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| Coulomb's Law |
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| It states that "the electrostatic force between two electric charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them ". |
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| If q1 and q2 are two point charges separated by a distance r, then the electrostatic force F between the charges is given by |
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| where k is a positive constant called 'Coulomb's constant'. This value depends on the medium between the two charges. |
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| (a) If the charges are in vacuum, then in SI Units, |
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| where e0 is the permittivity of space. |
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| The value of e0 = 8.854 x 10-12 C2/Nm2 which was obtained by experiments. Thus, K = 8.988 x 109 Nm2/C2. |
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| The unit of charge is Coulomb and is denoted by C. One coulomb of charge, is that charge which when placed one metre from another charge in vacuum experiences an electrical force of repulsion of magnitude 9 x 109 N . |
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(b) If the charges are immersed in matter, then e0 is replaced by e, a  e is called absolute electric permitivity of the dielectric medium. |
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 or relative permittivity . So Coulomb's law is written as |
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| (a) If q1q2 > 0, that is either both q1 and q2 are positive or both are negative, then the charges repel each other. |
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| (b) If q1q2 < 0, that is if the charges are dissimilar, then the charges attract each other. |
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| Thus, Coulomb's law agrees with Newton's third law. |
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