenthesis squared .")[$r_("±") = r sqrt("sin"^(2) #h(0.2em) θ + "cos"^(2) #h(0.2em) θ ∓ "cos" #h(0.2em) θ frac(d, r) + attach(( frac(d, 2 r) ), t: 2)) = r sqrt(1 ∓ "cos" #h(0.2em) θ frac(d, r) + attach(( frac(d, 2 r) ), t: 2)) .$] The last term in the root is small enough to be negligible (remember #math.equation(block: false, alt: "r ≫ d ,")[$r ≫ d ,$] and hence #math.equation(block: false, alt: "open parenthesis d / r close parenthesis squared")[$attach(( d "/" r ), t: 2)$] is extremely small, effectively zero to the level we will probably be measuring), leaving us with #math.equation(block: true, alt: "r sub ± equals r the square root of 1 ∓ cos θ the fraction d over r .")[$r_("±") = r sqrt(1 ∓ "cos" #h(0.2em) θ frac(d, r)) .$] Using the binomial approximation (a standard result from the mathematics of series, when #math.equation(block: false, alt: "α")[$α$] is small) #math.equation(block: true, alt: "the fraction 1 over the square root of 1 ∓ α approximately equals 1 plus or minus the fraction α over 2")[$frac(1, sqrt(1 ∓ α)) ≈ 1 ± frac(α, 2)$] and substituting this into our formula for #math.equation(block: false, alt: "V sub P")[$V_(P)$] , we get #math.equation(block: true, alt: "V sub P equals k sub e [ the fraction q over r open parenthesis 1 plus the fraction d cos θ over 2 r close parenthesis minus the fraction q over r open parenthesis 1 minus the fraction d cos θ over 2 r close parenthesis ] equals k sub e the fraction q d cos θ over r squared .")[$V_(P) = k_(e) [ frac(q, r) ( 1 + frac(d #h(0.2em) "cos" #h(0.2em) θ, 2 r) ) − frac(q, r) ( 1 − frac(d #h(0.2em) "cos" #h(0.2em) θ, 2 r) ) ] = k_(e) frac(q d #h(0.2em) "cos" #h(0.2em) θ, r^(2)) .$] This may be written more conveniently if we define a new quantity, the #strong[electric dipole moment], #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "p → equals q d → ,")[$arrow("p") = q arrow("d") ,$] ] where these vectors point from the negative to the positive charge. Note that this has magnitude #emph[qd]. This quantity allows us to write the potential at point #emph[P] due to a dipole at the origin as #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "V sub P equals k sub e the fraction p → times r ^ over r squared .")[$V_(P) = k_(e) frac(arrow("p") · hat("r"), r^(2)) .$] ] A diagram of the application of this formula is shown. #figure(figph[The figure shows two vectors r and p with an angle theta between them.], alt: "The figure shows two vectors r and p with an angle theta between them.", caption: [The geometry for the application of the potential of a dipole.]) There are also higher-order moments, for quadrupoles, octupoles, and so on. You will see these in future classes. === Potential of Continuous Charge Distributions We have been working with point charges a great deal, but what about continuous charge distributions? Recall from that #math.equation(block: true, alt: "V sub P equals k sub e ∑ the fraction q sub i over r sub i .")[$V_(P) = k_(e) display(∑ frac(q_(i), r_(i))) .$] We may treat a continuous charge distribution as a collection of infinitesimally separated individual points. This yields the integral #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #math.equation(block: true, alt: "V sub P equals k sub e ∫ the fraction d q over r")[$V_(P) = k_(e) display(∫ frac(d q, r))$] ] for the potential at a point #emph[P]. Note that #emph[r] is the distance from each individual point in the charge distribution to the point #emph[P]. As we saw in Electric Charges and Fields, the infinitesimal charges are given by #math.equation(block: true, alt: "d q equals { λ d l, open parenthesis one dimension close parenthesis; σ d A, open parenthesis two dimensions close parenthesis; ρ d V, open parenthesis three dimensions close parenthesis")[$d q = \{ λ #h(0.2em) d l & & ( "one dimension" ) \ σ #h(0.2em) d A & & ( "two dimensions" ) \ ρ #h(0.2em) d V & & ( "three dimensions" )$] where #math.equation(block: false, alt: "λ")[$λ$] is linear charge density, #math.equation(block: false, alt: "σ")[$σ$] is the charge per unit area, and #math.equation(block: false, alt: "ρ")[$ρ$] is the charge per unit volume. #examplebox("Example 4")[Potential of a Line of Charge][ Find the electric potential of a uniformly charged, nonconducting wire with linear density #math.equation(block: false, alt: "λ")[$λ$] (coulomb/meter) and length #emph[L] at a point that lies on a line that divides the wire into two equal parts. Strategy To set up the problem, we choose Cartesian coordinates in such a way as to exploit the symmetry in the problem as much as possible. We place the origin at the center of the wire and orient the #emph[y]-axis along the wire so that the ends of the wire are at #math.equation(block: false, alt: "y equals plus or minus L / 2")[$y = ± L "/" 2$]. The field point #emph[P] is in the #emph[xy]-plane and since the choice of axes is up to us, we choose the #emph[x]-axis to pass through the field point #emph[P], as shown. #figure(figph[The figure shows a line charge on the y-axis with its center at the origin. Point P is located on the x-axis at distance x away from the origin.], alt: "The figure shows a line charge on the y-axis with its center at the origin. Point P is located on the x-axis at distance x away from the origin.", caption: [We want to calculate the electric potential due to a line of charge.]) Solution Consider a small element of the charge distribution between #emph[y] and #math.equation(block: false, alt: "y plus d y")[$y + d y$]. The charge in this cell is #math.equation(block: false, alt: "d q equals λ d y")[$d q = λ #h(0.2em) d y$] and the distance from the cell to the field point #emph[P] is #math.equation(block: false, alt: "the square root of x squared plus y squared .")[$sqrt(x^(2) + y^(2)) .$] Therefore, the potential becomes #math.equation(block: true, alt: "V sub P, equals k sub e ∫ the fraction d q over r equals k sub e ∫ − L / 2 L / 2 the fraction λ d y over the square root of x squared plus y squared equals k sub e λ [ ln open parenthesis y plus the square root of y squared plus x squared close parenthesis ] − L / 2 L / 2; equals k sub e λ [ ln open parenthesis open parenthesis the fraction L over 2 close parenthesis plus the square root of open parenthesis the fraction L over 2 close parenthesis squared plus x squared close parenthesis minus ln open parenthesis open parenthesis minus the fraction L over 2 close parenthesis plus the square root of open parenthesis minus the fraction L over 2 close parenthesis squared plus x squared close parenthesis ]; equals k sub e λ ln [ the fraction L plus the square root of L squared plus 4 x squared")[$V_(P) & = k_(e) display(∫ frac(d q, r) = k_(e) display(∫_("−" L "/" 2)^(L "/" 2) frac(λ d y, sqrt(x^(2) + y^(2))) = k_(e) λ attach([ "ln" ( y + sqrt(y^(2) + x^(2)) ) ], t: L "/" 2, b: "−" L "/" 2))) \ & = k_(e) λ [ "ln" ( ( frac(L, 2) ) + sqrt(attach(( frac(L, 2) ), t: 2) + x^(2)) ) − "ln" ( ( − frac(L, 2) ) + sqrt(attach(( − frac(L, 2) ), t: 2) + x^(2)) ) ] \ & = k_(e) λ "ln" [ frac(L + sqrt(L^(2) + 4 x^(2)), "−" L + sqrt(L^(2) + 4 x^(2))) ] .$] Significance Note that this was simpler than the equivalent problem for electric field, due to the use of scalar quantities. Recall that we expect the zero level of the potential to be at infinity, when we have a finite charge. To examine this, we take the limit of the above potential as #emph[x] approaches infinity; in this case, the terms inside the natural log approach one, and hence the potential approaches zero in this limit. Note that we could have done this problem equivalently in cylindrical coordinates; the only effect would be to substitute #emph[r] for #emph[x] and #emph[z] for #emph[y]. ] #examplebox("Example 5")[Potential Due to a Ring of Charge][ A ring has a uniform charge density #math.equation(block: false, alt: "λ")[$λ$], with units of coulomb per unit meter of arc. Find the electric potential at a point on the axis passing through the center of the ring. Strategy We use the same procedure as for the charged wire. The difference here is that the charge is distributed on a circle. We divide the circle into infinitesimal elements shaped as arcs on the circle and use cylindrical coordinates shown. #figure(figph[The figure shows a ring of charge located on the xy-plane with its center at the origin. Point P is located on the z-axis at distance z away from the origin.], alt: "The figure shows a ring of charge located on the xy-plane with its center at the origin. Point P is located on the z-axis at distance z away from the origin.", caption: [We want to calculate the electric potential due to a ring of charge.]) Solution A general element of the arc between #math.equation(block: false, alt: "θ")[$θ$] and #math.equation(block: false, alt: "θ plus d θ")[$θ + d θ$] is of length #math.equation(block: false, alt: "R d θ")[$R d θ$] and therefore contains a charge equal to #math.equation(block: false, alt: "λ R d θ .")[$λ R d θ .$] The element is at a distance of #math.equation(block: false, alt: "the square root of z squared plus R squared")[$sqrt(z^(2) + R^(2))$] from #emph[P], and therefore the potential is #math.equation(block: true, alt: "V sub P equals k sub e ∫ the fraction d q over r equals k sub e ∫ 0 2 π the fraction λ R d θ over the square root of z squared plus R squared equals the fraction k sub e λ R over the square root of z squared plus R squared ∫ 0 2 π d θ equals the fraction 2 π k sub e λ R over the square root of z squared plus R squared equals k sub e the fraction q sub tot over the square root of z squared plus R squared .")[$V_(P) = k_(e) display(∫ frac(d q, r)) = k_(e) display(∫_(0)^(2 π) frac(λ R d θ, sqrt(z^(2) + R^(2)))) = frac(k_(e) λ R, sqrt(z^(2) + R^(2))) display(∫_(0)^(2 π) #h(0.2em) d θ) = frac(2 π k_(e) λ R, sqrt(z^(2) + R^(2))) = k_(e) frac(q_("tot"), sqrt(z^(2) + R^(2))) .$] Significance This result is expected because every element of the ring is at the same distance from point #emph[P]. The net potential at #emph[P] is that of the total charge placed at the common distance, #math.equation(block: false, alt: "the square root of z squared plus R squared")[$sqrt(z^(2) + R^(2))$]. ] #examplebox("Example 6")[Potential Due to a Uniform Disk of Charge][ A disk of radius #emph[R] has a uniform charge density #math.equation(block: false, alt: "σ")[$σ$], with units of coulomb meter squared. Find the electric potential at any point on the axis passing through the center of the disk. Strategy We divide the disk into ring-shaped cells, and make use of the result for a ring worked out in the previous example, then integrate over #emph[r] in addition to #math.equation(block: false, alt: "θ")[$θ$]. This is shown. #figure(figph[The figure shows a disk of charge located on the xy-plane with its center at the origin. Point P is located on the z-axis at distance z away from the origin.], alt: "The figure shows a disk of charge located on the xy-plane with its center at the origin. Point P is located on the z-axis at distance z away from the origin.", caption: [We want to calculate the electric potential due to a disk of charge.]) Solution An infinitesimal width cell between cylindrical coordinates #emph[r] and #math.equation(block: false, alt: "r plus d r")[$r + d r$] shown in will be a ring of charges whose electric potential #math.equation(block: false, alt: "d V sub P")[$d V_(P)$] at the field point has the following expression #math.equation(block: true, alt: "d V sub P equals k sub e the fraction d q over the square root of z squared plus r squared")[$d V_(P) = k_(e) frac(d q, sqrt(z^(2) + r^(2)))$] where #math.equation(block: true, alt: "d q equals σ 2 π r d r .")[$d q = σ 2 π r d r .$] The superposition of potential of all the infinitesimal rings that make up the disk gives the net potential at point #emph[P]. This is accomplished by integrating from #math.equation(block: false, alt: "r equals 0")[$r = 0$] to #math.equation(block: false, alt: "r equals R")[$r = R$]: #math.equation(block: true, alt: "V sub P, equals ∫ d V sub P equals k sub e 2 π σ ∫ 0 R the fraction r d r over the square root of z squared plus r squared ,; equals k sub e 2 π σ open parenthesis the square root of z squared plus R squared minus the square root of z squared close parenthesis .")[$V_(P) & = display(∫ d V_(P)) = k_(e) 2 π σ display(∫_(0)^(R) frac(r #h(0.2em) d r, sqrt(z^(2) + r^(2)))) , \ & = k_(e) 2 π σ ( sqrt(z^(2) + R^(2)) − sqrt(z^(2)) ) .$] Significance The basic procedure for a disk is to first integrate around #math.equation(block: false, alt: "θ")[$θ$] and then over #emph[r]. This has been demonstrated for uniform (constant) charge density. Often, the charge density will vary with #emph[r], and then the last integral will give different results. ] #examplebox("Example 7")[Potential Due to an Infinite Charged Wire][ Find the electric potential due to an infinitely long uniformly charged wire. Strategy Since we have already worked out the potential of a finite wire of length #emph[L] in , we might wonder if taking #math.equation(block: false, alt: "L → ∞")[$L → ∞$] in our previous result will work: #math.equation(block: true, alt: "V sub P equals lim L → ∞ k sub e λ ln open parenthesis the fraction L plus the square root of L squared plus 4 x squared over − L plus the square root of L squared plus 4 x squared close parenthesis .")[$V_(P) = limits("lim")_(L → ∞) #h(0.2em) k_(e) λ "ln" ( frac(L + sqrt(L^(2) + 4 x^(2)), "−" L + sqrt(L^(2) + 4 x^(2))) ) .$] However, this limit does not exist because the argument of the logarithm becomes \[2/0\] as #math.equation(block: false, alt: "L → ∞")[$L → ∞$], so this way of finding #emph[V] of an infinite wire does not work. The reason for this problem may be traced to the fact that the charges are not localized in some space but continue to infinity in the direction of the wire. Hence, our (unspoken) assumption that zero potential must be an infinite distance from the wire is no longer valid. To avoid this difficulty in calculating limits, let us use the definition of potential by integrating over the electric field from the previous section, and the value of the electric field from this charge configuration from the previous chapter. Solution We use the integral #math.equation(block: true, alt: "V sub P equals − ∫ R P E → times d l →")[$V_(P) = "−" display(∫_(R)^(P) arrow("E") · d arrow("l"))$] where #emph[R] is a finite distance from the line of charge, as shown. #figure(figph[The figure shows an infinite line charge on the z-axis. Points P and R are located on the x-axis.], alt: "The figure shows an infinite line charge on the z-axis. Points P and R are located on the x-axis.", caption: [Points of interest for calculating the potential of an infinite line of charge.]) With this setup, we use #math.equation(block: false, alt: "E → sub P equals 2 k sub e λ the fraction 1 over s s ^")[$arrow("E")_(P) = 2 k_(e) λ frac(1, s) hat("s")$] and #math.equation(block: false, alt: "d l → equals d s →")[$d arrow("l") = d arrow("s")$] to obtain #math.equation(block: true, alt: "V sub P minus V sub R equals − ∫ R P 2 k sub e λ the fraction 1 over s d s equals −2 k sub e λ ln the fraction s sub P over s sub R .")[$V_(P) − V_(R) = "−" display(∫_(R)^(P) #h(0.2em) 2 k_(e) λ frac(1, s) d s) = −2 k_(e) λ "ln" frac(s_(P), s_(R)) .$] Now, if we define the reference potential #math.equation(block: false, alt: "V sub R equals 0")[$V_(R) = 0$] at #math.equation(block: false, alt: "s sub R equals 1 m,")[$s_(R) = 1 #h(0.2em) "m,"$] this simplifies to #math.equation(block: true, alt: "V sub P equals −2 k sub e λ ln s sub P .")[$V_(P) = −2 k_(e) λ #h(0.2em) "ln" #h(0.2em) s_(P) .$] Note that this form of the potential is quite usable; it is 0 at 1 m and is undefined at infinity, which is why we could not use the latter as a reference. Significance Although calculating potential directly can be quite convenient, we just found a system for which this strategy does not work well. In such cases, going back to the definition of potential in terms of the electric field may offer a way forward. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ What is the potential on the axis of a nonuniform ring of charge, where the charge density is #math.equation(block: false, alt: "λ open parenthesis θ close parenthesis equals λ cos θ")[$λ ( θ ) = λ #h(0.2em) "cos" #h(0.2em) θ$] ? #solutionbox[ It will be zero, as at all points on the axis, there are equal and opposite charges equidistant from the point of interest. Note that this distribution will, in fact, have a dipole moment. ] ] === Summary - Electric potential is a scalar whereas electric field is a vector. - Addition of voltages as numbers gives the voltage due to a combination of point charges, allowing us to use the principle of superposition: #math.equation(block: false, alt: "V sub P equals k sub e ∑ 1 N the fraction q sub i over r sub i")[$V_(P) = k_(e) display(limits(∑)^(N)_(1) frac(q_(i), r_(i)))$]. - An electric dipole consists of two equal and opposite charges a fixed distance apart, with a dipole moment #math.equation(block: false, alt: "p → equals q d →")[$arrow("p") = q arrow("d")$]. - Continuous charge distributions may be calculated with #math.equation(block: false, alt: "V sub P equals k sub e ∫ the fraction d q over r")[$V_(P) = k_(e) display(∫ frac(d q, r))$]. === Conceptual Questions Compare the electric dipole moments of charges #math.equation(block: false, alt: "plus or minus Q")[$± Q$] separated by a distance #emph[d] and charges #math.equation(block: false, alt: "plus or minus Q / 2")[$± Q "/" 2$] separated by a distance #emph[d]/2. #solutionbox[ The second has 1/4 the dipole moment of the first. ] Would Gauss’s law be helpful for determining the electric field of a dipole? Why? In what region of space is the potential due to a uniformly charged sphere the same as that of a point charge? In what region does it differ from that of a point charge? #solutionbox[ The region outside of the sphere will have a potential indistinguishable from a point charge; the interior of the sphere will have a different potential. ] Can the potential of a nonuniformly charged sphere be the same as that of a point charge? Explain. === Problems A 0.500-cm-diameter plastic sphere, used in a static electricity demonstration, has a uniformly distributed 40.0-pC charge on its surface. What is the potential near its surface? #solutionbox[ #math.equation(block: true, alt: "V equals 144 V")[$V = 144 #h(0.2em) "V"$] ] How far from a #math.equation(block: false, alt: "1.00 - μ C")[$1.00 "-" μ "C"$] point charge is the potential 100 V? At what distance is it #math.equation(block: false, alt: "2.00 times 10 squared V?")[$2.00 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "V?"$] If the potential due to a point charge is #math.equation(block: false, alt: "5.00 times 10 squared V")[$5.00 #h(0.2em) × #h(0.2em) 10^(2) #h(0.2em) "V"$] at a distance of 15.0 m, what are the sign and magnitude of the charge? #solutionbox[ #math.equation(block: false, alt: "V equals the fraction k Q over r → Q equals 8.33 times 10 to the power −7 C")[$V = frac(k Q, r) → Q = 8.33 #h(0.2em) × #h(0.2em) 10^(−7) #h(0.2em) "C"$]; #linebreak() The charge is positive because the potential is positive. ] In nuclear fission, a nucleus splits roughly in half. (a) What is the potential #math.equation(block: false, alt: "2.00 times 10 to the power −14 m")[$2.00 #h(0.2em) × #h(0.2em) 10^(−14) #h(0.2em) "m"$] from a fragment that has 46 protons in it? (b) What is the potential energy in MeV of a similarly charged fragment at this distance? A research Van de Graaff generator has a 2.00-m-diameter metal sphere with a charge of 5.00 mC on it. Assume the potential energy is zero at a reference point infinitely far away from the Van de Graaff. (a) What is the potential near its surface? (b) At what distance from its center is the potential 1.00 MV? (c) An oxygen atom with three missing electrons is released near the Van de Graaff generator. What is its kinetic energy in MeV when the atom is at the distance found in part b? #solutionbox[ a. #math.equation(block: false, alt: "V equals 45.0 MV")[$V = 45.0 #h(0.2em) "MV"$]; #linebreak() b. #math.equation(block: false, alt: "V equals the fraction k Q over r → r equals 45.0 m")[$V = frac(k Q, r) → r = 45.0 #h(0.2em) "m"$]; #linebreak() c. #math.equation(block: false, alt: "Δ U equals 132 MeV")[$"Δ" U = 132 #h(0.2em) "MeV"$] ] An electrostatic paint sprayer has a 0.200-m-diameter metal sphere at a potential of 25.0 kV that repels paint droplets onto a grounded object. (a) What charge is on the sphere? (b) What charge must a 0.100-mg drop of paint have to arrive at the object with a speed of 10.0 m/s? (a) What is the potential between two points situated 10 cm and 20 cm from a #math.equation(block: false, alt: "3.0 - μ C")[$3.0 "-" μ "C"$] point charge? (b) To what location should the point at 20 cm be moved to increase this potential difference by a factor of two? #solutionbox[ #math.equation(block: false, alt: "V equals k Q / r")[$V = k Q / r$]; a. Relative to origin, find the potential at each point and then calculate the difference. #linebreak() #math.equation(block: false, alt: "Δ V equals 135 times 10 cubed V")[$"Δ" V = 135 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V"$]; #linebreak() b. To double the potential difference, move the point from 20 cm to infinity; the potential at 20 cm is halfway between zero and that at 10 cm. ] Find the potential at points #math.equation(block: false, alt: "P sub 1 , P sub 2 , P sub 3 , and P sub 4")[$P_(1) , P_(2) , P_(3) , #h(0.2em) "and" #h(0.2em) P_(4)$] in the diagram due to the two given charges. #figure(figph[The figure shows two charges, 5mC (located 4cm left from the center) and -10mC (located 4cm right from the center). Four Point P subscript 1, P subscript 2, P subscript 3 and P subscript 4 are located 2cm left, 2cm right, 3cm below and 3cm above the center.], alt: "The figure shows two charges, 5mC (located 4cm left from the center) and -10mC (located 4cm right from the center). Four Point P subscript 1, P subscript 2, P subscript 3 and P subscript 4 are located 2cm left, 2cm right, 3cm below and 3cm above the center.", caption: none) Two charges #math.equation(block: false, alt: "–2.0 µ C and + 2.0 µ C")[$–2.0 #h(0.2em) µ "C" #h(0.2em) "and" #h(0.2em) "+" 2.0 #h(0.2em) µ "C"$] are separated by 4.0 cm on the #emph[z]-axis symmetrically about origin, with the positive one uppermost. Two space points of interest #math.equation(block: false, alt: "P sub 1 and P sub 2")[$P_(1) #h(0.2em) "and" #h(0.2em) P_(2)$] are located 3.0 cm and 30 cm from origin at an angle #math.equation(block: false, alt: "30 °")[$30 "°"$] with respect to the #emph[z]-axis. Evaluate electric potentials at #math.equation(block: false, alt: "P sub 1 and P sub 2")[$P_(1) #h(0.2em) "and" #h(0.2em) P_(2)$] in two ways: (a) Using the exact formula for point charges, and (b) using the approximate dipole potential formula. #solutionbox[ a. #math.equation(block: false, alt: "V sub P 1 equals 7.4 times 10 to the power 5 V")[$V_(P 1) = 7.4 #h(0.2em) × #h(0.2em) 10^(5) #h(0.2em) "V"$] #linebreak() and #math.equation(block: false, alt: "V sub P 2 equals 6.9 times 10 cubed V")[$V_(P 2) = 6.9 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V"$]; #linebreak() b. #math.equation(block: false, alt: "V sub P 1 equals 6.9 times 10 to the power 5 V")[$V_(P 1) = 6.9 #h(0.2em) × #h(0.2em) 10^(5) #h(0.2em) "V"$] and #math.equation(block: false, alt: "V sub P 2 equals 6.9 times 10 cubed V")[$V_(P 2) = 6.9 #h(0.2em) × #h(0.2em) 10^(3) #h(0.2em) "V"$] ] (a) Plot the potential of a uniformly charged 1-m rod with 1 C/m charge as a function of the perpendicular distance from the center. Draw your graph from #math.equation(block: false, alt: "s equals 0.1 m to s equals 1.0 m")[$"s" = 0.1 #h(0.2em) "m" #h(0.2em) "to" #h(0.2em) "s" = 1.0 #h(0.2em) "m"$]. (b) On the same graph, plot the potential of a point charge with a 1-C charge at the origin. (c) Which potential is stronger near the rod? (d) What happens to the difference as the distance increases? Interpret your result.