Equations for point charges

Equations for point charges - positive V and q gives rise...

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Dr. Chang The following equations are all for point charges and they look similar: Magnitude Only F c is the Coulomb force exerted by q 1 on q 2 (or q 2 on q 1 ) when they are separated by a distance r. Note the 1/r 2 dependence. Direction of this force is determined by the nature of charges involved. Magnitude Only E is the electric field strength at a distance r from the point charge q. It also has a 1/r 2 dependence. Direction of the E field due to a +q is outward from the point charge whereas E for a –q is directed inward. Scalar quantity V is the electric potential created by a point charge q at a distance r away. q can be + or -. Therefore V can have positive or negative values. +q gives rise to
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Unformatted text preview: positive V and q gives rise to negative V. Scalar quantity U is the electric potential energy of a pair of point charges q 1 and q 2 separated by a distance r. q 1 and q 2 can carry + or sign in this equation. The pair would have no electric potential energy if they are infinitely apart (r = ). If one of them is a negative charge and r is finite, the pair has a negative electrical potential energy. Moving one of them to infinity would require work input equal to the original U in joules. Why? Since U f = 0 and U i is negative, U = U f- U i is positive in this case. Thus you can see work input is required to increase the electrical potential energy....
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