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Potential and E-field V(r) º - r ò E(r ') · dr ' origin r ò qV (r) º - origin r =- ò qE(r ') · dr ' Fon q · dr ' origin = external work done on charge to move it Potential and E-field V(r) º - r ò E(r ') · dr ' origin E(r) = -ÑV(r) The fact that V is well defined arises from ò E(r)·dl = 0 Which is exactly the same as (!) Ñ ´ E(r) = 0 Potential and E-field r ò V(r) = - E(r ') · dr ' origin E(r) = -ÑV(r) One way to find V(r) (given charges): V(r) = 1 4pe0 òòò r (r')  dt ' One way to find V(r) (given charges): V(r) = 1 4pe0 V(r) = òòò 1 4pe 0 r (r')  òòò dt ' dq  Voltage V = kq/r from a point charge ! So, what does V(r) º - r ò E(r ') · dr ' origin Have to do with ò E(r)·dl = 0 And why is that the same as Ñ ´ E(r) = 0 Fundamental theorem: æ df ö ò a çè dx ÷ø dx = f (b) - f (a) b Divergence theorem: òòò (Ñ· F)dt = òò F · dA V S Stoke’s (or “curl”) theorem: òò (Ñ ´ F) · dA = ò F · dl S 1.8 What is the curl of this vector field, V in the region shown below? V = c ĵ A. non-zero everywhere B. Zero at some points, non-zero others C. zero curl everywhere shown 1.8 What is the curl of this vector field, V in the region shown below? V = c ĵ A. non-zero everywhere B. Zero at some points, non-zero others C. zero curl everywhere shown 1.8 What is the curl of this vector field, V in the region shown below? V = c ĵ é 1 ¶vz ¶vf ù Ñ´V =ê ú ŝ ë s ¶f ¶z û é ¶vs ¶vz ù ˆ 1 é ¶(svf ) ¶vs ù +ê ú ẑ úf + ê ë ¶z ¶s û s ë ¶s ¶f û 2.43b/1.7b Which of the following could be a static physical Efield in a small region? I II A) Only I B) Only II C) Both D) Neither E) Cannot answer without further info E(r) = 1 4pe 0 òòò r(r ') Â̂  2 dt ' (with  = r - r' = (x - x', y - y', z - z')) It is also true that Â̂ 1 = -Ñ 2 Â Â æ ¶ ¶ ¶ö where Ñ = çè ¶x , ¶y , ¶z ÷ø Question: is the following mathematically ok? æ 1 1 ö 1 r(r ') E(r) = r(r ') ç -Ñ ÷ dt ' = -Ñ dt ' òòò òòò 4pe 0 Âø 4pe 0  è A) Yes B) No C) ??? A uniformly charged ring, in the xy plane, centered on the origin, has radius a and total charge Q. V(r =) = 0. What is the voltage at z on the z-axis? z kQ A) a kQ C) a 2 + z2 kQ B) z a E) None of these kQ D) 2 2 a +z A uniformly charged ring, in the xy plane, centered on the origin, has radius a and total charge Q. V(r =) = 0. What is the voltage at z on the z-axis? z kQ A) a kQ C) a 2 + z2  kQ B) z a dq E) None of these kQ D) 2 2 a +z A spherical shell has a uniform positive charge density on its surface. (There are no other charges around) What is the electric field inside the sphere? A: E=0 everywhere inside B: E is non-zero everywhere in the sphere C: E=0 only at the very center, but non-zero elsewhere inside the sphere. D: Not enough info given E(r) = -ÑV(r ) r ò V(r) º - E(r ') · dr ' origin Could this be a plotr of |E|(r)? Or V(r)? (for SOME physical situation?) A) Could be E(r), or V(r) B) Could be E(r), but can't be V(r) C) Can't be E(r), could be V(r) D) Can't be either E) ??? The voltage is zero at a point in space. You can conclude that : A) The E-field is zero at that point. B) The E-field is non-zero at that point C) You can conclude nothing at all about the Efield at that point The voltage is constant everywhere along a line in space. V=constant You can conclude that : A) The E-field has constant magnitude along that line. B) The E-field is zero along that line. C) You can conclude nothing at all about the magnitude of E along that line. y Consider an infinitesimal path element dL directed radially inward, toward the origin as shown. In spherical coordinates, the correct expression for dL is: A) d L = + dr r B) d L = - dr r C) Neither of these. cartesian : d L = dx xˆ + dy yˆ spherical: d L = dr r + r dq qˆ + r sin q df fˆ dL x Given a spherical SHELL with uniform surface charge density (no other charges anywhere else) what can you say about the potential V inside this sphere? (Assume as usual, V(∞)=0) A) V=0 everywhere inside B) V = non-zero constant everywhere inside C) V must vary with position, but is zero at the center. D) Could be A OR B! E) None of these, it’s something else! We usually choose V(r ∞) ≡ 0 when calculating the potential of a point charge to be V(r) = +κq/r. How does the potential V(r) change if we choose our reference point to be V(R)=0 where R is closer to +q than r. +q A B C D R r V(r) is positive but smaller than kq/r V(r) is positive but larger than kq/r V(r) is negative V(r) doesn’t change (V is independent of choice of reference) ∞ +Q +Q A B Two isolated spherical shells of charge, labeled A and B, are far apart and each has charge Q. Sphere B is bigger than sphere A. Which shell has higher voltage? [V(r=) = 0] A) Sphere A B) Sphere B C) Both have same voltage. Why is ò E × dl = 0 in electrostatics? a) Because Ñ´ E = 0 b) Because E is a conservative field c) Because the potential (voltage) between two points is independent of the path d) All of the above e) NONE of the above - it's not true! V A parallel plate capacitor is attached to a battery which maintains a constant voltage difference V between the capacitor plates. While the battery is attached, the plates are pulled apart. The electrostatic energy stored in the capacitor A) increases B) decreases C) stays constant.