Practice Problem Solutions

Transcription

1 Chapter 14 Fields and Forces Practice Problem Solutions Student Textbook page Conceptualize the Problem - Force, charge and distance are related by Coulomb s law. The electrostatic force, F, between the given charges q 1 =.4 µc F q =+5.3 µc r = 58 cm k = N m Coulomb s law can be applied to find the F = k q 1q r magnitude of the force. The electrostatic force between the charges is about 0.34 N. F = N m C C 58 cm 1.00 m F = N F 0.34 N 100 cm Charges in the microcoulomb range are expected to exert moderate forces on each other, so this result is reasonable.. Solution is similar to Practice Problem 1. The separation of the charges is about 0.80 m. 3. Solution is similar to Practice Problem 1. The magnitude of each charge is about C. 4. Solution is similar to Practice Problem 1. The force decreases by a factor of 4 = 16, to 0.50 N attractive. 5. Solution is similar to Practice Problem 1. After touching the objects together and increasing their separation to d, the force between them will be about 0.17 N. Practice Problem Solutions Student Textbook pages Conceptualize the Problem - Force, charge and distance are related by Coulomb s law. 37

2 - The same Coulomb force that repels the two protons must support the weight of the second proton. - The second proton must be placed above the first if its weight is to be supported by the repulsive force between the two protons. The distance between the protons, r, if the weight of the second proton is supported by their mutual repulsive force q 1 = C k = N m q = C g = 9.81 m/s m = kg Write down the equation that shows F = k q 1q r = mg the Coulomb force is the same as the r = kq 1q weight of the proton. mg kq1 q r = mg N m r = The second proton should be placed about 0.1 m directly above the force so that the force of repulsion supports its weight. Check the units: 1 N m kgm/s = N m kgm/s 1 = N m N r C kg9.81 m/s r = m r 0.1 m 1 = m 1 = m, as required. θ = 0 F T m g F e 7. Solution is similar to Practice Problem 1. The force on charge A is N[W73S]. The force on charge B is N[E63N]. The force on charge C is N[W36S]. 8. Solution is similar to Practice Problem 1. The net electrostatic force on q 1 is 8.7 N[E18 N] 9. Conceptualize the Problem - Draw a free-body diagram that shows all the forces that the suspended pith ball experiences: the tension in the string, the electrostatic force from the second pith ball and the force of gravity. - The suspended ball experiences no net acceleration in either the x- or y-directions. - Newton s second law can be applied in each the x- and y-directions. - Coulomb s law relates electrostatic force, charge and distance and can be used with the known variables in the problem to find the unknown charge. 373

3 The charge, q, on each of the pith balls m = 1.5 g q k = N m r =.6 cm g = 9.81 m/s F T θ = 0 Apply Newton s second law in the x: x- and y-direction. F e + F T sin θ = 0 F e = F T sin θ y: mg + F T cos θ = 0 F T = mg cos θ Substitute the second y equation into F e = F T sin θ F e = mg the first x. sin θ cos θ F e = mg tan θ Expand the term for the electrostatic force and solve for the charge. The charge on the pith balls will be C and each charge will have the same sign. F e = kq r = mg tan θ q = mgr tan θ k mgr q = tan θ k q =± 1.5 g 1kg 1000 g 9.81 m/s.6 cm 1.00 m 100 cm tan N m q =± C q ± C kgm/s Check the units: m N m = C 10. Solution is similar to Practice Problem 9. The charge on the pith balls will be C and each charge will have the same sign. Practice Problem Solutions Student Textbook pages Conceptualize the Problem - The electric field intensity is related to the force and charge. The magnitude and direction of the electric field, E, at the location of the charge 374

4 q = C k = N m C E F = 4.8 N[right] Find the electric field intensity by using the equation that defines electric field. E = F q 4.8 N[right] E = C E = N/C[right] The electric field is N/C[to the right] The electric field has units of N/C, as expected. Its direction is the same as the force on the positive charge. 1. Solution is similar to Practice Problem 11. The force on the charge will be N[W]. 13. Solution is similar to Practice Problem 11. The electric field is N/C[to the left]. 14. Solution is similar to Practice Problem 11. The charge is C. Practice Problem Solutions Student Textbook page Conceptualize the Problem - The definition of gravitational field intensity is the gravitational force per unit mass. The gravitational field intensity, g, at the surface of Mars F = 7.5 N[down] Note that [down] g m =.0 kg means [toward the planet s centre]. Find the gravitational field intensity by using the equation for field intensity and the given variables. g = F m g = 7.5 N[down].0 kg g = 3.75 N/kg[down] g 3.8 N/kg[down] The gravitational field intensity at the surface of Mars is 3.8 N/kg[down]. 375

5 Mars is known to have a lower mass than Earth, so it is expected to have a lower gravitational field intensity too. 16. Solution is similar to Practice Problem 15. The gravitational force on the object would be 5 N[down]. 17. Solution is similar to Practice Problem 15. The object s mass is 3.46 kg. 18. Solution is similar to Practice Problem 15. The gravitational field intensity would be.60 N/kg[down]. 19. Solution is similar to Practice Problem 15. The centripetal acceleration is.60 m/s [toward centre]. Practice Problem Solutions Student Textbook page Conceptualize the Problem - At any point outside of a charged sphere, the electric field is the same as it would be if the charge was concentrated at a point at the centre of the sphere. - The electric field is related to the source charge and distance. - The direction of the field is the direction in which a positive charge would move if it was placed at that point in the field. That is, the direction is radially outward from positive charges and toward negative charges. The electric field intensity, E q =.8 µc k = N m C E r = 18.0 cm Find the field intensity by using the equation for the special case of the field near a point charge. q E = k r E = C N m 18.0 cm 1.00 m E = N/C 100 cm The electric field intensity at the given distance from the sphere is N/C, and the negative sign indicates it is directed toward the sphere. Check the units: N m C m = N, as required. C E N/C 1. Solution is similar to Practice Problem 0. The sign and magnitude of the charge is C. 376

6 r AC y E Bat C θ E C θ A B 46 µc 8µC E net E Aat C x. Solution is similar to Practice Problem 0. The point P is about 0.3 m from the centre of the charge. 3. Solution is similar to Practice Problem 0. The number of electrons that should be removed is about Solution is similar to Practice Problem 0. The electric field intensity is N/C, directed toward the sphere. 5. Solution is similar to Practice Problem 0. The point M is about m from the centre of the charge. 6. Conceptualize the Problem - Since force vectors must be added vectorially and field vectors are force vectors divided by the unit charge, the field vectors must also be added vectorially. - The magnitude of the field vectors can be determined individually. - Draw a vector diagram showing the field intensity vectors at point C and then superimpose an x-y coordinate system on the drawing, with the origin at point C. The electric field intensity, E, at point C q A = 46 µc k = N m C E q B = 8 µc θ at point A r AB = 5.0 cm θ E r BC = 4.0 cm Calculate the distance between points A and C by using the Pythagorean theorem. Calculate the electric field intensity of both of the charges at point C, using the equation for the special case of the field intensity near a point charge. Find the angle at A between points B and C. r AC = r AB + r BC r AC = 5.0 cm cm r AC = 41 cm r AC = cm E q = k r E AatC = N m E AatC = N/C E BatC = N m E BatC = N/C tan θ = BC θ = tan AB 1 BC AB cm θ = tan 5.0 cm θ = C cm 1.00 m 100 cm C 4.0 cm 1.00 m 100 cm Use the method of components to find x-components: the resultant electric field vector. E A at Cx = N/C cos E A at Cx = N/C E B at Cx = 0 E netx = N/C 377

7 y-components: E A at Cy = N/C sin E A at Cy = N/C E B at Cy = N/C E nety = N/C Use the Pythagorean theorem to find E net = E netx + E nety the magnitude of the resultant vector. E net = N/C N/C E net = N/C E net = N/C Use the definition of the tangent function to find the direction of the electric field vector at point C. The electric field intensity at point C is N/C[81.4 above the +x-axis]. tan θ E = E nety E netx θ E = tan N/C N/C θ E = 81.4 Charge B is larger than charge A and closer than charge A to point C, so it contributes more to the electric field intensity. Based on the magnitude of the electric field intensity due to charge B, the final result seems reasonable. 7. Solution is similar to Practice Problem 6. The electric field intensity at point C is N/C[86.7 above the +x-axis]. 8. Solution is similar to Practice Problem 6. The electric field intensity at point D is N/C[3.7 above the x-axis]. 9. Solution is similar to Practice Problem 0. The electric field intensity at a point midway between the charges is N/C[towards the negative charge], or N/C. P r AP = 5.0 cm y E net E Aat P E Bat P θ E θ A θ P x r BP = 1.0 cm θ B A B 8.0 µc r AB = 13.0 cm 5.0 µc 30. Conceptualize the Problem - Since force vectors must be added vectorially and field vectors are force vectors divided by the unit charge, the field vectors must also be added vectorially. - The magnitude of the field vectors can be determined individually. - Draw a vector diagram showing the field intensity vectors at point P and then superimpose an x-y coordinate system on the drawing, with the origin at point P. This means that angles are measured from the positive x-axis. The electric field intensity, E, at point P q A = 8.0 µc k = N m C E q B = 5.0 µc θ E the electric r AP = 5.0 cm field direction r BP = 1.0 cm θ P at point P r AB = 13.0 cm θ A θ B 378

8 Determine the geometry of the triangle: calculate the individual angles. Note that because rap + r BP = 5.0 cm cm = rab or, r AB = 169 cm = 13.0 cm the triangle is a right triangle and the angle between sides AP and BP i.e. at point P is 90. If the above is not immediately noticed, the same result can be obtained from the cosine law: c = a + b ab cos θ Determine the angles of the other two sides from trigonometry. r AB = r AP + r BP r APr BP cos θ P θ P = cos 1 r AB r AP r BP r AP r BP θ P = cos cm 5.0 cm 1.0 cm 5.0 cm1.0 cm θ P = 90 cos θ A = r AP r AB θ A = cos cm 13.0 cm θ A = cos θ B = r BP r AB θ B = cos cm 13.0 cm θ B =.6 Calculate the electric field intensity of E = k q r both of the charges at point P, using the x-components: equation for the special case of the field E x = k q r cos θ intensity near a point charge. E A at Px = Use the method of components to find the resultant electric field vector C N m 5.0 cm 1.00 m cos cm E A at Px = N/C E B at Px = C N m 1.0 cm 1.00 m cos cm E B at Px = N/C E netx = N/C y-components: E y = k q r sin θ E A at Py = N m E A at Py = N/C E B at Py = N m E B at Py = N/C E nety = N/C Use the Pythagorean theorem to find the E net = E netx + E nety magnitude of the resultant vector. Use the definition of the tangent function to find the direction of the electric field vector at point P. The electric field intensity at point P is N/C[73.6 above the +x-axis] C 5.0 cm 1.00 m sin cm C 1.0 cm 1.00 m sin cm E net = N/C N/C E net = N/C E net = N/C E net N/C tan θ E = E nety E netx θ E = tan N/C N/C θ E =

9 Both the x- and y-components of the resultant electric field vector are positive, so the direction of the electric field should be between 0 and 90 above the x-axis. Charge A is closer to point P than charge B, and it has a greater magnitude, so it should contribute more to the electric field intensity at point P than charge B, and it does. Practice Problem Solutions Student Textbook page Conceptualize the Problem - Since the point in question is outside the surface of Earth, the gravitational field there is the same as it would be if Earth s mass was concentrated at a point at Earth s centre. Therefore, the equation for the gravitational field intensity near a point mass applies. The gravitational field intensity, g, at a point beyond Earth s surface r = N m m G = kg g M E = kg Use the equation for the gravitational field intensity near a point source. The gravitational field intensity at this distance above Earth s surface is about N/kg, directed toward Earth s centre. g = G M E [toward centre] r g = 11 N m kg kg [toward centre] m g = N/kg[toward centre] g N/kg[toward centre] The distance from Earth s centre is about 13. times greater than the radius of Earth; therefore, it is expected that the gravitational field intensity at this point will be roughly 13. = times lower than at the surface. show good agreement: 9.81 N/kg/173.3 = N/kg. 3. Solution is similar to Practice Problem 31. The radius of Saturn is about m. 33. Solution is similar to Practice Problem 31. The acceleration due to gravity on the surface of Venus is about 8.09 N/kg[toward centre]. 34. Conceptualize the Problem - The data about the falling object can be used with the appropriate kinematics equation to find the acceleration due to gravity. - The acceleration due to gravity is the same as the gravitational field intensity. - The gravitational field intensity relates the gravitational mass and the radius. The mass, M, of the planet 380

10 11 N m m = 3.60 kg G = kg g t =.60 s v 1 = 0 m/s M y = 1.86 m r = m Use a kinematics equation that relates y = v 1 t + 1 at vertical displacement to acceleration, y = at time and initial velocity. Rearrange to find the acceleration due a = y t to gravity. a = 1.86 m.60 s a = m/s Use the equation for the magnitude of the gravitational field intensity near a point source. Rearrange to find the mass. Note that the acceleration calculated above is equal to the gravitational field intensity, a = g. The mass of the planet is kg. g = G M r M = r g G M = m m/s 11 N m M = kg M kg kg The gravitational field intensity is more than ten times smaller than that of Earth and the radii of the planet and Earth are similar, so it is expected that the mass of the planet will be smaller than Earth s by about a factor of ten, and it is. 35. Solution is similar to Practice Problem 31. The gravitational field intensity is N/kg[toward centre]. 36. Solution is similar to Practice Problem 31. The gravitational field intensity is 8.09 N/kg[toward centre]. 37. Solution is similar to Practice Problem 31. The mass of Neptune is about kg. Practice Problem Solutions Student Textbook pages Conceptualize the Problem - Two charges are close together and therefore they exert a force on each other. - Work must be done on or to the charges in order to bring them close to each other. - Since work was done on or by a charge, it has electric potential energy. The electric potential energy, E Q, stored between the charges 381

11 q 1 =+.6 µc q = 3. µc r = 1.60 m k = N m Write the equation for electric potential energy between two charges. E Q = k q 1q r The electric potential energy stored in the field between the charges is J. E Q = E Q = J E Q J E Q N m C C 1.60 m The units cancel to give N m = J. The sign is negative, indicating the electric potential energy is negative. A negative sign is correct for unlike charges, because work was done by one of the charges to get it from infinity to its present distance from the second charge. 39. Solution is similar to Practice Problem 38. The kinetic energy per charge is 0.18 J. 40. Solution is similar to Practice Problem 38. The separation of the charges is m. 41. Solution is similar to Practice Problem 38. The charges will have the magnitude of C, and will have the same sign, either both positive or both negative. Practice Problem Solutions Student Textbook pages Solution is similar to Practice Problem 0. The electric field intensity is about N/C, away from the charge. 43. Conceptualize the Problem - A charge creates an electric field. - At any point in the field, you can describe an electric potential difference between that point and a location an infinite distance away. - Electric potential difference is a scalar quantity and depends only on the distance from the source charge and not the direction. The distance, r, from a positive point charge at which the electric potential difference will have a particular value q = 8. C k = N m r V = 5.0 V 38

12 Use the equation for the electric potential difference at a point a distance r from a point charge. V = k q r The distance from the point charge is m. r = k q V r = N m r = m r m 8. C 5.0 V N m C V = N mm 1 C V = Jm 1 C J/C = m Note that this distance is 300 times greater than Earth s radius, or 0.1 times the distance between Earth and the Sun. 44. Solution is similar to Practice Problems 38 and 43. The electric potential energy of the system is J. 45. Solution is similar to Practice Problem 43. The charge has a sign and magnitude of C. 46. Solution is similar to Sample Problem on student textbook pages 313 and 314. The points of zero potential are 6. cm and 7 cm to the right of charge A. 47. Solution is similar to Practice Problem 43. The electric potential difference at point P is V. 48. Conceptualize the Problem - The electric potential difference between two points is the algebraic difference between the individual potential differences of the points. The electric potential difference, V, between the two points V X =+4.8 V V V Y = 3. V Use algebraic subtraction to determine the potential difference between the two points. V = V X V Y V = 4.8 V 3. V V = 8.0 V The electric potential difference between the two points is 8.0 V. As the electric potential difference at one point is positive and the other is negative, the electric potential difference between the two points is expected to be greater than that at either point. 383

13 49. Solution is similar to Practice Problem 43. The electric potential difference at point M is V. 50. Solution is similar to Practice Problem 43. The electric potential difference at point P is V. 4.0 µc A d B d d P d D d C d 6.0 µc 51. Conceptualize the Problem - The electric potential difference due to the combination of charges is the algebraic sum of the electric potential differences due to each point alone. - Designate charge A as the +4.0 µc charge, C as the 6.0 µc charge and B and D as the other two corners of the square. Let P be the intersection of the diagonals. - Due to the symmetry of the charge arrangement, points B and D will always have the same potential. Therefore, it is only necessary to consider one of them we consider point B in this solution. - The size of the square is not given. Let the length of the sides be given by d and carry it through the calculation perhaps it will cancel out. The charge, q P, required at the intersection of the diagonals of the square to make the potential difference zero at the unoccupied corners of the square q A = 4.0 µc q C = 6.0 µc k = N m d q P V at B or D Calculate the total potential difference V at B = V AatB + V CatB + V PatB = 0 at point B from each of the charges, k q A d + k q C d + k q P d = 0 including the unknown charge, together. Use the Pythagorean theorem to find the q A + q C + q P = 0 distance from the unknown charge, q P, to point B: r = d + d = d or, r = d. Note that the size of the square, d, cancels out of the calculation on the right. Solve for the unknown charge. q p = q A q C q p = 4.0 µc 6.0 µc The required charge is C. q p = C q p C The magnitude of the charge at point P is less than those at A or C, and the charge is closer to B or D, so the result seems reasonable. 5. Conceptualize the Problem - The work done per unit charge to move a charge from one point in the field to another is equal to the difference of the electric potentials at each of those two points. 384

14 - The difference of electric potentials is written as the final potential minus the initial potential. The electric potential difference of point B, V B q =.0 C V B W AB = 8.0 J V A = 6.0 V Write down the equation for work done per unit charge. Solve for V B. The electric potential difference of point B was.0 V. W AB = V q final V initial W AB = V q A V B V B = V A W AB q V B = 6.0 V 8.0 J.0 C V B =.0 V The electric potential difference of point B will be higher or lower than that at point A. If it was higher, the work done would be negative because the charge is positive, therefore it is lower. 53. Solution is similar to Practice Problem 5. The kinetic energy of the charge will be 1 J. 54. Solution is similar to Practice Problem 43. The potential difference between the two points is V. 55. Solution is similar to Practice Problem 5. a The potential difference between point A and B is V. b The work required to move the second charge would be J. c A is at a higher potential. 56. Solution is similar to Sample Problem on student textbook pages 676 and 677. The points of zero potential are 5.3 cm and 16 cm to the right of charge A. 57. Solution is similar to Sample Problem on student textbook pages 676 and 677. The points of zero potential lie on a line that is perpendicular to the line that connects the charges and runs through the point midway between the two charges. 58. Solution is similar to Sample Problem on student textbook pages 676 and 677. The points of zero potential are 3.4 cm above the origin and 4 cm below the origin. 59. Solution is similar to Sample Problem on student textbook pages 676 and 677. q can have any magnitude and its distance from q 1 is given by.0 cm q µ where q is in µ C and the distance is in cm. 385

15 60. Solution is similar to Sample Problem on student textbook pages 676 and 677. The point of zero potential difference will be 4.0 cm to the right of the negative charge. Chapter 14 Review Answers to Problems for Understanding Student Textbook pages The force of repulsion is N N m F = C 1 C1 C m F = N 19. The force between the two free electrons is N repulsive N m F = C C C m F N 0. The centres of the balls are m apart. F = kq 1q r kq r = 1 q F N m r =± C C C N r = m Use the positive root, since r is a distance. 1. The net force on each charge is as follows. FB on A A + B FA on B + FC on A FC on B FA on C FB on C C F AonB = F BonA = N m C C 1.0 m F AonB = F BonA = 0.07 N F AonC = N m F ConA = C C C.0 m F AonC = F ConA = 0.07 N F AonB = N m F BonA = C C C 1.0 m F BonC = F ConB = 0.16 N Let right be the positive direction and left be the negative direction. F net on A = 0.07 N N F net on B = 0.07 N N F net on C = 0.07 N 0.16 N F net on A = N F net on B = 0.9 N F net on C = 0.4 N 386

16 . The force on A is 3.8 N in a direction 153 clockwise from the line segment CA. The force on B is 4.4 N in a direction 157 counterclockwise from the line segment CB. The force on C is 3.8 N in a direction 154 clockwise from the line segment BC. FB on A 10 FA FC on A.0 N α.4 N A+4.0 µc m 0.30 m FB on C N γ 60 C+6.0 µc 0.30 m 60 B +5.0 µc 3.0 N β FC on B 10 FC FA on C.4 N.0 N FA on B FB F AonB = F BonA = N m C C 0.30 m F AonB = F BonA =.0 N F AonC = N m F ConA = C C C 0.30 m F AonC = F ConA =.4 N F BonC = N m F ConB = C C C 0.30 m F BonC = F ConB = 3.0 N Use the law of cosines and the law of sines. Force on A F A =.0 N +.4 N.0.4 cos 10 N F A = N F A = 3.8 N Force on B F B =.0 N N.03.0 cos 10 N F B = 19 N F A = 4.4 N sin α = sin α = 33 sin β = sin β = 3 387

17 Force on C F C = 3.0 N +.4 N cos 10 N F = 1.96 N F C = 4.7 N sin γ = sin γ = 6 3. The electrostatic force is about times stronger than the gravitational force. F G = Gm 1m N m r = C kg kg m = N F Q = kq 1q r = N m C C m = N F Q F g F Q F g = N N Approximate opposite charges for Earth and the Moon would be C and C, respectively. Assume that the charges on Earth q E and the Moon q M would be proportional to their respective volumes. 3qM q E = re r M q E = m 3qM m q E = 49.3q M For the Coulombic force to be equal to the gravitational force, Gm Em M r N m kg kg = kg q M = q M = C = kq Eq M r N m 49.3q M q M and q E = 49.3q M q E = C Note: You might want to calculate relative charges using surface areas rather than volumes. 5. The ratio of the electric force to the gravitational force is kq e q e = kq r Gm e m e r kq Gm = Gm N N kq = Gm 6. The charge on the 0.30 g pith ball is 57 C. F a = F g qe = mg kg 9.81 N q = kg N C q = 57 C negative, since upward force from a downward field 388

18 7. The electric force exerted on the 3.0 g Ping-Pong ball is N away from the comb. tan 10.0 = F Q mg F Q = mg tan 10.0 F Q = kg F Q = N 9.8 N kg FQ 10.0 mg FT FQ 8. a The mass of Uranus is kg. g = Gm U r m U = gr U + h G g = Gm U r U + h m U = 8.71 N kg m m 11 N m kg m U = kg b The gravitational field intensity is 8.81 N/kg at the surface of Uranus. Since g r, g m m surface = m 8.71 N kg g surface = 8.81 N kg c A 100 kg person would weigh N on Uranus. F = mg F = 8.81 N/kg kg F = N 9. The planet would have a surface gravitational field intensity /9 as strong as Earth s. Since g M r, g p = 3 g E g p = 9 g E 389

19 30. a The force between the two particles would be N. F Q = kg 1q F Q = r N m C m F Q = N The gravitational force is negligible, as shown in answer 3. b The speed of the electron is m/s. F c = mv r v = F c r m N m v = kg v = m s c The electric field that the electron experiences is N/C toward the proton. E = kq r N m E = C C m E = N C toward proton d The electric potential difference that the electron experiences is 7. V. V = Er 5.9 V = N C m V = 7. V 31. The mass that the electron should have is kg, which is times larger than the actual mass N m kg m r = m = N m C r N m C C 11 N m m = kg ratio = kg kg ratio = F = q E, so tripling the charge will triple the force; changing from positive to negative charge will reverse the direction of the force. Therefore, F = N[W]. kg 390

20 33. The separation is 0.51 m. E Q = kq 1q r r = kg 1q E Q N m r = C C C 0.16 J r = m 34. The electric field at 1, is N/C in a direction 53 clockwise from the perpendicular. d 1 = 1 + d 1 = and d = + 5 d = N m E 1 = C C 5m E1 = N C away from 0, 0 tan θ = 1 = 1, so θ = N m E = C 8m E = N C toward 3, 0 tan φ =,soφ = 45 θ + φ = 71.6 Enet = N N cos 7 N E net = N C sin α = sin α = 6 The direction of E net is θ + α = 53 clockwise from the perpendicular at 1,. E1 θ + φ θ d 1 = 5 θ 1 α φ Enet E d = µc 5 µc a The change in potential energy is q V = C 9 4 V q V = J b Some of the potential energy of the charge is converted into other forms of energy, such as kinetic energy or heat. 391

21 36. The work done is W = q V W = C 8 V 4 V W = J The negative sign indicates that work is done on the charge by the field. 37. a Assuming that the charge is effectively all located at the centre of the sphere V = kq r N m V = C C m V = V b Yes, the electrons on a conducting surface will move until distributed such that the surface is equipotential. c The charges are approximately 63 nc on the larger sphere and 1 nc on the smaller sphere. The two spheres will have approximately the same amount of charge per unit area and area is proportional to r, so q 1 r 1 = q r q 1 = q q 1 = 5.33q Since q 1 + q = C, 5.33q + q = C q = C and q = 75 nc 1 nc q q = 1 nc 1 = 63 nc 38. a The electric field vectors at the midpoint will be equal but opposite; therefore, the net electric field equals zero. The potential difference between P and infinity is V net = kq r V net = N m V net = V C m N m C 0.50 m b With one of the charges negative and the other positive, the electric field vectors will both point away from the positive charge and their magnitudes will add, so N m E Q = C C 0.50 m EQ = N away from positive charge C The two scalar quantities for the electric potential difference will be equal but opposite, so the net electric potential difference adds to zero. c Changes in variables affect vector and scalar quantities differently. 39

22 39. a.3 J of work must be done. W = E Q W = kq 1q r N m W = C C C m W =.3 J b The potential difference is V. V = W q.3 J V = C V = V N m C C m c Point X, which is closer to the positive point charge, is at the higher potential. 40. a The potential difference is V. V = kq kq r r V = N m C 10 6 C m m V = V b Being closer to the positive charge, point R is at the higher potential. 393