lista do Cap. 21

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Using Coulomb’s Law
Coulomb’s law is a force law, and forces are vectors. It has 
been many chapters since we made much use of vectors and 
vector addition, but these mathematical techniques will be 
essential in our study of electricity and magnetism.
There are two important observations regarding Coulomb’s law:
1. Coulomb’s law applies only to point charges. A point 
charge is an idealized material object with charge and 
mass but with no size or extension. For practical purposes,
two charged objects can be modeled as point charges if 
they are much smaller than the separation between them.
2. Electric forces, like other forces, can be 
superimposed. If multiple charges
1, 2, 3, . . . are present, the net electric force on charge j due to
all other charges is
 Fnet = F1 on j + F2 on j + F3 on j + . . . (25.4)
where each of the Fi on j is given by Equation 25.2 or 25.3.
These conditions are the basis of a strategy for using 
Coulomb’s law to solve electrostatic force problems.