Electromagnetics Flashcards
Which of the following scenarios can be attributed to electromagnetic interference?
A. Malfunctioning heart defibrillator in a hospital
B. Static charge accumulation in electronic devices
C. Jammed GPS systems
D. All of the above
Correct Option: D
Solution:
Defibrillators, GPS systems, and electronic devices contain components that are susceptible to electromagnetic interference.
The magnetic saturation curve limits the voltage at which a generator or motor can operate. Which of the following statements regarding saturation curves is not true?
A.
As field current increases, the hysteresis effect limits the increase in the flux produced.
B. Poles that allow the production of more flux permit higher operating voltages.
C. More flux at a constant field voltage can be produced by increasing the number of poles.
D. Saturation does not depend upon the type of steel used in the poles.
Correct Option: D
Solution:
The magnetic permeabilities of steel vary greatly.
How can electromagnetic compatibility be achieved?
A. By shielding and increasing the distance between the emitter and the receptor.
B. By adding coupling paths between the emitter and the receptor.
C. By increasing the system bandwidth.
D. None of the above.
Correct Option: A
Solution:
Adding coupling paths between the emitter and the receptor increases electromagnetic interference.
Similarly, increasing system bandwidth can result in higher susceptibility to electromagnetic interference.
Shielding and increasing distance between emitter and receptor help improve electromagnetic compatibility.
A coaxial cable has a surge impedance of 50 ohm at 100 khz frequency. The inductance of the cable is 0.08 µH/ft. Assuming the cable acts as a lossless transmission line, the capacitance of the cable is:
A. 32 pf/ft
B. 0.032 pf/ft
C. 0.32 pf/ft
D. 3.2 pf/ft
Correct Option: A
Solution:
Refer to “Lossless Transmission Lines” in the NCEES FE Reference Handbook>Electrical and Computer Engineering> Difference Equations.
Characteristic impedance is also known as surge impedance and is given by Z0= sqrt(L/C)
A travelling wave on a transmission line has a frequency of 10 Ghz and is travelling at the speed of light in a vacuum. The distance travelled by the wave in one period is:
A. 0.2 m
B. 30 m
C. 0.03 m
D. 0.15 m
Correct Option: C
Solution:
Refer to “Lossless Transmission Lines” in the NCEES FE Reference Handbook>Electrical and Computer Engineering> Difference Equations..
lambda = U/ f
The wavelength of the travelling wave is defined as the distance the signal will travel in one period.
From the NCEES FE Reference Handbook>Units and Conversion Factors>Fundamental Constants
The speed of light in a vacuum = 3 x 10^8 m/s
Hence wavelength = (3x 10^8 m/s) 10 e Ghz = 0.03 m
References:
NCEES FE Reference Handbook>Units and Conversion Factors>Fundamental Constants
NCEES FE Reference Handbook>Electrical and Computer Engineering> Difference Equations.
How does the voltage across the secondary winding of a transformer change if the number of turns in the secondary winding is quadrupled keeping the same turns in the primary?
A. Secondary voltage remains the same.
B. Primary voltage is doubled.
C. Secondary voltage is quadrupled
D. None of the above.
Correct Option: C
Solution:
Refer to “Induced voltage” in the NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics.
Induced voltage is directly proportional to the number of turns.
A step-down transformer core has magnetic flux density of 1.5 T. The area of cross section is 0.01 m2. What is the flux in the core?
A. 15 weber
B. 0.015 weber
C. 1.5 weber
D. 105 weber
Correct Option: B
Solution:
Refer to “Magnetic fields” in the NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics..
Hence, Ø = B × A
Therefore, flux = 1.5 × 0.01 = 0.015 weber
References:
NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics.
A current-carrying conductor is placed in the magnetic field, and it experiences a force of 200 N. Calculate the magnetic flux density of the field perpendicular to the conductor if the length of the conductor is 2 m and current in the conductor is 300 amperes.
A. 0.3 T
B. 60 T
C. 0.5 T
D. 40 T
Correct Option: A
Solution:
Refer to “Magnetic fields” in the NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics..
F = B X IL
Hence B = 200/ (300X2)
References:
NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatic
Two parallel wires carry current in opposite directions. Which statement is true?
A. The wires are attracted.
B. The wires are repelled.
C. The wires exert no net forces on each other.
D. None of the above
Correct Option: B
Solution:
Refer to the NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics Each wire will create a magnetic field. See the figure below.
For each wire ILx B
is pointing away from the other wire. The wires will repel. Using the righthand rule, we can derive that the magnetic fields are opposing and, therefore, will repel.
References:
NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics
Which of the following is not true?
A. Transformers do not work at DC because only time-varying voltages produce a magnetic field.
B. The divergence of a time-varying magnetic field is always zero.
C. A capacitor may produce a magnetic field around it with zero conduction current through it.
D. The electric field in a perfect conductor is zero.
Correct Option: A
Solution:
Refer to the NCEES FE Reference Handbook>Electrical and Computer Engineering>Electrostatics.
A: DC voltages do produce a magnetic field. However, this magnetic field does not vary with time. By Faraday’s law, this static magnetic field does not induce a voltage.
B: This is one of Maxwell’s equations, sometimes called Gauss’s law for magnetism, that indicates the nonexistence of magnetic monopoles.
C: A capacitor can have a changing electric field that generates a magnetic field. This is indicated in Ampère’s law with Maxwell’s addition, one of Maxwell’s equations. The term with the change in electric field (D field) is called displacement current.
D: The electric field would move any charges immediately resulting in no net electric field.
A transmission line with 75Ω characteristic impedance is terminated with a 300Ω load. If a DC voltage of 5 V is applied at the source, what is the voltage at the load after a single reflection occurs? Assume a lossless transmission line and zero source impedance.
A. 3 V
B. 4 V
C. 5 V
D. 8 V
Correct Option: D
Solution:
The load impedance does not match the characteristic impedance of the transmission line, so reflections will occur. At the load we will have both the incident (forward traveling) and reflected voltage. We can first solve for the reflection coefficient, which will tell us the ratio of these two.
Refer to the NCEES FE Reference Handbook>Electrical and Computer Engineering> Difference Equations
A lossless transmission line is operating at 5.8 GHz. It has a 50Ω characteristic impedance and is terminated with a load of unknown impedance. Which transmission line length will give an input impedance most nearly equal to the load impedance? Assume the velocity of propagation is 0.77 times the speed of light.
A. 0.5 cm
B. 3 cm
C. 6 cm
D. 7 cm
Correct Option: C
Solution:
Refer to the NCEES FE Reference Handbook>Electrical and Computer Engineering> Difference Equations. Input impedance is the following:
