Física Básica Experimental I Cuestiones Tema VII. Electrostática. Soluciones incluidas. 1.

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1 1. A cubical surface with no charge enclosed and with sides 2.0 m long is oriented with right and left faces perpendicular to a uniform electric field E of ( N/C) î. The net electric flux E through this surface is approximately A) zero D) N m 2 /C B) N m 2 /C E) N m 2 /C C) N m 2 /C 2. How many electrons must be transferred to a body to produce a charge of 125 nc? A) D) B) E) C) Two small spheres attract one another electrostatically. This can occur for a variety of reasons. Which of the following statements must be true? A) At least one sphere is charged. D) Both have the same charge. B) Neither is charged. E) None of these is correct. C) Both are charged. 4. If you bring a positively charged insulator near two uncharged metallic spheres that are in contact and then separate the spheres, the sphere on the right will have A) no net charge. D) either a positive or negative charge. B) a positive charge. E) None of these is correct. C) a negative charge. Ans: B 5. A positive charge of 63.0 nc is 15.0 cm from a negative charge of 45.0 nc. The force on one of the charges due to the other is approximately A) N D) N B) N E) N C) N 6. Charges q1 and q2 exert repulsive forces of 15 N on each other. What is the

2 repulsive force when their separation is increased so that their final separation is 160% of their initial separation? A) 1.3 N B) 5.9 N C) 9.8 N D) 8.7 N E) 6.4 N Ans: B 7. If all the charges are 15 cm from the origin (the crossing point of the vertical and horizontal lines) in the above figure and Q = +3.0 C, then calculate the magnitude of the net force on a charge of +Q placed at the origin. A) 22.8 N B) 10.2 N C) 26.0 N D) 187 N E) None of the above Ans: C 8. The SI units of electric fields can be expressed as A) C/m2 B) C/s C) V C D) N E) V/m 9. The electric field at point A is zero. What is charge Q1? A) +32 C B) 32 C C) The field cannot be zero at A for any value of Q1. D) +16 C E) 16 C Section: 21-5 Topic: Electric Field Lines Type: Factual 10.

3 An electric dipole consists of a positive charge separated from a negative charge of the same magnitude by a small distance. Which, if any, of the diagrams best represents the electric field lines around an electric dipole? A) 1 B) 2 C) 3 D) 4 E) None of these is correct. 11. Consider a uniform electric field E = (5.0 kn/c) î. What is the flux of this field through a square of side 20 cm if the normal to its plane makes a 45º angle with the x axis? A) 71 N m 2 /C D) 0.35 kn m 2 /C B) 0.14 kn m 2 /C E) 0.19 kn m 2 /C C) 0.28 kn m 2 /C Ans: B 12. A surface is so constructed that, at all points on the surface, the E vector points inward. Therefore, it can be said that A) the surface encloses a net positive charge. B) the surface encloses a net negative charge. C) the surface encloses no net charge. D) the surface vector at all points on the surface is necessarily parallel to the electric field vector E. E) the surface vector at all points on the surface is necessarily perpendicular to the electric field vector E. Ans: B 13. The figure shows a surface enclosing the charges 2q and q. The net flux through the surface surrounding the two charges is A) q/ 0 B) 2q/ 0 C) q/ 0 D) zero E) None of these is correct. 14. A horizontal surface of area m2 has an electric flux of 123 N.m2/C passing through it at an angle of 25 to the horizontal. If the flux is due to a uniform electric field, calculate the magnitude of the latter. A) 907 N/C D) N/C B) 423 N/C E) 383 N/C C) N/C 15. nc/m3. What is the electric field at r = 3.0 cm?

4 A) 36.0 N/C B) 230 N/C C) 140 N/C D) 565 N/C E) 450 N/C Section: 22-3 Topic: Calculating E from Gauss's Law Type: Numerical 16. An infinite plane of surface charge density = nc/m2 lies in the yz plane at the origin, and a second infinite plane of surface charge density = 8.00 nc/m2 lies in a plane parallel to the yz plane at x = 4.00 m. The electric field at x = 3.50 m is approximately A) 226 N/C B) 339 N/C C) 904 N/C D) 452 N/C E) zero Ans: C 17. The voltage between the cathode and the screen of a television set is 22 kv. If we assume a speed of zero for an electron as it leaves the cathode, what is its speed just before it hits the screen? A) m/s D) m/s B) m/s E) m/s C) m/s 18. The concept of difference in electric potential is most closely associated with A) the mechanical force on an electron. B) the number of atoms in one gram-atom. C) the charge on one electron. D) the resistance of a certain specified column of mercury. E) the work per unit quantity of electricity. 19. Charges Q and q (Q q), separated by a distance d, produce a potential VP = 0 at point P. This means that A) no force is acting on a test charge placed at point P. B) Q and q must have the same sign. C) the electric field must be zero at point P. D) the net work in bringing Q to distance d from q is zero. E) the net work needed to bring a charge from infinity to point P is zero.

5 20. Two parallel metal plates 5.0 cm apart have a potential difference between them of 75 V. The electric force on a positive charge of C at a point midway between the plates is approximately A) N D) N B) N E) N C) N 21. The electrostatic potential as a function of distance along a certain line in space is shown in graph (1). Which of the curves in graph (2) is most likely to represent the electric field as a function of distance along the same line? A) 1 B) 2 C) 3 D) 4 E) Two charges Q1 and Q2 are at rest a distance of 66 cm apart. How much work must be done to slowly move the charges to a separation of 33 cm? (Q 1 = C and Q 2 = C) A) J D) J B) J E) J C) J 23. The electric potential in a region of space is given by V(x) = 50 V + (15 V/m) x. The electric field in this region is A) 50 V î D) (15 V/m) î B) (15 V/m)x î E) C) (50 V/m + 15 V/m) î 24. (15 V/m) î

6 An isolated hollow spherical conductor of radius R is carrying a charge of Q. Graph (1) represents the potential V as a function of the distance r from the center of the sphere. The graph that represents the electric field as a function of distance is A) 2 B) 3 C) 4 D) 5 E) The figure shows portions of four equipotential surfaces whose potentials are related as follows: V1 > V2 > V3 > V4. The lines represent four paths (A A', B B', C C', D D') along which equal test charges are moved. The work involved can be said to be A) the greatest for path A A'. D) the greatest for path D D'. B) the greatest for path B B'. E) the same for all paths. C) the greatest for path C C'. 26. Two protons in a nucleus of 238 U are m apart. The electrostatic potential energy of the pair is approximately A) zero B) J C) J D) J E) J 27.

7 Which of the curves in the graph represents the electrostatic potential energy of a small negative charge plotted as a function of its distance from a positive point charge? A) 1 B) 2 C) 3 D) 4 E) The work required to bring a positively charged body from very far away is greatest for point. A) A B) B C) C D) D E) E 29. A parallel-plate capacitor has square plates of side 8.0 cm separated by 0.80 mm. If you charge this capacitor to 15 V, the amount of charge transferred from one plate to the other is A) 71 nc B) 7.1 nc C) 1.1 pc D) 1.1 nc E) 7.1 pc Section: 24-2 Topic: Capacitance Type: Conceptual 30. In which of these configurations is (are) the electric field(s) uniform? A) 1 B) 2 C) 3 D) 1 and 3 E) 1, 2, and If you increase the charge on a parallel-plate capacitor from 3 µc to 9 µc and

8 increase the plate separation from 1 mm to 3 mm, the energy stored in the capacitor changes by a factor of A) 27 B) 9 C) 3 D) 8 E) 1/ V battery, what would be the total energy stored in the capacitors? A) J D) J B) J E) J C) J