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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. 
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static electricity, electrostatics, electric charge, electrically neutral, electric force, electroscope, electrostatic induction, Coulomb’s law, coulomb (C), negative charge, positive charge
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Review problems and questions |
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- Describe the similarities and differences between the interactions of magnetic poles and the interactions of electric charges.
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Similarities: - Both produce a force that can either repel or attract.
- Both are noncontact forces that act through fields.
- Both have opposite states (positive and negative charges versus north and south magnetic poles).
- Like poles or charges repel, while unlike poles or charges attract.
- Induced charge is attractive, similar to magnetized materials.
- Both get weaker with distance (although magnetic force falls off faster).
Differences:- Isolated electric charges (positive or negative) exist, but no isolated magnetic poles (north or south) exist.
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- 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?
- The electroscope is initially neutral and becomes charged.
- The electroscope is initially charged with the same polarity as the rod.
- The electroscope is initially charged with the opposite polarity of the rod.
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Choice c is correct. The fact that the leaves start apart means that the electroscope is initially charged. The fact that they come together when the rod comes near means that the charge is opposite the charge on the rod. This opposite charge is attracted up into the metal ball and rod, reducing the net charge on the leaves and thereby reducing their mutual repulsion.
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- 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 C and +1 C,
- +1 C and −1 C, or
- −1 C and −1 C.
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Answer: - The magnitude of the force is 9×109 N. Because the charges have the same sign, they repel each other.
- Because the numbers are the same as the previous case, the force between the charges is the same, 9×109 N. The charges have different signs, however, so they attract each other.
- Again, the force between the charges is 9×109 N. This time, the charges have the same sign, so they repel each other.
Solution: - Asked: magnitude and direction of electric force Fe
Given: object 1 of charge q1 = +1 C; object 2 of charge q2 = +1 C;
charges separated by distance r = 1 m; Coulomb’s constant ke = 9 ×109 N m2/C2
Relationships: Solve: Answer: The magnitude of the force is 9×109 N. Because the charges have the same sign, they repel each other. - Because the numbers are the same as the previous case, the force between the charges is the same, 9×109 N. The charges have different signs, however, so they attract each other.
- Again, the force between the charges is 9×109 N. This time, the charges have the same sign, so they repel each other.
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- 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.
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Wet objects often become reasonable conductors of electricity. If he were outside in a rainstorm, the water on the kite string could easily have conducted the lightning’s electricity to him.
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- How is it possible for a charged object to attract a neutral object?
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In electrical induction, a charged body brought near a neutral body can cause the electrical charges in the neutral body to slightly separate. The like charges in the neutral object are repelled, while the unlike charges are attracted to the charged body. These unlike charges then cause attraction between the charged body and the neutral body.
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- 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.
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The Earth and Moon are electrically neutral because the atoms that make them up typically contain equal amounts of positive and negative charge. Since they contain no net charge, they exert no electric force on each other.
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- 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?
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They should be placed a distance 3D apart. Tripling the distance reduces the force from 90 to 10 N, a factor of 1/9:
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