L05_viet_E-fields and Charged Particles in E-Fields

L05_viet_E-fields and Charged Particles in E-Fields -...

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Physics 122 Electricity and Magnetism Lecture 5 E-fields and Charged Particles in E-Fields
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05/13/09 Physics 122 - Lecture 5 2 E-Field of a Charged Sphere Therefore, the electric field outside the surface of a uniformly spherically charged shell of charge Q with radius R is : sphere 2 0 1 ˆ 4 Q E r r πε = r The electric field inside the spherical-shell is zero (because the sum of the forces cancel). If η = Q/4 twice as big as the E field of an infinite plane of charge. (independent of R)
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05/13/09 Physics 122 - Lecture 5 3 Example : A Charged Sphere A sphere with a radius R = 0.1 m has a charge Q = 20.0 nC on its surface. What is the electric field at a distance of 1.0 m from the sphere? Note that the electric field does not depend on the radius of the sphere (because the E-field of a charged sphere is the same as that of the same charge at a point at the center of the sphere.) Measuring the external E- field tells you the charge but not the radius of the sphere. sphere 2 0 9 2 2 -8 2 2 1 4 (9.0 10 N m /C )(2.0 10 C)/(1.0 m) 1.8 10 N/C Q E r πε = = =
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05/13/09 Physics 122 - Lecture 5 4 A Parallel-Plate Capacitor The parallel plate capacitor is an important component of electronic circuits. It is constructed of two oppositely charged electrodes separated by a small gap. The net charge of the capacitor is zero. We can model the parallel plate capacitor as two parallel infinite charged planes placed a distance d apart. Let the x axis go from + to -. Then we have only to superpose the fields that we have previously calculated. We find that in the two
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L05_viet_E-fields and Charged Particles in E-Fields -...

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