Electrical Circuits Lab Viva Questions and Answers - ObjectiveBooks

Electrical Circuits Lab Viva Questions and Answers

Question No. 01: What id difference between power and energy?
Answer: Energy is defined as the capacity of a physical system to perform work. In the context of electric circuits, energy (w) is related to power by the following relationship p = vi =dw/dt
So the difference is that power is the rate of change of energy.

Question No. 02: What do you mean by Phasors?
Answer: Addition of two out-of-phase sinusoidal signals is rather complicated in the time domain. An example could be the sum of voltages across a series connection of a resistor and an inductor. Phasors simplify this analysis by considering only the amplitude and phase components of the sine wave. Moreover, they can be solved using complex algebra or treated vectorially using a vector diagram.

Question No. 03: What is Corkscrew Rule?
Answer: As current flows along a wire, the magnetic field rotates in the direction of a corkscrew.

Question No. 04: Differentiate between Kirchhoff’s First law and Kirchhoff’s Second law?
Kirchhoff’s First law: 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 “IR” drop in the circuit

Question No. 05: How to calculate Energy stored in an inductance?
Answer: Energy stored =½ L I2 Joules, where L is in henries and I is in amps

Question No. 06: What is a capacitor and what does it do?
Answer: Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates.

Question No. 07: What is Current Divider Rule (CDR)?
Answer: Current divider rule provides a useful relationship for determining the current through individual circuit elements that are connected in parallel.

Question No. 08: What is Voltage Divider Rule?
Answer: Voltage divider rule provides a useful formula to determine the voltage across any resistor when two or more resistors are connected in series with a voltage source.

Question No. 09: What is Current?
Answer: Current can be defined as the motion of charge through a conducting material. The unit of current is Ampere whilst charge is measured in Coulombs.

Question No. 10: Could you measure Voltage in series?
Answer: No, Voltage is always measured across (in parallel with) a circuit element.

Question No. 11: What is Kirchoff’s Current Law (KCL)?
Answer: The algebraic sum of all the currents entering or leaving a node in an electric circuit is equal to zero. In other words, the sum of currents entering is equal to the sum of currents leaving the node in an electric circuit.

Question No. 12: Define Super node?
Answer: A super node exists when an ideal voltage source appears between any two nodes of an electric circuit. The usual way to solve this is to write KCL equations for both nodes and simply add them together into one equation ignoring the voltage source in question. However, this would mean one less equation than the number of variables (node voltages) present in the circuit. A constraint equation can be easily specified given by the magnitude of the ideal voltage source present between the nodes and the respective node voltages. The following example will help clarify this scenario.

Question No. 13: What does Ampere turns shows?
Answer: mmf A coil of N turns carrying a current I amps gives an mmf of NI ampere turns In a vacuum, a magnetizing force of 1 ampere turn / metre produces a magnetic field of 1.26 × 10-6 tesla.

Question No. 14: What is the difference between an inductor and a capacitor?
An inductor is a passive electrical device employed in electrical circuits for its property of inductance. An inductor can take many forms.
capacitor is an electrical/electronic device that can store energy in the electric field between a pair of conductors (called "plates").

Question No. 15: How does a capacitor store an electrical charge?
Answer: Electrical Energy in Capacitor stores in Potential Charge form. The energy stored in a capacitor is almost entirely in the electric field produced between the plates. It takes energy from a battery or some other power source to move electrons to one of the plates and away from the other.

Question No. 16: Define Ampere?
Answer: The quantity of total charge that passes through an arbitrary cross-section of a conducting material per unit second is defined as an Ampere.
I =Q/t
or, Q = It
Where, Q is the symbol of charge measured in Coulombs (C), I is the current in amperes (A) and t is the time in seconds (s).

Question No. 17: Define Ohm’s Law for A.C (Alternating Current)?
Answer: Everything else would remain same only the resistance will be replaced with Impedance, which is defined as the opposition to the flow of A.C.

Question No. 18: What do you mean by dependent and independent voltage sources?
Answer: In general, there are two main types of DC sources
  1. Independent (Voltage and Current) Sources
  2. Dependent (Voltage and Current) Sources
An independent source produces its own voltage and current through some chemical reaction and does not depend on any other voltage or current variable in the circuit. The output of a dependent source, on the other hand, is subject to a certain parameter (voltage or current) change in a circuit element. Herein, the discussion shall be confined to independent sources only.

Question No. 19: Differentiate between Magnetic field and Magnetic Flux?
Answer: Magnetic field is B = μ H where B is in tesla and H = 1.26 × 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 φ = 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 =1.26NItA × 10-6/ L weber.

Question No. 20: Differentiate between Induced emf and Self inductance?
Answer: Induced emf, E = - N dφ/dt where E is in volts, N is number of turns and dφ/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 × 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).

Question No. 21: What is the function of the capacitor?
Answer: A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e. insulator).

Question No. 22: Could you measure current in parallel?
Answer: No, Current is always measured through (in series with) a circuit element.

Question No. 23: State and define Ohm’s Law?
Answer: It is the most fundamental law used in circuit analysis. It provides a simple formula describing the voltage-current relationship in a conducting material.
Statement: The voltage or potential difference across a conducting material is directly proportional to the current flowing through the material.
V ∝ I
V = RI or I =V/R
or R =V/I
Where, the constant of proportionality R is called the resistance or electrical resistance, measured in ohms (Ω).

Question No. 24: Briefly explain the purpose of Inductor in an electric circuit?
Answer: An inductor is a piece of conducting wire generally wrapped around a core of a ferromagnetic material. Like capacitors, they are employed as filters as well but the most well known application is their use in AC transformers or power supplies that converts AC voltage levels.

Question No. 25: What is Superposition Theorem?
Answer: Superposition theorem is extremely useful for analyzing electric circuits that contains two or more active sources. In such cases, the theorem considers each source separately to evaluate the current through or voltage across a component. The resultant is given by the algebraic sum of all currents or voltages caused by each source acting independently. Superposition theorem can be formally stated as follows:
“The current through or voltage across any element in a linear circuit containing several sources is the algebraic sum of the currents or voltages due to each source acting alone, all other sources being removed at that time.”

Question No. 26: What is Force on a conductor in a magnetic field?
Answer: F = B I L Newtons, where B in tesla, I in amps and L in meters.

Question No. 27: What is an inductor and what does it do?
Answer: An inductor is a passive electronic component which is capable of storing electrical energy in the form of magnetic energy. Basically, it uses a conductor that is wound into a coil, and when electricity flows into the coil from the left to the right, this will generate a magnetic field in the clockwise direction.

Question No. 28: At what base you will tell the direction of rotation of DC Motors and Generators?
Answer: Motors obey the left hand rule and generators the right hand rule.

Question No. 29: What is the difference between Voltages or Potential Difference?
Answer: Voltage or potential difference between two points in an electric circuit is 1V if 1J (Joule) of energy is expended in transferring 1 C of charge between those points.
It is generally represented by the symbol V and measured in volts (V). Note that the symbol and the unit of voltage are both denoted by the same letter; However, it rarely causes any confusion.
The symbol V also signifies a constant voltage (DC) whereas a time-varying (AC) voltage is represented by the symbol v or v (t)

Question No. 30: What does the term “Voltage Regulation” means?
Answer: Voltage regulation (VR) is an important measure of the quality of a source. It is used to measure the variation in terminal voltage between no load (IL =0, open circuit) and full load (IL = IFL)

Question No. 31: State and define Norton’s Theorem?
Answer: Thevenin’s equivalent circuit is a practical voltage source. In contrast, Norton’s equivalent circuit is a practical current source. This can be formally stated as:
“Any two-terminal, linear circuit, of resistors and sources, can be replaced by a single current source in parallel with a resistor.”
To determine Norton’s equivalent circuit, Norton current, IN, and Norton resistance, RN , are required. The following steps outline the procedure required:
  1. Remove the load resistance, RL.
  2. IN is the SC current through the load terminals and
  3. RN is the resistance across the load terminals with all sources replaced by their internal resistances. Clearly RN = RTH.

Question No. 32: What does the term Power Factor shows?
Answer: The term cos φ is called the power factor and is an important parameter in determining the amount of actual power dissipated in the load. In practice, power factor is used to specify the characteristics of a load.
For a purely resistive load φ = 0 Degree, hence Unity Power Factor
For a capacitive type load I leads V, hence Leading power factor
For an inductive type load I lags V, hence Lagging power factor
Clearly, for a fixed amount of demanded power P, at a constant load voltage V, a higher power factor draws less amount of current and hence low I2R losses in the transmission lines. A purely reactive load where φ → 900 and cos φ → 0 will draw an excessively large amount of current and a power factor correction is required.

Question No. 33: What is the energy stored in the magnetic field of the inductor?
Answer: The formula for the energy stored in a magnetic field is E = ½ LI 2. The energy stored in a magnetic field is equal to the work needed to produce a current through the inductor. Energy is stored in a magnetic field.

Question No. 34: What is the function of Capacitor in Electrical Circuits?
Answer: A capacitor is a passive circuit element that has the capacity to store charge in an electric field. It is widely used in electric circuits in the form of a filter.

Question No. 35: What is DC Current source? Differentiate between ideal and non-ideal current sources
Answer: A current source, unlike the DC voltage source, is not a physical reality. However, it is useful in deriving equivalent circuit models of semiconductor devices such as a transistor. It can also be subdivided into ideal and non-ideal categories.

The Ideal Current Source By definition, an ideal current source that produces a current which is independent of the variations in load. In other words the current supplied by an ideal current source does not change with the load voltage.

Non-Ideal or Practical Current Source The current delivered by a practical current source falls off with an increase in load or load voltage.

Question No. 36: What does the term Super-mesh means?
Answer: A super-mesh exists when an ideal current source appears between two meshes of an electric circuit. In such a situation, like super-node, mesh equations are written for the meshes involved and added giving a single equation. Again, there would be one less equation than the number of variables (mesh currents) and hence a constraint equation is needed. This would be based on the magnitude of the ideal current source present between the two meshes and their mesh currents.

Question No. 37: How to calculate Energy stored in a capacitance?
Answer: Energy stored = ½ C V2 Joules, where C is in farads and V in volts

Question No. 38: How many Types of Circuit Loads are there in a Common Electrical Circuit?
Answer: A load generally refers to a component or a piece of equipment connected to the output of an electric circuit. In its fundamental form, the load is represented by any one or a combination of the following:
  1. Resistor (R)
  2. Inductor (L)
  3. Capacitor (C)
A load can either be of resistive, inductive or capacitive nature or a blend of them. For example, a light bulb is a purely resistive load where as a transformer is both inductive and resistive. A circuit load can also be referred to as a sink since it dissipates energy whereas the voltage or current supply can be termed as a source.

Question No. 39: Differentiate between ideal and non-ideal voltage sources?
Answer: The Ideal Voltage Source An ideal voltage source which has a terminal voltage which is independent of the variations in load. In other words, for an ideal voltage source, the supply current alters with changes in load but the terminal voltage, VL always remains constant. Non-Ideal or Practical Voltage Source For a practical source, the terminal voltage falls off with an increase in load current.

Question No. 40: What is the method to solve circuit using Source Transformation?
Answer: In an electric circuit, it is often convenient to have a voltage source rather than a current source (e.g. in mesh analysis) or vice versa. This is made possible using source transformations.
It should be noted that only practical voltage and current sources can be transformed. In other words, a Thevenin’s equivalent circuit is transformed into a Norton’s one or vice versa. The parameters used in the source transformation are given as follows.
Thevenin parameters: VTH, RTH =⇒ RN = RTH, IN = VTH/RTH
Norton parameters: IN, RN =⇒ RTH = RN, VTH = RN IN
Any load resistance, RL will have the same voltage across, and current through it when connected across the terminals of either source.

Question No. 41: Differentiate between Real and Apparent Power?
Answer: It is important to highlight that in AC circuits, the product of voltage and current yields the apparent power which is measured in volt-amperes or VA
KW which is also written ad Kilo-Watt is the real power that is actually converted to the useful work.
KVAR is also termed as Kilo-Volt Reactive this power is used for magnetic field excitation and flows back and forth between source and load.

Question No. 42: At what factor Resistance R of a conductor depends?
Answer: 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 coefficient 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

Question No. 43: Why Inductors are installed in electrical Circuits?
Answer: An inductor is a piece of conducting wire generally wrapped around a core of a ferromagnetic material. Like capacitors, they are employed as filters as well but the most well known application is their use in AC transformers or power supplies that converts AC voltage levels.

Question No. 44: What is Maximum Power Transfer Theorem?
Answer: As discussed in the section on Thevenin’s theorem, any DC network of sources and resistances can be replaced by a single voltage source in series with a resistance connected across the load:

The maximum power transfer theorem states that the power delivered to the load is maximum when the load resistance, RL is equal to the internal (source) resistance, Rs of the DC power supply.

In other words, it can be said that the load resistance must match the Thevenin’s resistance for maximum power transfer to take place i.e., (Rs = RTH) = RL
When this occurs, the voltage across the load resistance will be Vs/2 and the power delivered to the load is given by which clearly demonstrates maximum power delivered when Rs = RL. Under this condition, the maximum power will be:

Question No. 45: What does Bode Plots shows?
Answer: Bode plots are graphical ways 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.

Question No. 46: Elaborate Thevenin’s Theorem?
Answer: Thevenin’s theorem provides a useful tool when solving complex and large electric circuits by reducing them to a single voltage source in series with a resistor. It is particularly advantageous where a single resistor or load in a circuit is subject to change. Formally, the Thevenin’s theorem can be stated as:
“Any two-terminal linear electric circuit consisting of resistors and sources can be replaced by an equivalent circuit containing a single voltage source in series with a resistor connected across the load.”
The following steps outline the procedure to simplify an electric circuit using Thevenin’s theorem where VTH and RTH are the Thevenin’s voltage and Thevenin’s resistance respectively.
  1. Remove the load resistance RL.
  2. VTH is the open circuit (OC) voltage across the load terminals
  3. RTH is the resistance across the load terminals with all sources replaced by their internal resistances.

Question No. 47: Differentiate between Low Pass, High pass and Band Pass filter?
Answer: 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 through 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.

Question No. 48: Distinguish between self-induction and mutual induction.
  1. Self-Induction is the characteristic of the coil itself.
  2. When the main current in the coil decreases, the induced current opposes the decay of current in the coil.
  3. When the main current in the coil increases, the induced current opposes the growth of current in the coil.
Mutual induction
  1. Mutual induction is the characteristic of a pair of coils.
  2. When the main current in the coil decreases, induced current developed in the neighboring coil opposes the decay of current in the coil.
  3. When the main current in the coil increases, the induced current developed in the neighboring coil opposes the growth of current in the coil.