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ELEC 210

SECOND SEMESTER EXAMINATION REPLACEMENTS 2020/21

ELECTROMAGNETICS

1.    a)    Explain what is meant by the following terms in electromagnetics:

i)      Absolute potential

ii)     Magnetic flux density

b)   You have four point charges, 2q, q, -q, and -2q. If possible, describe how you would place a closed surface that encloses at least the charge 2q (and perhaps other charges) and through which the net electric flux is

i) 0

ii) +3q/ɛ0

c)    The membrane of a cell is electrically equivalent to a parallel plate capacitor of area  1.0  mm2    and  thickness  1.0  μm.  The  relative  permittivity  of the membrane is 8 and the potential difference across the membrane is 90 mV. Determine:

i)     the capacitance

ii)    the magnitude of the electric field in the membrane 

iii)   the stored electrical energy 

iv)    the magnitude of the separated charge

Total 21

2.    a)    i)       Explain what is meant by the term displacement current. 

ii)      Give  a  relationship  between   conduction   current  and  displacement current, defining each term in your expression.

b) An electric field in a vacuum is given by the expression:

E(r, t) = E0  × ay × sin(k ×  (x-c×t))

i)       Show that this expression satisfies the wave equation

ii)     Determine the corresponding magnetic field B(r, t)

c)    i)       Sketch the magnetic field lines around a long straight wire carrying a direct current of 2 A.

ii)     What  is  the  value of magnetic field intensity at a distance of 0.5 m from the wire?

d)    State the differential form of Maxwell’s equations for electric and magnetic fields using only the electrical field intensity and magnetic field intensity. Identify units of any parameters which are not electric or magnetic fields.

Total 44

3. a) i) Explain the term Transmission Line in electromagnetics.