This set consists of Tough Electromagnetic Theory Questions and Answers.

1. Copper behaves as a

a) Conductor always

b) Conductor or dielectric depending on the applied electric field strength

c) Conductor or dielectric depending on the frequency

d) Conductor or dielectric depending on the electric current density

View Answer

Explanation: The loss tangent for copper is very large due to its high conductivity. This shows that the copper behaves as a conductor in all conditions.

2. In a good conductor the phase relation between the tangential components of electric E and the magnetic field H is as follows

a) E and H are in phase

b) E and H are out of phase

c) H leads E by 90

d) E leads H by 45

View Answer

Explanation: In a conductor, the intrinsic impedance gives the phase relation between E and H. For a conductor, the electric field and magnetic field are in 45 degree phase difference. E and H are 45 leading.

3. For an electromagnetic wave incident from one medium to a second medium, total internal reflection takes place when

a) Angle of incidence is equal to the Brewster angle with E field perpendicular to the plane of incidence

b) Angle of incidence is equal to the Brewster angle with E field parallel to the plane of incidence

c) Angle of incidence is equal to the critical angle with the wave moving from the denser to rarer medium

d) Angle of incidence is equal to the critical angle with the wave moving from the rarer to denser medium

View Answer

Explanation: Total internal reflection takes place when the angle of incidence is greater than the critical angle. Also the wave should move from the denser medium to a rarer medium.

4. For maximum power transfer, a lossless transmission line 50 ohm is to be matched to a resistive load impedance of 100 ohm. The characteristic impedance of the wavelength/4 transformer is

a) 70.7

b) 50

c) 100

d) Infinity

View Answer

Explanation: For maximum power transfer, Z

_{in}= Z

_{0}

^{2}/Z

_{L}. On substituting for the given values, we get the characteristic impedance as 70.7 ohm.

5. In an impedance Smith chart , a clockwise movement along a constant resistance circle gives rise to

a) Decrease in reactance

b) Increase in reactance

c) No change in reactance

d) No change in impedance

View Answer

Explanation: In clockwise direction, along the constant resistance circle gives rise to an increase in the value of reactance.

6. A transmission line is distortionless if

a) RL = 1/GC

b) RL = GC

c) LG = RC

d) RG = LC

View Answer

Explanation: Condition for distortionless line is R/L = G/C. In other words, the rise time constant is equal to the fall time constant. Hence RC = LG.

7. A lossless line having 50 ohm characteristic impedance and length wavelength/4 is short circuited at one end connected to an ideal voltage source of 1V at the other end. The current drawn from the voltage sources is

a) 0

b) 0.02

c) Infinity

d) 50

View Answer

Explanation: For a quarter wave transformer, the input impedance is given by Z

_{in}= Z

_{0}

^{2}/Z

_{L}. The load impedance will be zero in case of short circuit. Thus the input impedance will be infinite. The current drawn is I = V/Z

_{L}= 1/∞ = 0.

8. The capacitance per unit length and the characteristic impedance of a lossless transmission line are C and Z respectively. The velocity of a travelling wave on the transmission line is

a) ZC

b) 1/ZC

c) Z/C

d) C/Z

View Answer

Explanation: The characteristic impedance of the Z = √(L/C) and the velocity of propagation is V = 1/√(LC). Thus we get V = 1/ZC.

9. The minimum distance of the stub from the load side is 5 cm. Calculate the guided wavelength of the transmission line.

a) 5 cm

b) 2.5 cm

c) 10 cm

d) 1.25 cm

View Answer

Explanation: The minimum distance of the stub from the load line is given by V

_{min}= wavelength/2. On substituting the given value, we get the guided wavelength as 10 cm.

10. One end of a lossless transmission line having the characteristic impedance of 75 ohm and length of 1 cm is short circuited. At 3 GHz, the input impedance at the other end of the transmission line is

a) 0

b) Resistive

c) Inductive

d) Capacitive

View Answer

Explanation: The input impedance is given by Z

_{IN}= j Z

_{0}tan 2pi l/wavelength. For short circuited line, Z

_{L}= 0. On substituting the given values we get the input impedance as j54.49 ohm. Since it’s a short circuit, there is no real component, hence no resistance. It will have only an imaginary unit “j” which indicates an inductive reactance. Hence, the short circuit introduces an inductive behavior at the input of the transmission line.

**Sanfoundry Global Education & Learning Series – Electromagnetic Theory.**

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