课程名称︰电磁学 课程性质︰物理系必修 课程教师︰贺培铭 开课学院：理学院 开课系所︰物理系 考试日期（年月日）︰2011/11/10 考试时限（分钟）：180 mins 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 若答案完全正确，没有计算过程不扣分；但答案未化约到最简形式会酌量扣分 1. (a) (5%) What is the volume charge density ρ(r) of an infinitely long cylinder of radius R with a constant surface charge density σ ? The central axis of the cylinder with the z-axis. Express your answer in cylindrical coordibates. (b) (10%) Find A,B in the following identity 2 2 d d d x ----- δ(x-a) = A ---- δ(x-a) + B --- δ(x-a) 2 2 dx dx dx 2. (15%) A solid ball of radius a with total charge q is covered by a concentric spherical conducting shell from radius r = a to b. The total charge on the conductor is Q, and the volume charge density is constant from r = 0 to a. (图就是两个同心圆，小的半径a，大的半径b) (a) Fibd the total charge at the surface at r= b. (b) Find the electric potential V(r) in all 3 regions (i) r > b , (ii) b > r > a , (iii) a > r. 3. (15%) An infinitely long rectangular metal pipe (sides a & b) extends along the z direction. The metal plates at x = ±a/2 are grounded, V(±a/2,y) = 0, and that at y = ±b/2 is given by the function V(x, ±b/2) = A sin(2πx/a) for a given constant A. (a) Find V(x,y) inside the pipe. (Hint:{sin(nπ(x + a/2)/a} n=1~∞ is a complete basis of functions on the domain [-a/2 , a/2]. ) (b) Find the charge density σ(x) at the boundary y = (b/2)^- (on the inner surface of the conductor). 图： XY截面上的长方形，四边：x=±a/2 , y=±b/2 4. (15%) The multipole expansion of the electric potential is ^ ^ ^ 1 Q p。r r_i r_j Q_ij V(r) = ------- [ ---- + ------ + Σ ------------ + ....] 4πε_0 r r^2 i,j 2 r^3 where Q_ij = ∫dτ' ρ(r') [3 r_i' r_j' - (r')^2 δ_ij] For the charge distribution of 2 point charges of equal charge q at (x,y,z) = (1,0,0),(-1,0,0), find (1) its monopole, dipole and quadrupole moments, (2) an approximate value of the potential V(0,0,z) on the z axis for large z , ignoring terms that are of order 1/z^4 or smaller. 5. (10%) A point charge q is placed at the center (r=0) of a spherical region of radius R. The potential V(R,θ) is given on the boundary and 1 we treat the integrals ν_ι =∫ V(R,θ)P_ι(cosθ)dcosθ as given numbers. -1 Find the electric potential V(r,θ) for r > R. 6. (30%) Which of the following statements is(are) correct? (a) For an arbitrary function f(x) defined on the region [0,π], Fourier's trick allows us to ffind coeddicients f_n such that f(x) equals the sum ∞ Σ f_n sin(nx) for all x ∈(0,π) even when f(x) has discontinuity. n=1 (b) The total charge on a conductor is always 0 because the charge density ρ = 0 throughout the conductor. (c) If we hide a charge q inside a conducting shell, there is no way to find the value of q by measuring the electric field outside the conducting shell. (d) To solve the electric potential for a point charge q inside a grounded metal box, we only need 6 image charge , one for each side of the box. (e) A×B is a vector field if A is a vector field and B is a pseudo vector field. (f) If ▽。F = 0 for a vector field F, there always exsits a scalar field U such that F = - ▽U. (g) The electric energy for a system of point charges is always positive because W = ε/2 ∫E^2 dτ and E^2 is always positive. (h) Across a boundary with surface charge density σ≠0, there is always a discontinuity in the electric potential V. (i) The followinf integral vanishes. ^ r ∫ ▽。(----) dτ= 0 All space r^2 (j) The capacitance of a capacitor is defined by C=Q/V and its electric energy is given by W = 1/2 V^2 / C . --

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