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# Coulombs Law

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### Coulombs Law

1. 1. Coulomb’s Law Electric Force and Electric Field
2. 2. Coulomb's Law <ul><li>Coulomb determined </li></ul><ul><ul><li>Force is attractive if charges are opposite sign </li></ul></ul><ul><ul><li>Force is proportional to the product of the charges q 1 and q 2 and is along the lines joing them </li></ul></ul><ul><ul><li>Force inversely proportional to the square of distance between the charges </li></ul></ul><ul><li>i.e. </li></ul><ul><ul><li>|F 12 |  |Q 1 | |Q 2 | / r 12 2 </li></ul></ul><ul><ul><li>or </li></ul></ul><ul><ul><li>|F 12 |= k |Q 1 | |Q 2 | / r 12 2 </li></ul></ul>
3. 3. Coulomb's Law <ul><li>Units of constant can be determined from Coulomb's Law </li></ul><ul><li>Colomb (and others since) have determined this constant which (in a vacuum) in SI units is </li></ul><ul><ul><li>k = 8.987.5x10 9 Nm 2 C -2 </li></ul></ul><ul><li>k is normally expressed as k = 1/4  0 </li></ul><ul><ul><li>where is the permittivity of free space </li></ul></ul>
4. 4. Coulomb's Law
5. 5. Vector form of Coulomb’s Law Q1 Q2 r 12 r 12 is a vector connecting the two charges. is a unit vector in this direction + + + -
6. 6. Sample Problem Resultant F on P = F BP – F AP = 2.62  10 -5 N. Direction: To the left (attraction) P B A + - + 6 cm 4 cm 2 nC 5 nC 1 nC
7. 7. Force from many charges Superposition
8. 8. Force from many charges Q1 Force on charge is vector sum of forces from all charges Principle of superposition + - Q2 + Q4 - Q3