# Chapter 3 Review

#### Key Terms

**electric dipole**

system of two equal but opposite charges a fixed distance apart

**electric dipole moment**

quantity defined as

for all dipoles, where the vector points from the negative to positive charge

**electric potential**

potential energy per unit charge

**electric potential difference**

the change in potential energy of a charge

moved between two points, divided by the charge.

**electric potential energy**

potential energy stored in a system of charged objects due to the charges

**electron-volt**

energy given to a fundamental charge accelerated through a potential difference of one volt

**electrostatic precipitators**

filters that apply charges to particles in the air, then attract those charges to a filter, removing them from the airstream

**equipotential line**

two-dimensional representation of an equipotential surface

**equipotential surface**

surface (usually in three dimensions) on which all points are at the same potential

**grounding**

process of attaching a conductor to the earth to ensure that there is no potential difference between it and Earth

**ink jet printer**

small ink droplets sprayed with an electric charge are controlled by electrostatic plates to create images on paper

**photoconductor**

substance that is an insulator until it is exposed to light, when it becomes a conductor

**Van de Graaff generator**

machine that produces a large amount of excess charge, used for experiments with high voltage

**voltage**

change in potential energy of a charge moved from one point to another, divided by the charge; units of potential difference are joules per coulomb, known as volt

**xerography**

dry copying process based on electrostatics

#### Key Equations

Potential energy of a two-charge system | |

Work done to assemble a system of charges | |

Potential difference | or |

Electric potential | |

Potential difference between two points | |

Electric potential of a point charge | |

Electric potential of a system of point charges | |

Electric dipole moment | |

Electric potential due to a dipole | |

Electric potential of a continuous charge distribution | |

Electric field components | |

Del operator in Cartesian coordinates | |

Electric field as gradient of potential | |

Del operator in cylindrical coordinates | |

Del operator in spherical coordinates |

#### Summary

#### 3.1 Electric Potential Energy

- The work done to move a charge from point to in an electric field is path independent, and the work around a closed path is zero. Therefore, the electric field and electric force are conservative.
- We can define an electric potential energy, which between point charges is , with the zero reference taken to be at infinity.
- The superposition principle holds for electric potential energy; the potential energy of a system of multiple charges is the sum of the potential energies of the individual pairs.

#### 3.2 Electric Potential and Potential Difference

- Electric potential is potential energy per unit charge.
- The potential difference between points and , , that is, the change in potential of a charge moved from to , is equal to the change in potential energy divided by the charge.
- Potential difference is commonly called voltage, represented by the symbol :

- An electron-volt is the energy given to a fundamental charge accelerated through a potential difference of . In equation form,

#### 3.3 Calculations of Electric Potential

- Electric potential is a scalar whereas electric field is a vector.
- Addition of voltages as numbers gives the voltage due to a combination of point charges, allowing us to use the principle of superposition: .
- An electric dipole consists of two equal and opposite charges a fixed distance apart, with a dipole moment .
- Continuous charge distributions may be calculated with .

#### 3.4 Determining Field from Potential

- Just as we may integrate over the electric field to calculate the potential, we may take the derivative of the potential to calculate the electric field.
- This may be done for individual components of the electric field, or we may calculate the entire electric field vector with the gradient operator.

#### 3.5 Equipotential Surfaces and Conductors

- An equipotential surface is the collection of points in space that are all at the same potential. Equipotential lines are the two-dimensional representation of equipotential surfaces.
- Equipotential surfaces are always perpendicular to electric field lines.
- Conductors in static equilibrium are equipotential surfaces.
- Topographic maps may be thought of as showing gravitational equipotential lines.

#### 3.6 Applications of Electrostatics

- Electrostatics is the study of electric fields in static equilibrium.
- In addition to research using equipment such as a Van de Graaff generator, many practical applications of electrostatics exist, including photocopiers, laser printers, ink jet printers, and electrostatic air filters.

#### Answers to Check Your Understanding

3.1.

3.2. It has kinetic energy of

at point

and potential energy of

which means that as

approaches infinity, its kinetic energy totals three times the kinetic energy at

since all of the potential energy gets converted to kinetic.

3.3. positive, negative, and these quantities are the same as the work you would need to do to bring the charges in from infinity

3.4.

3.5.

electrons

3.6. It would be going in the opposite direction, with no effect on the calculations as presented.

3.7. Given a fixed maximum electric field strength, the potential at which a strike occurs increases with increasing height above the ground. Hence, each electron will carry more energy. Determining if there is an effect on the total number of electrons lies in the future.

3.8.

recall that the electric field inside a conductor is zero. Hence, any path from a point on the surface to any point in the interior will have an integrand of zero when calculating the change in potential, and thus the potential in the interior of the sphere is identical to that on the surface.

3.9. The x-axis the potential is zero, due to the equal and opposite charges the same distance from it. On the

-axis, we may superimpose the two potentials; we will find that for

again the potential goes to zero due to cancellation.

3.10. It will be zero, as at all points on the axis, there are equal and opposite charges equidistant from the point of interest. Note that this distribution will, in fact, have a dipole moment.

3.11. Any, but cylindrical is closest to the symmetry of a dipole.

3.12. infinite cylinders of constant radius, with the line charge as the axis

#### Conceptual Questions

#### 3.1 Electric Potential Energy

1. Would electric potential energy be meaningful if the electric field were not conservative?

2. Why do we need to be careful about work done *on* the system versus work done *by* the system in calculations?

3. Does the order in which we assemble a system of point charges affect the total work done?

#### 3.2 Electric Potential and Potential Difference

4. Discuss how potential difference and electric field strength are related. Give an example.

5. What is the strength of the electric field in a region where the electric potential is constant?

6. If a proton is released from rest in an electric field, will it move in the direction of increasing or decreasing potential? Also answer this question for an electron and a neutron. Explain why.

7. Voltage is the common word for potential difference. Which term is more descriptive, voltage or potential difference?

8. If the voltage between two points is zero, can a test charge be moved between them with zero net work being done? Can this necessarily be done without exerting a force? Explain.

9. What is the relationship between voltage and energy? More precisely, what is the relationship between potential difference and electric potential energy?

10. Voltages are always measured between two points. Why?

11. How are units of volts and electron-volts related? How do they differ?

12. Can a particle move in a direction of increasing electric potential, yet have its electric potential energy decrease? Explain.

#### 3.3 Calculations of Electric Potential

13. Compare the electric dipole moments of charges

separated by a distance

and charges

separated by a distance

.

14. Would Gauss’s law be helpful for determining the electric field of a dipole? Why?

15. In what region of space is the potential due to a uniformly charged sphere the same as that of a point charge? In what region does it differ from that of a point charge?

16. Can the potential of a nonuniformly charged sphere be the same as that of a point charge? Explain.

#### 3.4 Determining Field from Potential

17. If the electric field is zero throughout a region, must the electric potential also be zero in that region?

18. Explain why knowledge of

is not sufficient to determine

What about the other way around?

#### 3.5 Equipotential Surfaces and Conductors

19. If two points are at the same potential, are there any electric field lines connecting them?

20. Suppose you have a map of equipotential surfaces spaced

apart. What do the distances between the surfaces in a particular region tell you about the strength of the

in that region?

21. Is the electric potential necessarily constant over the surface of a conductor?

22. Under electrostatic conditions, the excess charge on a conductor resides on its surface. Does this mean that all of the conduction electrons in a conductor are on the surface?

23. Can a positively charged conductor be at a negative potential? Explain.

24. Can equipotential surfaces intersect?

#### 3.6 Applications of Electrostatics

25. Why are the metal support rods for satellite network dishes generally grounded?

26. (a) Why are fish reasonably safe in an electrical storm? (b) Why are swimmers nonetheless ordered to get out of the water in the same circumstance?

27. What are the similarities and differences between the processes in a photocopier and an electrostatic precipitator?

28. About what magnitude of potential is used to charge the drum of a photocopy machine? A web search for “xerography” may be of use.

#### Problems

#### 3.1 Electric Potential Energy

29. Consider a charge

fixed at a site with another charge

(charge

mass

) moving in the neighboring space. (a) Evaluate the potential energy of

when it is

from

(b) If

starts from rest from a point

from

what will be its speed when it is

from

? (Note:

is held fixed in its place.)

30. Two charges

and

are placed symmetrically along the

-axis at

Consider a charge

and mass

moving along the

-axis. If

starts from rest at

what is its speed when it reaches

?

31. To form a hydrogen atom, a proton is fixed at a point and an electron is brought from far away to a distance of

the average distance between proton and electron in a hydrogen atom. How much work is done?

32. (a) What is the average power output of a heart defibrillator that dissipates

of energy in

? (b) Considering the high-power output, why doesn’t the defibrillator produce serious burns?

#### 3.2 Electric Potential and Potential Difference

33. Find the ratio of speeds of an electron and a negative hydrogen ion (one having an extra electron) accelerated through the same voltage, assuming non-relativistic final speeds. Take the mass of the hydrogen ion to be

34. An evacuated tube uses an accelerating voltage of

to accelerate electrons to hit a copper plate and produce X-rays. Non-relativistically, what would be the maximum speed of these electrons?

35. Show that units of

and

for electric field strength are indeed equivalent.

36. What is the strength of the electric field between two parallel conducting plates separated by

and having a potential difference (voltage) between them of

?

37. The electric field strength between two parallel conducting plates separated by

is

(a) What is the potential difference between the plates? (b) The plate with the lowest potential is taken to be zero volts. What is the potential

from that plate and

from the other?

38. The voltage across a membrane forming a cell wall is

and the membrane is

thick. What is the electric field strength? (The value is surprisingly large, but correct.) You may assume a uniform electric field.

39. Two parallel conducting plates are separated by

, and one of them is taken to be at zero volts. (a) What is the electric field strength between them, if the potential

from the zero volt plate (and

from the other) is

? (b) What is the voltage between the plates?

40. Find the maximum potential difference between two parallel conducting plates separated by

of air, given the maximum sustainable electric field strength in air to be

41. An electron is to be accelerated in a uniform electric field having a strength of

(a) What energy in

is given to the electron if it is accelerated through

? (b) Over what distance would it have to be accelerated to increase its energy by

?

42. Use the definition of potential difference in terms of electric field to deduce the formula for potential difference between

and

for a point charge located at the origin. Here

is the spherical radial coordinate.

43. The electric field in a region is pointed away from the

-axis and the magnitude depends upon the distance

from the axis. The magnitude of the electric field is given as

where

is a constant. Find the potential difference between points

and

explicitly stating the path over which you conduct the integration for the line integral.

44. Singly charged gas ions are accelerated from rest through a voltage of

. At what temperature will the average kinetic energy of gas molecules be the same as that given these ions?

#### 3.3 Calculations of Electric Potential

45. A

-diameter plastic sphere, used in a static electricity demonstration, has a uniformly distributed

charge on its surface. What is the potential near its surface?

46. How far from a

point charge is the potential

? At what distance is it

?

47. If the potential due to a point charge is

at a distance of

what are the sign and magnitude of the charge?

48. In nuclear fission, a nucleus splits roughly in half. (a) What is the potential

from a fragment that has

protons in it? (b) What is the potential energy in

of a similarly charged fragment at this distance?

49. A research Van de Graaff generator has a

-diameter metal sphere with a charge of

on it. (a) What is the potential near its surface? (b) At what distance from its center is the potential

? (c) An oxygen atom with three missing electrons is released near the Van de Graaff generator. What is its energy in

when the atom is at the distance found in part b?

50. An electrostatic paint sprayer has a

-diameter metal sphere at a potential of

that repels paint droplets onto a grounded object. (a) What charge is on the sphere? (b) What charge must a

drop of paint have to arrive at the object with a speed of

?

51. (a) What is the potential between two points situated

and

from a

point charge? (b) To what location should the point at

be moved to increase this potential difference by a factor of two?

52. Find the potential at points

and

in the diagram due to the two given charges.

53. Two charges

and

are separated by

on the

-axis symmetrically about origin, with the positive one uppermost. Two space points of interest

and

are located

and

from origin at an angle

with respect to the

-axis. Evaluate electric potentials at

and

in two ways: (a) Using the exact formula for point charges, and (b) using the approximate dipole potential formula.

54. (a) Plot the potential of a uniformly charged

rod with

charge as a function of the perpendicular distance from the centre. Draw your graph from

to

(b) On the same graph, plot the potential of a point charge with a

charge at the origin. (c) Which potential is stronger near the rod? (d) What happens to the difference as the distance increases? Interpret your result.

#### 3.4 Determining Field from Potential

55. Throughout a region, equipotential surfaces are given by

The surfaces are equally spaced with

for

for