课程名称︰电磁学一
课程性质︰必修
课程教师︰林晃岩
开课学院：电机资讯学院
开课系所︰电机工程学系
考试日期（年月日）︰104/4/24
考试时限（分钟）：110分钟
试题 :
Electromagnetics (I) Midterm Examination April 24, 2015
1. (a) Write down the Maxwell equations in differential form. (b) Show that
these equations are consistent with the law of conservation of charge.(15%)
2. (a) What is the Biot-Savart law? State its mathematical expression. (b) Is
it exactly validfor computing the magnetic field due to a time-varying
current distribution? (14%)
3. A point charge Q moves with velocity V in a vacuum region characterized by
electric field of intensity E and magnetic field of flux density B. (a) Find
the force acting on charge Q. (b) Find the power delivered by E field to
charge Q. (c) Show that the power delivered by B field to charge Q is zero.
(d) If the aforementioned electric and magnetic fields are the corresponding
fields of a traveling uniform plane wave, show that the ratio of the
magnitude of magnetic force to that of electric force, experienced by charge
Q, is less than or equal to |v|/c, where c is the speed of light in vacuum.
(16%)
4. A magnetic flux density is given in the xz-plane by
B = B_0 sin[π(x + v_0 t)]a_y, where v_0 is much smaller than the speed of
light. Consider a rigid square loop situated in the xz-plane with its
vertices at (x, 0, 3), (x, 0, 4), (x+1, O, 4), and (x+1, 0, 3). (a) Find the
expression for the emf induced around the loop in the sense defined by
connecting the above points in succession. (b) Let us move (without
rotation) the loop with the velocity v to produce zero induced emf on it.
How should we choose v? (14%)
5. A current density due to flow of charges is given by
J = -(3x a_x + y a_y +z^2 a_z). (a) Byusing surface integrals directly,
find the displacement current emanating from the closed surface of the
cubical box bounded by the planes x = ±1, y = ±1, and z = ±1. (b) Redo
(a) by using an appropriate volume integral. (14%)
6. Consider a vector field A = y sinx a_x - cosx a_y. (a) Evaluate the line
integral ∫A．dlfrom point (x1, y1, z1) to point (x2, y2, z2) along the
straight line connecting them. (b) Find ▽ x A. (c) Use the results of (a)
and (b) to verify Stokes' theorem. (15%)
J_0[1-exp(-10^6t)]a_y, t≧ 0
7. An infinite plane sheet of current density J_S={
0 , t < 0
A/m lies in the z = 0 plane in free space. (a) Find E_y(z, t) as a function
of t in the z = 300 m plane.(b) Find the asymptotic result of (a) as t
approaches infinity. (c) As t approaches infinity,the surface current
density becomes almost constant, producing an almost constant tangential
electric field as well as an almost constant tangential magnetic field. This
seems contradictory to the fact that a DC current produces a static magnetic
field only. Please explain this paradox briefly. (12%)