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Unformatted text preview: L22E from Distributed ChargesThe electric field of a point charge QisEpoints awayfrom a positivepoint charge,towardsa negativepoint charge.rrQkEeˆ2=1. Left.2. Down.3. Right.4. Up.5. The electric field is zero.At the position of the dot, the electric field pointsSeveral charges – Just add the E fields vectorially “principle of superposition”The electric dipole – on perpendicular axis onlyp = q sFour charges are at the corners of a square of side a = 1.50 m. Charge q= Continuous Charge DistributionsIn real situations, charge is often distributed smoothlyover some volume, surface, or line.In these cases use charge densityrather thencharge.So far we’ve discussed the fields of point charges.A charge of 4 nC exists on a semicircular insulatingrod. The electric field at the point O points in whatdirection?1) Left2) Right3) Up4) Down5) Other6) I have no ideaCharge DensityThe volume charge density(ρ) is the charge per unit volume (C/m3). The surface charge density( σ) is the charge per unit area (C/m2). The linear charge density(λ) is the charge per unit length (C/m). Sample Problem:Estimate the charge density inside a proton (radius L 1.2 fm).Sample Problem:Q= 2.50 nC is spread uniformly over the surface of a cube with 1cm long edges. What is the surface charge density?Sample Problem:Q= 2.50 nC is spread uniformly on a hoop of diameter 1.00 m. What is the linear charge density?Sample Problem:Estimate the charge density inside a proton (radius ~ 1.2 fm).Solution. The charge is Q= +e= 1.60e19 C.The volume is V= (4π/3)(1.2e15 m)3= 7.24e45 m3.Assuming the charge to be distributed uniformly,ρ =Q/V= 2.2e25 C/m3....
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 Spring '08
 Souder
 Charge, Electric charge, Fundamental physics concepts, uniform electric field

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