 # Eletromagnetismo - Hayt - 7ª Ed - Soluções - chapter06 7th solution

(Parte 1 de 2)

CHAPTER 6.

6.1. Atomic hydrogen contains 5.5×1025 atoms/m3 at a certain temperature and pressure. When an electric ﬁeld of 4 kV/m is applied, each dipole formed by the electron and positive nucleus has an eﬀective length of 7.1 × 10−19 m. a) Find P: With all identical dipoles, we have χe = P 6.2. Find the dielectric constant of a material in which the electric ﬂux density is four times the polarization. 6.3. A coaxial conductor has radii a =0 .8m ma nd b =3 m and a polystyrene dielectric for ρ aρ V/m

Then

b) Find Vab and χe: Use

ρ aρ C·m

6.4. Consider a composite material made up of two species, having number densities N1 and N2 molecules/m3 respectively. The two materials are uniformly mixed, yielding a total number density of N = N1 + N2. The presence of an electric ﬁeld E, induces molecular dipole moments p1 and p2 within the individual species, whether mixed or not. Show that the dielectric constant of the composite material is given by r = f r1 +(1−f) r2, where f is the number fraction of species 1 dipoles in the composite, and where r1 and r2 are the dielectric constants that the unmixed species would have if each had number density N.

We may write the total polarization vector as

In terms of the susceptibilities, this becomes Ptot = 0 [fχe1 +( 1 − f)χe2]E, where χe1 and χe2 are evaluated at the composite number density, N.N ow

Identifying r as shown, we may rewrite it by adding and subracting f:

e) The angle θ1 between E1 and a normal to the surface: Use

6.6. The potential ﬁeld in a slab of dielectric material for which r =1 .6i sg iven by V = −5000x. a) Find D, E, and P in the material.

50az V/m, ﬁnd E2:W e need to ﬁnd the components of E1 that are normal and tangent to the boundary, and then apply the appropriate boundary conditions. The normal component will be EN1 = E1 · n.T aking f = x − y +2 z, the unit vector that is normal to the surface is

This normal will point in the direction of increasing f, which will be away from the origin, or into region 2 (you can visualize a portion of the surface as a triangle whose vertices are on the

the surface. Then

a) Find D2: One approach is to ﬁrst ﬁnd E2. This will have the same y and z (tangential) components as E1, but the normal component, Ex, will diﬀer by the ratio r1/ r2; this arises from Dx1 = Dx2 (normal component of D is continuous across a non-charged

6.9. Let the cylindrical surfaces ρ =4 cm and ρ =9 cm enclose two wedges of perfect dielectrics, r1 =2 for 0 <φ<π /2, and r2 =5 for π/2 <φ< 2π.I f E1 = (2000/ρ)aρ V/m, ﬁnd: a) E2: The interfaces between the two media will lie on planes of constant φ,t o which E1 is parallel. Thus the ﬁeld is the same on either side of the boundaries, and so E2 = E1.

b) the total electrostatic energy stored in a 1m length of each region: In general we have wE =( 1/2) r 0E2.S oi n region 1:

ρdρdφdz = π

In region 2, we have

ρdρdφdz = 15π

b) After connecting a 6 V battery across the capacitor, calculate E, D, Q, and the total stored electrostatic energy: First,

c) With the source still connected, the dielectric is carefully withdrawn from between the plates. With the dielectric gone, re-calculate E, D, Q, and the energy stored in the capacitor.

d) If the charge and energy found in (c) are less than that found in (b) (which you should have discovered), what became of the missing charge and energy? In the absence of friction in removing the dielectric, the charge and energy have returned to the battery that gave it.

6.1. Capacitors tend to be more expensive as their capacitance and maximum voltage, Vmax, increase. The voltage Vmax is limited by the ﬁeld strength at which the dielectric breaks down, EBD. Which of these dielectrics will give the largest CVmax product for equal plate areas: (a) air: r =1 , EBD =3 MV/m; (b) barium titanate: r = 1200, EBD =3 MV/m; (c) silicon dioxide: r =3 .78, EBD =1 6 MV/m; (d) polyethylene: r =2 .26, EBD =4 .7 MV/m? Note that Vmax = EBDd, where d is the plate separation. Also, C = r 0A/d, and so VmaxC = r 0AEBD, where A is the plate area. The maximum CVmax product is found through the maximum rEBD product. Trying this with the given materials yields the winner, which is barium titanate.

6.12. An air-ﬁlled parallel-plate capacitor with plate separation d and plate area A is connected to a battery which applies a voltage V0 between plates. With the battery left connected, the plates are moved apart to a distance of 10d. Determine by what factor each of the following quantities changes:

a) V0: Remains the same, since the battery is left connected.

b) C:A s C = 0A/d, increasing d by a factor of ten decreases C by a factor of 0.1.

c) E:W e require E × d = V0, where V0 has not changed. Therefore, E has decreased by a factor of 0.1.

f) ρS:A s Q is reduced by 0.1, ρS reduces by 0.1. This is also consistent with D having been reduced by 0.1.

assuming charge density ρs on the top plate. D will, as usual, be x-directed, originating at the top plate and terminating on the bottom plate. The key here is that D will be constant over the distance between plates. This can be understood by considering the x-varying dielectric as constructed of many thin layers, each having constant permittivity. The permittivity changes from layer to layer to approximate the given function of x. The approximation becomes exact as the layer thicknesses approach zero. We know that D, which is normal to the layers, will be continuous across each boundary, and so D is constant over the plate separation distance, and will be given in magnitude by ρs. The electric ﬁeld magnitude is now

The voltage beween plates is then

ρs dx

6.14. Repeat Problem 6.12 assuming the battery is disconnected before the plate separation is increased: The ordering of parameters is changed over that in Problem 6.12, as the progression of thought on the matter is diﬀerent.

a) Q: Remains the same, since with the battery disconnected, the charge has nowhere to go.

b) ρS:A s Q is unchanged, ρS is also unchanged, since the plate area is the same. c) D:A s D = ρS,i t will remain the same also. d) E: Since E = D/ 0, and as D is not changed, E will also remain the same.

e) V0:W e require E × d = V0, where E has not changed. Therefore, V0 has increased by a factor of 10.

f) C:A s C = 0A/d, increasing d bya factor of ten decreases C bya factor of 0.1. The same result occurs because C = Q/V0, where V0 is increased by 10, whereas Q has not changed.

meter of surface area if: a) r3 is that of air; b) r3 = r1;c ) r3 = r2;d ) region 3 is silver: The combination will be three capacitors in series, for which

So that

Evaluating this for the four cases, we ﬁnd a) C =3 .05nF for r3 =1 , b) C =5 .21nF for r3 =2 .5, c) C =6 .32nF for r3 =4 , and d) C =9 .83nF if silver (taken as a perfect conductor) forms region 3; this has the eﬀect of removing the term involving r3 from the original formula (ﬁrst equation line), or equivalently, allowing r3 to approach inﬁnity.

6.16. A parallel-plate capacitor is made using two circular plates of radius a, with the bottom plate on the xy plane, centered at the origin. The top plate is located at z = d, with its center on the z axis. Potential V0 is on the top plate; the bottom plate is grounded. Dielectric having radially-dependent permittivity ﬁlls the region between plates. The permittivity is given by

(ρ)= 0(1 + ρ/a). Find: a) E: Since does not vary in the z direction, and since we must always obtain V0 when integrating E between plates, it must follow that E = −V0/daz V/m.

6.17. Two coaxial conducting cylinders of radius 2 cm and 4 cm have a length of 1m. The region between the cylinders contains a layer of dielectric from ρ = c to ρ = d with r =4 . Find the capacitance if a) c =2 cm, d =3 cm: This is two capacitors in series, and so

4 ln b) d =4 cm, and the volume of the dielectric is the same as in part a:H aving equal volumes requires that 32 − 2 =4 2 − c2, from which c =3 .32 cm. Now

4 ln

6.18. (a) If we could specify a material to be used as the dielectric in a coaxial capacitor for which the permittivity varied continuously with radius, what variation with ρ should be used in order to maintain a uniform value of the electric ﬁeld intensity?

Gauss’s law tells us that regardless of the radially-varying permittivity, D =( aρs/ρ)aρ, where a is the inner radius and ρs is the presumed surface charge density on the inner cylinder. Now which indicates that must have a 1/ρ dependence if E is to be constant with radius.

b) Under the conditions of part a,h ow do the inner and outer radii appear in the expression for the capacitance per unit distance? Let = g/ρ where g is a constant. Then E = aρs/g aρ and the voltage between cylinders will be

aρsg aρ · aρ dρ = aρsg where b is the outer radius. The capacitance per unit length is then C =2 πaρs/V0 = 2πg/(b − a), or a simple inverse-distance relation.

6.19. Two conducting spherical shells have radii a =3 cm and b =6 cm. The interior is a perfect dielectric for which r =8 .

a) Find C:F or a spherical capacitor, we know that:

b) A portion of the dielectric is now removed so that r =1 .0, 0 <φ<π /2, and r =8 , π/2 <φ< 2π. Again, ﬁnd C:W e recognize here that removing that portion leaves us with two capacitors in parallel (whose C’s will add). We use the fact that with the dielectric completely removed, the capacitance would be C( r =1 )=5 3.3/8=6 .67pF. With one-fourth the dielectric removed, the total capacitance will be

6.20. Show that the capacitance per unit length of a cylinder of radius a is zero: Let ρs be the surface charge density on the surface at ρ = a. Then the charge per unit length is Q =2 πaρs.

The electric ﬁeld (assuming free space) is E =( aρs)/( 0ρ)aρ. The potential diﬀerence is evaluated between radius a and inﬁnite radius, and is

aρs

The capacitance, equal to Q/V0,i s therefore zero.

We then have

6.2. Two #16 copper conductors (1.29-m diameter) are parallel with a separation d between axes. Determine d so that the capacitance between wires in air is 30 pF/m.

We use C

The above expression evaluates the capacitance of one of the wires suspended over a plane at mid-span, h = d/2. Therefore the capacitance of that structure is doubled over that required (from 30 to 60 pF/m). Using this,

6.23. A 2 cm diameter conductor is suspended in air with its axis 5 cm from a conducting plane.

Let the potential of the cylinder be 100 V and that of the plane be 0 V. Find the surface charge density on the:

a) cylinder at a point nearest the plane: The cylinder will image across the plane, producing an equivalent two-cylinder problem, with the second one at location 5 cm below the plane. We will take the plane as the zy plane, with the cylinder positions at x = ±5.

6.23a. (continued) and

b) plane at a point nearest the cylinder: At x = y =0 ,

[ aax

from which

6.24. For the conductor conﬁguration of Problem 6.23, determine the capacitance per unit length. This is a quick one if we have already solved 6.23. The capacitance per unit length will be

6.25 Construct a curvilinear square map for a coaxial capacitor of 3-cm inner radius and 8-cm outer radius. These dimensions are suitable for the drawing.

a) Use your sketch to calculate the capacitance per meter length, assuming R =1 : The sketch is shown below. Note that only a 9◦ sector was drawn, since this would then be duplicated 40 times around the circumference to complete the drawing. The capacitance is thus

b) Calculate an exact value for the capacitance per unit length: This will be

6.26 Construct a curvilinear-square map of the potential ﬁeld about two parallel circular cylinders, each of 2.5 cm radius, separated by a center-to-center distance of 13cm. These dimensions are suitable for the actual sketch if symmetry is considered. As a check, compute the capacitance per meter both from your sketch and from the exact formula. Assume R =1 .

Symmetry allows us to plot the ﬁeld lines and equipotentials over just the ﬁrst quadrant, as is done in the sketch below (shown to one-half scale). The capacitance is found from the formula C =( NQ/NV ) 0, where NQ is twice the number of squares around the perimeter of the half-circle and NV is twice the number of squares between the half-circle and the left vertical plane. The result is

We check this result with that using the exact formula:

6.27. Construct a curvilinear square map of the potential ﬁeld between two parallel circular cylinders, one of 4-cm radius inside one of 8-cm radius. The two axes are displaced by 2.5 cm. These dimensions are suitable for the drawing. As a check on the accuracy, compute the capacitance per meter from the sketch and from the exact expression:

where a and b are the conductor radii and D is the axis separation.

The drawing is shown below. Use of the exact expression above yields a capacitance value of C =1 1.5 0 F/m. Use of the drawing produces:

6.28. A solid conducting cylinder of 4-cm radius is centered within a rectangular conducting cylinder with a 12-cm by 20-cm cross-section.

a) Make a full-size sketch of one quadrant of this conﬁguration and construct a curvilinearsquare map for its interior: The result below could still be improved a little, but is nevertheless suﬃcient for a reasonable capacitance estimate. Note that the ﬁve-sided region in the upper right corner has been partially subdivided (dashed line) in anticipation of how it would look when the next-level subdivision is done (doubling the number of ﬁeld lines and equipotentials).

b) Assume = 0 and estimate C per meter length: In this case NQ is the number of squares around the full perimeter of the circular conductor, or four times the number of squares shown in the drawing. NV is the number of squares between the circle and the rectangle, or 5. The capacitance is estimated to be

6.29. The inner conductor of the transmission line shown in Fig. 6.14 has a square cross-section 2a × 2a, while the outer square is 5a × 5a. The axes are displaced as shown. (a) Construct ag ood-sized drawing of the transmission line, say with a =2 .5 cm, and then prepare a curvilinear-square plot of the electrostatic ﬁeld between the conductors. (b) Use the map to calculate the capacitance per meter length if =1 .6 0. (c) How would your result to part b change if a =0 .6 cm? a) The plot is shown below. Some improvement is possible, depending on how much time one wishes to spend.

b) From the plot, the capacitance is found to be

c) If a is changed, the result of part b would not change, since all dimensions retain the same relative scale.

6.30. For the coaxial capacitor of Problem 6.18, suppose that the dielectric is leaky, allowing current to ﬂow between the inner and outer conductors, while the electric ﬁeld is still uniform with radius.

a) What functional form must the dielectric conductivity assume? We must have constant current through any cross-section, which means that J = I/(2πρ)aρ A/m2, where I is the radial current per unit length. Then, from J = σE, where E is constant, we require a1 /ρ dependence on σ,o rl et σ = σ0/ρ, where σ0 is a constant.

b) What is the basic functional form of the resistance per unit distance, R?F rom Problem

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