Electric Potential

Homework Assignment

Questions: 1, 3, 6, 8, 13

Problems: X1, X2, X3, X3, 17, 18, 21, 29, 31, 33, 36, 45

from the FIFTH edition of Serway & Beichner

Distinguish between electric potential and electrical potential energy.

Q3: Give a physical explanation of the fact that the potential energ of a pair of like charges is positive whereas the potential energy of a pair of unlike charges is negative.

Q6: Describe the equipotential surfaces for
(a) an infinite line of charge and
(b) a uniformly charged sphere

Q8: The electric field inside a hollow, uniformly charged sphere is zero. Does this imply that the potential is zero inside the sphere?

Q13: Why is it important to avoid sharp edges or points on conductors used in high-voltage equipment?


25.X1 The gap between electrodes in a spark plug is 0.0060 cm. To produce an electric spark in a gasoline-air mixture, an electric field of 3.0 x 106 V/m must be achieved. When starting the car, what minimum voltage must be supplied by the ignition circuit?

25.X2 A deuteron (a nucleus that consists of one proton and one neutron) is accelerated through a 2.7 kV potential divverence.
(a) How much energy does it again?
(b) How fast is it going if it starts from rest?

25.X3 Consider two points in an electric field. The potential at P1 is V1 = - 30 V, and the potential at Pw is V2 = + 150 V. How much work is done by an external force in moving a chargee = - 4.7 microCoulombs from p2 to P1?

25.X4 The magnitude of the electric field between two charged parallel plates separated by 1.8 cm is 2.4 x 10 4 N/C. Find the potential difference between the two plates. How much kinetic energy is gained by a deuteron in accelerating from the positive to the negative plate?

25.X5 An electron in the beam of a typical television picture tube is accelerated through a potential difference of 20 kV before striking the face of the tube.
(a) What is the energy of this electron, in electron volts, and what is its speed when it strikes the screen?
(b) How much momentum is imparted to the screen by the electron?

25.X6 At what distance from a point charge of 8.0 microCoulombs does the electric potential equal 3.6 x 104 V?

25.29 A small spherical object carries a charge of 8.0 nanoCoulombs. At what distance from the center of the object is the potential equal to 100 V? 50 V? 25 V? Is the spacing of the equipotentials proportional to the change in V?

25.18 A charge + 1 q is at the origin. A charge - 2 q is at x = 2.0 m on the x-axis. For what finite value(s) of x is
(a) the electric field zero?
(b) the electric ppotential zero?

25.31 In Rutherford’s famous scattering experiments that led to the planetary model of the atom, alpha particles (charge + 2 e, mass = 6.6 x 10 - 27 kg) were fired at a gold nucleus (charge + 79 e). An alpha particle initiallly very far from the gold nucleus is fired at 2.0 x 107 m/s directly toward the center of the nucleus. How close does the alpha particle get to this center before turning around?

25.33 Calculate the energy required to assemble the array of charges shown in Figure P25.33 where a = 0.20 m, b = 0.40 m, and q = 6.0 microCoulombs.

25.36 The potential in a region between x = 0 and x = 6.0 m is V = a + b x where a = 10 V and b = - 7.0 V/m. Determine
(a) the potential at x = 0, 3.0 m, and 6.0 m and
(b) the magnitude and direction of the electric field at x = 0, 3.0 m, and 6.0 m.

25.45 Calculate the electric potential at point P on the axis of the annulus shown in Figure P25.45, which has a uniform charge density sigma.


Electric Potential

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(c) Doug Davis, 2002; all rights reserved