Electrostatic Forces and Fields

The classical picture of light as an oscillating electromagnetic field provides a reasonably satisfactory basis for discussing the spectroscopic properties of molecules, provided that we take the quantum mechanical nature of matter into account. To develop this picture, let s start by reviewing some of the principles of classical electrostatics. 

Charged particles exert forces that conventionally are described in terms of electric and magnetic fields. Consider two particles with charges qi and located at positions r and r2 in a vacuum. According to Coulomb s law, the electrostatic force acting on particle 1 is 

Fields are additive if the system contains additional charged particles the field at ri is the sum of the fields from all the other particles. 

In the cgs system is equal to l/4 r so that the proportionality constants in Eqs. (3.1) and (3.2) are unity. Because this simplifies the equations of electromagnetism, the cgs system continues to be widely used. Appendix A4 gives a table of equivalent units in 

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Two profound implications of Equation (20.15) lead to a general method for calculating electrostatic forces and fields. First, the right-hand side of Equation (20.15) does not depend on r. For the electric flux through a spherical container, the r dependence of E cancels with the r- dependence of the spherical surface area. Because of this cancelation, the electric held flux out of a sphere is independent of its radius. It doesn t matter how large you choose your imaginary balloon to be, the flux is the same through spherical balloons of any radius. Second, we show now that the shape of the balloon doesn t matter either. 

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