325_Sp2011_2_Coulolb's_Law_and_Electric_Feild

325_Sp2011_2_Coulolb's_Law_and_Electric_Feild - ©...

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Unformatted text preview: © Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin Coulomb’s Law and Electric Field 1 2. Coulomb’s Law and Electric Field Intensity EE325 Mikhail Belkin © Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin Coulomb’s Law and Electric Field 2 Forces between charges • let’s start with the “fundamental” interaction between “point charges” • 1600: Gilbert publishes first major classification of electric and non-electric materials • Charles Augustin Coulomb (1736-1806) – 1761-1781 initially served as a military engineer – 1785 – 1791: wrote seven important treatises on electricity and magnetism, submitted to the Académie des Sciences (France) – developed a theory of attraction and repulsion between bodies of the opposite and same electrical charge – demonstrated an inverse square law for such forces – examined perfect conductors and dielectrics • suggested that there was no perfect dielectric, proposing that every substance has a limit above which it will conduct electricity – put forward the case for action at a distance between electrical charges in a similar way as Newton 's theory of gravitation was based on action at a distance between masses © Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin Coulomb’s Law and Electric Field 3 Typical equipment used in Coulomb’s days Electrostatic frictional generator Coulomb’s torsion balance © Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin Coulomb’s Law and Electric Field 4 Key results • force (a VECTOR quantity!) between two simple charged objects is directly along the line that joins the two objects – force is attractive for oppositely-charged objects – force is repulsive for like-charged objects • the magnitude of the force decreases as the square of the distance between the charges • the magnitude of the force is proportional to the product of the two charges – in “free” space ( 29 1 2 2 12 4 o Q Q F R πε = r ( 29 ( 29 1 2 1 2 12 12 2 2 12 12 12 4 4 ˆ o o Q Q Q Q R F r R R R πε πε = = r r r • in vector form • demos – electron-proton demo: http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html © Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin Coulomb’s Law and Electric Field 5 Units / dimensions • force ∝ (charge) 2 / [ ε o *(distance) 2 ] – let’s use MKS: – force: Newton = kg*(m / sec 2 ) (i.e., mass*acceleration) – charge: Coulomb (fundamental) – abbreviated as “Coul” or “C” – distance: meter (fundamental) ε o : Coul 2 / N*m 2 = Coul 2 sec 2 / kg*m 3 • more familiar units for ε 0 (F/m): “recall” that – Q = C*V C = Q/V Farad = Coul / Volt – e*Volt = energy Volt = energy / charge – energy = force*distance Volt = force*distance/ charge • Volt = N*m/Coul – Farad = Coul / [N*m/Coul] = Coul 2 / N*m...
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This note was uploaded on 04/03/2012 for the course EE 325 taught by Professor Brown during the Fall '08 term at University of Texas.

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325_Sp2011_2_Coulolb's_Law_and_Electric_Feild - ©...

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