Lec1 - Winter 2008 Lecture 1 Electric Forces and Fields due...

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Winter 2008 Page 1 of 4 Lecture 1 Electric Forces and Fields due to point charges Coulomb developed an experimental law to relate the force applied on a charge due to the presence of another charge. This law indicates that electrical charges can create force on other charges at a distance. This action at a distance behavior will be used in this course to develop principles of energy storages and loss based on the application of force to fixed charged structures. Coulomb’s law states that the force F between two point charges Q 1 and Q 2 acts along the straight line path joining the two charges and is directly proportional to the product of the two charges and is inversely proportional to the square of the distance R between the charges. This is written as: R a R Q Q F ˆ 4 2 2 1 pe = . The values of the charges are in coulombs (an SI unit). The constant e is the permittivity of the environment, for air ( 29 m F o 12 10 854 . 8 - = = e e . The unit vector R R a R = ˆ is the unit vector that follows the direct straight line path linking the charges. It is often convenient to describe the effect that a single charge has on the environment. This if often described as the electric field ( E ). This is obtained by dividing the force relationship (Coulombs’ law) acting on one of the charges by the value of the second charge. Since the electric field is proportional to the force we can write it as R a R Q Q F E ˆ 4 2 1 2 pe = = (units may be newtons per coulomb or volts per meter). In both of the above relationships we can note that the effect of a charge on the environment is to push similarly valued charges away, and to attract opposite charges. For example if two identical 10 micro-coulomb
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This note was uploaded on 04/17/2008 for the course ECSE 351 taught by Professor Davis during the Winter '08 term at McGill.

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Lec1 - Winter 2008 Lecture 1 Electric Forces and Fields due...

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