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Lesson_3_-_Electric_Force_and_Field - EE3321...

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+ EE3321 ELECTROMAGNETIC FIELD THEORY Lecture 3 Highlights 1. Coulomb’s Law As reported by the Ancient Greek philosopher Thales of Miletus around 600 BC, charge could be accumulated by rubbing fur on various substances, such as amber (or “electron” in Greek which is spelled ήλεκτρον). The Greeks noted that the charged amber buttons could attract light objects such as hair. They also noted that if they rubbed the amber for long enough, they could even get a spark to jump. The law of electrostatic attraction and repulsion, developed in the 1780s by French physicist Charles Augustine de Coulomb, may be stated in scalar form as follows: The magnitude of the electrostatic force between two point electric charges is directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance between the charges. Thus, the magnitude of the force exerted by two charges over each other is given by F = k Q 1 Q 2 Newtons (N= kg m s -2 ) R 2 k = __1___ = 9 x 10 9 N m 2 / (As) 2 4πε o ε o = 8.85 x 10 -12 F/m permittivity of free space Coulomb’s Law falls into the category of inverse-square laws found in physics. The diagram below shows how the law works. Lines represent the “flux” emanating from the source. The total number of flux lines depends on the strength of a charge and is constant with increasing distance. A greater density of flux lines
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(lines per unit area) means a stronger force field. The density of flux lines is inversely proportional to the square of the distance from the source because the surface area of a sphere increases with the square of the radius. Thus the strength of the field is inversely proportional to the square of the distance from the source. 2. Electric Charge Convention Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between a moving charge and an
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