Basic Electrical Engineering Interview Questions 5

• What does Bode Plots shows?

Bode plots are a graphical way to display the behavior of a circuit over a wide range of frequencies. By plotting the amplitude and phase versus the logarithm of frequency, each unit of change on the ω axis is equal to a factor of 10 also called a decade of frequency. Also, there may be a wide distribution in the amplitude response over a specified range of frequencies. The usual way is to plot the amplitude in dB and phase in degrees or radians versus the logarithm of frequency.

• Differentiate between Low Pass, High pass and Band Pass filter?

Filters form a vital part in electrical networks especially where a particular frequency range is of prime concern. For instance, a radio station is broadcasting a transmission at a frequency of 100 MHz. This means that it is required to design a receiving filter which allows only 100 MHz frequency to pass hrough whilst other frequencies are filtered out. An ideal filter will attenuate all signals with frequencies less than and greater than 100 MHz thus providing the best channel sound quality without any distortion.
Low Pass Filter
A low pass Filter allows low frequencies to pass through the circuit whereas high frequencies are severely attenuated or blocked.
High Pass Filter
A high pass filter, as the name suggests, allows high frequencies to pass through the circuit whilst low frequencies are attenuated or blocked. The cut-off point or bandwidth concept is the same as in the low pass filter.
Band Pass Filter
A band pass filter permits a certain band of frequencies to pass through the network which is adjusted by the designer. It is simply an amalgamation of a low pass and a high pass filter.

• What is Corkscrew Rule?

As current flows along a wire, the magnetic field rotates in the direction of a corkscrew.

• What does Ampere turns shows?

mmf A coil of N turns carrying a current I amps gives an mmf of N I ampere turns In a vacuum, a magnetizing force of 1 ampere turn / metre produces a magnetic field of 1.26*10^-6 tesla.

• Differentiate between Magnetic field and Magnetic Flux?

Magnetic field is B = µ H where B is in tesla and H = 1.26 x 10^-6 times ampere turns/metre MMF in a solenoid of N turns and current I mmf = (4 m / 10) N I Gilberts.
Whereas Magnetic flux ø= B A where ø is in weber, B is in tesla and A is in square meters.
Magnetic flux in a uniform closed magnetic circuit, length L meters and cross section A square meters is
=1.26NItAx10^-6/Lweber.

• What is Force on a conductor in a magnetic field ?

F = B I L Newtons where B in tesla, I in amps and L in meters

• What is force on parallel conductors ?

F = [2 I2 / d] 10^-' Newtons/meter where I is in amps and d is in meters With currents in opposite directions, the force is pushing the conductors apart

• What do you mean by Pull of Electromagnet?

Pu11= B² 10⁷ / (8 ) Newtons per m² of magnet force where B is in tesla

•  How would you calculate Power loss in a resistor?

W = I² R = V² / R

• At what factor Resistance R of a conductor depends?

R = p L (1 + αT) / A ohms where p is resistivity in ohms per cm cube, L cm is the length, A in cm2 is the cross sectional area, α is temp co-efficient and T is the temperature in degrees Celsius.
Several sources give Copper p = 1.7 * 10^-6 ohms per cm cube and α = 0.004. At very low Temperatures, the resistance of some materials falls to zero

• Differentiate between Kirchhoff’s First law and Kirchhoff’s Second law?

Kirchhoff’s First law is the total current leaving a point on an electrical circuit = total current entering
Kirchhoff’s Second law The sum of the voltages round any circuit = net "I R" drop in the circuit

• Differentiate between Induced emf and Self inductance?

Induced emf , E = - N dø/dt where E is in volts, N is number of turns and dø/dt is in Wb/sec This equation is the foundation on which Electrical Engineering is based.
Whereas Self Inductance E = - L dI/dt where E is in volts, L is inductance in henneries and dI/dt is in amps/sec Self inductance of a coil wound on a ring of permeability is L = 1.26 N² μ A / S x 10^-6 Henneries where N is number of turns, A is cross sectional area in m² and S meters is the length of the magnetic circuit. Experimental results for a coil of length S meters, diameter d meters and radial thickness t meters with at core indicate L = 3 d² N² / (1.2 d + 3.5 S + 4 t) micro Henneries. (t = 0 for a single layer coil).

• How to calculate Energy stored in an inductance?

Energy stored =1/2 L I² Joules where L is in henries and I is in amps

• How to calculate Energy stored in a capacitance

= 1/2 C V² Joules where C is in farads and V in volts

• At what base you will tell the direction of rotation of DC Motors and Generators?

Motors obey the left hand rule and generators the right hand rule