Section 2 review
There are two kinds of electric charges, positive and negative. Electrostatics is the study of electric charge at rest and electric current is electric charge in motion. Matter contains electric charge inside every atom. Most objects around us, however, have little or no net charge because each atom has exactly the same amount of positive and negative charge, which cancel each other. Electric charge is measured in units of coulombs (C). Electric charges exert forces on each other that can be calculated using Coulomb’s law. Charges of similar sign (+/+ or −/−) repel each other whereas charges of opposite sign (+/−) attract each other. Read the text aloud
static electricity, electrostatics, electric charge, electrically neutral, electric force, electroscope, electrostatic induction, Coulomb’s law, coulomb (C), negative charge, positive charge

F e = k e q 1 q 2 r 2

Review problems and questions

  1. Describe the similarities and differences between the interactions of magnetic poles and the interactions of electric charges. Read the text aloud Show
  1. The leaves of an electroscope are initially apart from each other. When a charged rod is brought near the electroscope, the leaves get closer together. Which of the following is true?
    1. The electroscope is initially neutral and becomes charged.
    2. The electroscope is initially charged with the same polarity as the rod.
    3. The electroscope is initially charged with the opposite polarity of the rod. Read the text aloud Show
  1. Two charged objects are located 1 m apart. Calculate the magnitude and describe the direction for the electric force between them if the two charges are
    1. +1 C and +1 C,
    2. +1 C and −1 C, or
    3. −1 C and −1 C. Read the text aloud Show
  1. If Benjamin Franklin had actually flown a metal key on a kite in the rain during an electrical storm, many people think that he would have died on the spot. Describe why he might have been killed. Read the text aloud Show
  1. How is it possible for a charged object to attract a neutral object? Read the text aloud Show
  1. The atoms that make up the Earth and the Moon contain enormous amounts of charge as well as mass. The Earth and Moon exert strong gravitational forces on each other, and yet they exert no electrical force on each other. Explain how this is possible. Read the text aloud Show
  1. Two charged spheres placed a distance D apart exert a repulsive force of 90 N on each other. How far apart should they be placed to reduce this repulsive force to 10 N? Read the text aloud Show

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