Lecture%2019C - Physics1B Lecture19C ElectricFields...

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Physics 1B Lecture 19C
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Electric Fields This is a two-step process: Source charge q [Coul] E field [N/Coul] Test charge q o [Coul] creates exerts force on   The electric field can point in towards q OR away from q (depending on the  sign of the source charge).   The electric force can point along the electric field OR opposite the electric  field (depending on the sign of the test charge).
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Electric Fields We relate the electric force and the electric field by the following  equation:   This equation is always valid (not just for point charges).   Note that the electric field is a vector (it has both magnitude and direction).   If q o  is negative then the electric force and the electric field point in opposite  directions.   Also note that the SI unit for the electric field is: [Newton/Coulomb]. 
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Electric Fields For a point charge (or a spherically symmetric object) the magnitude of  the electric field will be given by:   A charge will never feel a force from its own electric field (Newton’s 3rd  Law).  If the charge configuration is more complicated than just a point charge, you  cannot use this equation.   For a configuration with N point charges, the electric field is found from  the superposition principle  N total E E E E + + + = ... 2 1
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Electric Fields For a positive source charge we get the following  electric field at point P:  The electric field produced by a positive source  charge is directed away from it.   A positive test charge would be repelled from  the positive source charge.   The electric field is essentially the electric force that would act upon a  positive test charge of magnitude 1 Coulomb if it were placed there.
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Electric Fields For a negative source charge we get the following  electric field at point A:   The electric field produced by a negative  source charge is directed towards it.   A positive test charge would be attracted to  the negative source charge.   Please note that the electric field exists whether or not there is  a test charge present.
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Electric Fields Example Two charged particles are fixed to an x axis:  particle 1 of charge  –200nC at x = 6.00cm and particle 2 of charge +200nC at  x = 21.0cm.  What is the net electric field midway between the  particles?   Answer   The coordinate system is already defined for you. q 1 =–200nC q 2 =+200nC +x x=0 x=21cm x=6cm X
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Electric Fields   Answer Next, let’s list the quantities that we know: q 1  = –2.0x10 –7 Coul q 2  = +2.0x10 –7 Coul r 1  = 15cm/2 = 7.5cm r 2  = 15cm/2 = 7.5cm Let’s calculate the electric field due to each source charge separately.  First, 
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This note was uploaded on 01/30/2012 for the course PHYSICS 1B 1B taught by Professor Grosmain during the Winter '10 term at UCSD.

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Lecture%2019C - Physics1B Lecture19C ElectricFields...

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