Chapter 26 Problems 1 , 2, 3 = straightforward, intermediate, challenging Section 26. 1 Definition of Capacitance 1. (a) How much charge is on each plate of a 4. 00-pF capacitor when it is connected toa 12. 0-V battery? (b) If this same capacitor is connected toa 1. 50-V battery, what charge is stored? 2. Two conductors having net charges of +10. 0 pc and -10. 0 pc have a potential difference of 10. 0 V between them. (a) Determine the capacitance of the system. (b) What is the potential difference between the two conductors if the charges on each are increased to +100 pc and -100 pC?
Section 26. 2 Calculating Capacitance . An isolated charged conducting sphere of radius 12. 0 cm creates an electric field of 4. 90 x 104 NIC at a distance 21. 0 cm from its center. (a) What is its surface charge density? (b) What is its capacitance? 4. (a) If a drop of liquid has capacitance 1. 00 pF, what is its radius? (b) If another drop has radius 2. 00 mm, what is its capacitance? (c) What is the charge on the smaller drop if its potential is 100 V? 5. Two conducting spheres with diameters of 0. 400 m and 1. 00 m are separated by a distance that is large compared with the diameters.
The spheres are connected by a thin wire and are charged to 7. 00 pC. a) How is this total charge shared between the spheres? (Ignore any charge on the wire. ) (b) What is the potential of the system of spheres when the reference potential is taken to be V = O at r = 6. Regarding the Earth and a cloud layer 800 m above the Earth as the “plates” of a capacitor, calculate the capacitance. Assume the cloud layer has an area of 1. 00 km2 and that the air between the cloud and the ground is pure and dry. Assume charge builds up on the cloud and on the ground until a uniform electric field of 3. 0 x 106 NIC throughout the space between them makes the air break down and conduct electricity as a lightning bolt. What is the maximum charge the cloud can hold? 7. An air-filled capacitor consists of two parallel plates, each with an area of 7. 60 cm2, separated by a distance of 1. 80 mm. A 20. 0-V potential difference is applied to these plates. Calculate (a) the electric field between the plates, (b) the surface charge density, (c) the capacitance, and (d) the charge on each plate. capacitor has a plate area of 21. 0 x 10-12 m2.
Determine the plate separation of such a capacitor (assume a parallel-plate confguration). The order of magnitude of the diameter of an atom is 10-10 m = 0. 1 nm. Express the plate separation in nanometers. . When a potential difference of 150 V is applied to the plates ofa parallel-plate capacitor, the plates carry a surface charge density of 30. 0 nC/cm2. What is the spacing between the plates? 10. A variable air capacitor used in a radio tuning circuit is made of N semicircular plates each of radius R and positioned a distance d from its neighbors, to which it is electrically connected.
As shown in Figure P26. 10, a second identical set of plates is enmeshed with its plates halfway between those of the first set. The second set can rotate as a unit. Determine the capacitance as a function of the angle of rotation B, here B = O corresponds to the maximum capacitance. Figure P26. 10 1 1 . A 50. 0-m length of coaxial cable has an inner conductor that has a diameter of 2. 58 mm and carries a charge of 8. 10 pC. The surrounding conductor has an inner diameter of 7. 27 mm and a charge of -8. 10 pC. (a) What is the capacitance of this cable? b) What is the potential difference between the two conductors? Assume the region between the conductors is air. 12. A 20. 0-pF spherical capacitor is composed of two concentric metal spheres, one having a radius twice as large as the other. The region between the spheres is a vacuum. Determine the volume of this region. 13. An air-filled spherical capacitor is constructed with inner and outer shell radii of 7. 00 and 14. 0 cm, respectively. (a) Calculate the capacitance of the device. (b) What potential difference between the spheres results in a charge of 4. 00 pc on the capacitor? 14.
A small object of mass m carries a charge q and is suspended by a thread between the vertical plates of a parallel-plate capacitor. The plate separation is d. If the thread makes an angle B with the vertical, what is the potential difference between the plates? Find the capacitance of the Earth. Suggestion: The outer conductor of the 15. “spherical capacitor” may be considered as a conducting sphere at infinity where V approaches zero. ) 16. Two capacitors, Cl = 5. 00 pF and C2 = 12. 0 pF, are connected in parallel, and the resulting combination is connected to a 9. 00-V battery. (a) What is the equivalent capacitance of the combination?
What are (b) the potential difference across each capacitor and (c) the charge stored on each capacitor? 17. What If? The two capacitors of Problem 16 are now connected in series and to a 9. 00-V battery. Find (a) the equivalent capacitance of the combination, (b) the otential difference across each capacitor, and (c) the charge on each capacitor. 18. Evaluate the equivalent capacitance of the configuration shown in Figure P26. 18. All the capacitors are identical, and each has capacitance C. Figure P26. 18 19. Two capacitors when connected in parallel give an equivalent capacitance of 9. 0 pF and give an equivalent capacitance of 2. 00 pF when connected in series. What is the capacitance of each capacitor? 20. Two capacitors when connected in parallel give an equivalent capacitance of Cp and an equivalent capacitance of Cs when connected in series. What is the capacitance of each capacitor? 1. Four capacitors are connected as shown in Figure P26. 21. (a) Find the equivalent capacitance between points a and b. (b) Calculate the charge on each capacitor if AVab = 15. 0 V. Figure P26. 21 22. Three capacitors are connected to a battery as shown in Figure P26. 22.
Their capacitances are Cl = 3C, C2 = C, and C3 = SC. (a) What is the equivalent capacitance of this set of capacitors? (b) State the ranking of the capacitors according to the charge they store, from largest to smallest. (c) Rank the capacitors according to the potential differences across them, from largest to smallest. (d) What If? If C3 is ncreased, what happens to the charge stored by each of the capacitors? Figure P26. 22 and 20. 0 V. Capacitor Cl is first charged by the closing of switch Sl . Switch Sl is then opened, and the charged capacitor is connected to the uncharged capacitor by the closing of S2.
Calculate the initial charge acquired by Cl and the final charge on each capacitor. Figure P26. 23 24. According to its design specification, the timer circuit delaying the closing of an elevator door is to have a capacitance of 32. 0 pF between two points A and B. (a) When one circuit is being constructed, the inexpensive but durable capacitor nstalled between these two points is found to have capacitance 34. 8 PF. To meet the specification, one additional capacitor can be placed between the two points. Should it be in series or in parallel with the 34. 8-pF capacitor? What should be its capacitance? b) What If? The next circuit comes down the assembly line with capacitance 29. 8 pF between A and B. What additional capacitor should be installed in series or in parallel in that circuit, to meet the specification? 25. A group of identical capacitors is connected first in series and then in parallel. The combined capacitance in parallel is 100 times larger than for the series onnection. How many capacitors are in the group? 26. Consider three capacitors Cl, C2, C3, and a battery. If Cl is connected to the battery, the charge on Cl is 30. 8 pC. Now Cl is disconnected, discharged, and connected in series with C2.
When the series combination of C2 and Cl is connected across the battery, the charge on Cl is 23. 1 pC. The circuit is disconnected and the capacitors discharged. Capacitor C3, capacitor Cl , and the battery are connected in series, resulting in a charge on Cl of 25. 2 pC. If, after being disconnected and discharged, Cl, C2, and C3 are connected in series with one another and with the attery, what is the charge on Cl? 27. Find the equivalent capacitance between points a and b for the group of capacitors connected as shown in Figure P26. 27. Take Cl = 5. 00 pF, C2 = 10. 0 pF, and c = 2. 00 PF.
Figure P26. 27 28. For the network described in the previous problem, if the potential difference between points a and b is 60. 0 V, what charge is stored on C3? capacitors shown in Figure P26. 29. Figure P26. 29 30. Some physical systems possessing capacitance continuously distributed over space can be modeled as an infinite array of discrete circuit elements. Examples are a microwave waveguide and the axon of a nerve cell. To practice analysis of an infinite array, determine the equivalent capacitance C between terminals X and Y of the infinite set of capacitors represented in Figure P26. 30.
Each capacitor has capacitance CO. Suggestion: Imagine that the ladder is cut at the line AB, and note that the equivalent capacitance of the infinite section to the right of AB is also C. Figure P26. 30 Section 26. 4 Energy Stored in a Charged Capacitor (a) A 3. 00-pF capacitor is connected toa 12. 0-V battery. How much energy is 31. stored in the capacitor? (b) If the capacitor had been connected to a 6. 00-V battery, ow much energy would have been stored? 32. The immediate cause of many deaths is ventricular fibrillation, uncoordinated quivering of the heart as opposed to proper beating.
An electric shock to the chest can cause momentary paralysis of the heart muscle, after which the heart will sometimes start organized beating again. A defibrillator (Fig. 26. 14) is a device that applies a strong electric shock to the chest over a time of a few milliseconds. The device contains a capacitor of several microfarads, charged to several thousand volts. Electrodes called paddles, about 8 cm across and coated with conducting paste, are eld against the chest on both sides of the heart. Their handles are insulated to prevent injury to the operator, who calls, “Clear! and pushes a button on one paddle to discharge the capacitor through the patient’s chest. Assume that an energy of 300 J is to be delivered from a 30. 0- pF capacitor. To what potential difference must it be charged? 33. Two capacitors, Cl = 25. 0 pF and C2 = 5. 00 pF, are connected in parallel and charged with a 100-V power supply. (a) Draw a circuit diagram and calculate the total energy stored in the two capacitors. (b) What If? What potential difference would be equired across the same two capacitors connected in series in order that the combination stores the same amount of energy as in (a)?
Draw a circuit diagram of this circuit. what fraction does the stored energy change (increase or decrease) when the plate separation is doubled? 35. As a person moves about in a dry environment, electric charge accumulates on his body. Once it is at high voltage, either positive or negative, the body can discharge via sometimes noticeable sparks and shocks. Consider a human body well separated from ground, with the typical capacitance 1 50 PF. (a) What charge on the ody will produce a potential of 10. 0 kV? (b) Sensitive electronic devices can be destroyed by electrostatic discharge from a person.
A particular device can be destroyed by a discharge releasing an energy of 250 p]. To what voltage on the body does this correspond? 36. A uniform electric field E = 3 000 V/m exists within a certain region. What volume of space contains an energy equal to 1. 00 x 10-7 J? Express your answer in cubic meters and in liters. 37. A parallel-plate capacitor has a charge Q and plates of area A. What force acts on one plate to attract it toward the other plate? Because the electric field between he plates is E = Q /AEO, you might think that the force is F = QE = Q2/AEO.
This is wrong, because the field E includes contributions from both plates, and the field created by the positive plate cannot exert any force on the positive plate. Show that the force exerted on each plate is actually F = Q2/2EOA. (Suggestion: Let C = EOA/x for an arbitrary plate separation x; then require that the work done in separating the two charged plates be W = f F dx. ) The force exerted by one charged plate on another is sometimes used in a machine shop to hold a workpiece stationary. 38. The circuit in Figure P26. 8 consists of two identical parallel metal plates connected by identical metal springs to a 100-V battery.
With the switch open, the plates are uncharged, are separated by a distance d = 8. 00 mm, and have a capacitance C = 2. 00 PF. When the switch is closed, the distance between the plates decreases by a factor of 0. 500. (a) How much charge collects on each plate and (b) what is the spring constant for each spring? (Suggestion : Use the result of Problem 37. ) Figure P26. 38 39. Review problem. A certain storm cloud has a potential of 1. 00 x 108 V relative to a tree. If, during a lightning storm, 50. C of charge is transferred through this potential difference and 1. 0% of the energy is absorbed by the tree, how much sap in the tree can be boiled away? Model the sap as water initially at 30. 00C. Water has a specific heat of 4 186 J/kg0C, a boiling point of 1000C, and a latent heat of vaporization of 2. 26 x 106 J/kg. potential difference AV and connected in parallel. Then the plate separation in one of the capacitors is doubled. (a) Find the total energy of the system of two capacitors before the plate separation is doubled. (b) Find the potential difference across each capacitor after the plate separation is doubled. c) Find the total energy of the system after the plate separation is doubled. d) Reconcile the difference in the answers to parts (a) and (c) with the law of conservation of energy. Show that the energy associated with a conducting sphere of radius R and 41. charge Q surrounded by a vacuum is U = keQ2/2R. 42. Consider two conducting spheres with radii RI and R2. They are separated by a distance much greater than either radius. A total charge Q is shared between the spheres, subject to the condition that the electric potential energy of the system has the smallest possible value. The total charge Q is equal to ql + q2, where ql epresents the charge on the first sphere and q2 the charge on the second.
Because the spheres are very far apart, you can assume that the charge of each is uniformly distributed over its surface. You may use the result of Problem 41 . (a) Determine the values of ql and q2 in terms of Q , RI, and R2. (b) Show that the potential difference between the spheres is zero. (We saw in Chapter 25 that two conductors Joined by a conducting wire will be at the same potential in a static situation. This problem illustrates the general principle that static charge on a conductor will distribute itself o that the electric potential energy of the system is a minimum. ) Section 26. 5 Capacitors with Dielectrics 43.
Determine (a) the capacitance and (b) the maximum potential difference that can be applied to a Teflon-filled parallel-plate capacitor having a plate area of 1. 75 cm2 and plate separation of 0. 040 0 mm. 44. (a) How much charge can be placed on a capacitor with air between the plates before it breaks down, if the area of each of the plates is 5. 00 cm2? (b) What If? Find the maximum charge if polystyrene is used between the plates instead of air. 45. A commercial capacitor is to be constructed as shown in Figure 26. 7a. This particular capacitor is made from two strips of aluminum separated by a strip of paraffin-coated paper.
Each strip of foil and paper is 7. 00 cm wide. The foil is 0. 004 00 mm thick, and the paper is 0. 025 0 mm thick and has a dielectric constant of 3. 70. What length should the strips have, if a capacitance of 9. 50 x 10-8 F is desired before the capacitor is rolled up? (Adding a second strip of paper and rolling the capacitor effectively doubles its capacitance, by allowing charge storage on both sides of each strip of foil. ) 46. The supermarket sells rolls of aluminum foil, of plastic wrap, and of waxed agnitude estimates for its capacitance and its breakdown voltage. 47. A parallel-plate capacitor in air has a plate separation of 1. 0 cm and a plate area of 25. 0 cm2. The plates are charged to a potential difference of 250 V and disconnected from the source. The capacitor is then immersed in distilled water. Determine (a) the charge on the plates before and after immersion, (b) the capacitance and potential difference after immersion, and (c) the change in energy of the capacitor. Assume the liquid is an insulator. 48. A wafer of titanium dioxide (K = 173) of area 1. 00 cm2 has a thickness of 0. 100 mm. Aluminum is evaporated on the parallel faces to form a parallel-plate capacitor. (a) Calculate the capacitance. (b) When the capacitor is charged with a 12. -V battery, what is the magnitude of charge delivered to each plate? (c) For the situation in part (b), what are the free and induced surface charge densities? (d) What is the magnitude of the electric field? 49. Each capacitor in the combination shown in Figure P26. 49 has a breakdown voltage of 15. 0 V. What is the breakdown voltage of the combination? Figure P26. 49 Section 26. 6 Electric Dipole in an Electric Field 50. A small rigid object carries positive and negative 3. 0-nC charges. It is oriented so that the positive charge has coordinates (-1. 20 mm, 1. 10 mm) and the negative charge is at the point (1. 0 mm, -1. 30 mm). (a) Find the electric dipole moment of the object. The object is placed in an electric field E = (7 8007 – 4 9001) NIC. (b) Find the torque acting on the object. (c) Find the potential energy of the object-field system when the object is in this orientation. (d) If the orientation of the object can change, find the difference between the maximum and minimum potential energies of the system. 51. A small object with electric dipole moment p is placed in a nonuniform electric ield E = E(x)i. That is, the field is in the x direction and its magnitude depends on the coordinate x.
Let B represent the angle between the dipole moment and the x direction. (a) Prove that the dipole feels a net force in the direction toward which the field increases. (b) Consider a spherical balloon centered at the origin, with radius 15. 0 cm and carrying charge 2. 00 pC. Evaluate dE/ dx at the point (16 cm, O, O). Assume a water droplet at this point has an induced Section 26. 7 An Atomic Description of Dielectrics 52. A detector of radiation called a Geiger tube consists of a closed, hollow, onducting cylinder with a fine wire along its axis. Suppose that the internal diameter of the cylinder is 2. 0 cm and that the wire along the axis has a diameter of 0. 200 mm. The dielectric strength of the gas between the central wire and the cylinder is 1. 20 x 106 Vim. Calculate the maximum potential difference that can be applied between the wire and the cylinder before breakdown occurs in the gas. 53. The general form of Gauss’s law describes how a charge creates an electric field in a material, as well as in vacuum. It is where E = KEO is the permittivity of the material. (a) A sheet with charge Q uniformly istributed over its area A is surrounded by a dielectric.
Show that the sheet creates a uniform electric field at nearby points, with magnitude E = Q /2AE. (b) Two large sheets of area A, carrying opposite charges of equal magnitude Q, are a small distance d apart. Show that they create uniform electric field in the space between them, with magnitude E = Q /AE. (c) Assume that the negative plate is at zero potential. Show that the positive plate is at potential Qd/AE. (d) Show that the capacitance of the pair of plates is AE/d = KAEO/d. Additional Problems 54. For the system of capacitors shown in Figure P26. , find (a) the equivalent capacitance of the system, (b) the potential across each capacitor, (c) the charge on each capacitor, and (d) the total energy stored by the group. Figure P26. 54 55. Four parallel metal plates Pl, P2, P3, and P4, each of area 7. 50 cm2, are separated successively by a distance d = 1. 19 mm, as shown in Figure P26. 55. Pl is connected to the negative terminal of a battery, and P2 to the positive terminal. The battery maintains a potential difference of 12. 0 V. (a) If P3 is connected to the negative terminal, what is the capacitance of the three-plate system Pl P2P3? (b) What is the charge on P2? ) If P4 is now connected to the positive terminal of the battery, what is the capacitance of the four-plate system Pl P2P3P4? (d) What is the charge on 56. One conductor of an overhead electric transmission line is a long aluminum wire 2. 40 cm in radius. Suppose that at a particular moment it carries charge per length 1. 40 pC/m and is at potential 345 kV. Find the potential 12. 0 m below the wire. Ignore the other conductors of the transmission line and assume the electric field is everywhere purely radial. 57. Two large parallel metal plates are oriented horizontally and separated by a distance 3d.
A conducting wire Joins them, and initially each plate carries no charge. Now a third identical plate carrying charge Q is inserted between the two plates, parallel to them and located a distance d from the upper plate, as in Figure P26. 57. (a) What induced charge appears on each of the two original plates? (b) What potential difference appears between the middle plate and each of the other plates? Each plate has area A. Figure P26. 57 58. A 2. 00-nF parallel-plate capacitor is charged to an initial potential difference AVI = 100 V and then isolated. The dielectric material between the plates is mica, with a dielectric constant of 5. . (a) How much work is required to withdraw the mica sheet? (b) What is the potential difference of the capacitor after the mica is withdrawn? 59. A parallel-plate capacitor is constructed using a dielectric material whose dielectric constant is 3. 00 and whose dielectric strength is 2. 00 x 108 V/m. The desired capacitance is 0. 250 pF, and the capacitor must withstand a maximum potential difference of 4 000 V. Find the minimum area of the capacitor plates. 60. A 10. 0-pF capacitor has plates with vacuum between them. Each plate carries a charge of magnitude 1 000 pC. A particle with charge -3. 00 pc and mass 2. 0 x 10-16 g is fired from the positive plate toward the negative plate with an initial speed of 2. 00 x 106 m/s. Does it reach the negative plate? If so, find its impact speed. If not, what fraction of the way across the capacitor does it travel? 61. A parallel-plate capacitor is constructed by filling the space between two square plates with blocks of three dielectric materials, as in Figure P26. 61. You may assume that e d. (a) Find an expression for the capacitance of the device in terms of the plate area A and d, KI, Q, and K3. (b) Calculate the capacitance using the values A = 1. 00 crn2, d = 2. 00 mm, KI = 4. 90, Q = 5. 60, and 2. 10.