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As the example implies, gravitational force is completely negligible on a small scale, where the interactions of individual charged particles are important. On a large scale, such as between the Earth and a person, the reverse is true. Most objects are nearly electrically neutral, and so attractive and repulsive Coulomb forces nearly cancel. Gravitational force on a large scale dominates interactions between large objects because it is always attractive, while Coulomb forces tend to cancel.

Test prep for ap courses

For questions 25–27, suppose that the electrostatics force between two charges is F .

What will be the force if the distance between them is halved?

  1. 4 F
  2. 2 F
  3. F /4
  4. F /2

(a)

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Which of the following is false?

  1. If the charge of one of the particles is doubled and that of the second is unchanged, the force will become 2 F .
  2. If the charge of one of the particles is doubled and that of the second is halved, the force will remain F .
  3. If the charge of both the particles is doubled, the force will become 4 F .
  4. None of the above.
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Which of the following is true about the gravitational force between the particles?

  1. It will be 3.25×10 −38 F .
  2. It will be 3.25×10 38 F .
  3. It will be equal to F .
  4. It is not possible to determine the gravitational force as the masses of the particles are not given.

(d)

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Two massive, positively charged particles are initially held a fixed distance apart. When they are moved farther apart, the magnitude of their mutual gravitational force changes by a factor of n . Which of the following indicates the factor by which the magnitude of their mutual electrostatic force changes?

  1. 1/ n 2
  2. 1/ n
  3. n
  4. n 2
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  1. What is the electrostatic force between two charges of 1 C each, separated by a distance of 0.5 m?
  2. How will this force change if the distance is increased to 1 m?

(a) 3.60×10 10 N, (b) It will become 1/4 of the original value; hence it will be equal to 8.99×10 9 N

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  1. Find the ratio of the electrostatic force to the gravitational force between two electrons.
  2. Will this ratio change if the two electrons are replaced by protons? If yes, find the new ratio.
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Section summary

  • Frenchman Charles Coulomb was the first to publish the mathematical equation that describes the electrostatic force between two objects.
  • Coulomb's law gives the magnitude of the force between point charges. It is
    F = k | q 1 q 2 | r 2 , size 12{F=k { {q rSub { size 8{1} } q rSub { size 8{2} } } over {r rSup { size 8{2} } } } } {}

    where q 1 and q 2 are two point charges separated by a distance r , and k 8.99 × 10 9 N · m 2 / C 2

  • This Coulomb force is extremely basic, since most charges are due to point-like particles. It is responsible for all electrostatic effects and underlies most macroscopic forces.
  • The Coulomb force is extraordinarily strong compared with the gravitational force, another basic force—but unlike gravitational force it can cancel, since it can be either attractive or repulsive.
  • The electrostatic force between two subatomic particles is far greater than the gravitational force between the same two particles.

Conceptual questions

[link] shows the charge distribution in a water molecule, which is called a polar molecule because it has an inherent separation of charge. Given water's polar character, explain what effect humidity has on removing excess charge from objects.

A schematic representation of the outer electron cloud of a neutral water molecule is shown. Three atoms are placed on the vertices of a triangle. The hydrogen atom has positive q charge and the oxygen atom has minus two q charge, and the angle between the line joining each hydrogen atom with the oxygen atom is one hundred and four degrees. The cloud density is shown more at the oxygen atom.
Schematic representation of the outer electron cloud of a neutral water molecule. The electrons spend more time near the oxygen than the hydrogens, giving a permanent charge separation as shown. Water is thus a polar molecule . It is more easily affected by electrostatic forces than molecules with uniform charge distributions.

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Practice Key Terms 3

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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