Basic Electrical Engineering Core Concepts

Posted on May 1, 2025 in Electromechanical Engineering

Basic Electrical and Electronics Engineering

Short Questions

1. Define Voltage, Current, and Power

i) Voltage:

  • Definition: Voltage is the electrical potential difference between two points in a circuit. It represents the energy required to move a unit charge between those points.
  • Unit: Volt (V)

ii) Current:

  • Definition: Electric current is the rate of flow of electric charge through a conductor or circuit. It measures how many charges pass through a point in a given time.
  • Unit: Ampere (A)

iii) Power:

  • Definition: Electrical power is the rate at which electrical energy is transferred or converted. It is the product of voltage and current.
  • Unit: Watt (W)
  • Formula: P = V × I
    Where P is power, V is voltage, and I is current.

2. Define Cycle and Time Period

i) Cycle:

  • Definition: A cycle is one complete oscillation or repetition of a periodic wave or signal. It refers to a full sequence of events that repeats itself at regular intervals in a periodic phenomenon.
  • Example: In an alternating current (AC) wave, one cycle includes the complete positive and negative alternations of the wave.

ii) Time Period:

  • Definition: The time period is the time it takes to complete one full cycle of a periodic wave or oscillation. It is the duration of one complete repetition.
  • Unit: Second (s)
  • Formula: T = 1 / f
    Where T is the time period and f is the frequency (the number of cycles per second).

3. Define RMS Value for an AC Signal

RMS (Root Mean Square) Value for an AC Signal:

The RMS value of an alternating current (AC) signal is a measure of the effective value of the fluctuating signal. It is equivalent to the value of a DC current that would produce the same amount of heat or power in a resistor as the AC signal.

4. Define Amplitude, Frequency, and Wavelength

i) Amplitude:

  • Definition: Amplitude is the maximum displacement or distance a wave reaches from its equilibrium or resting position. It represents the peak value of the wave.
  • Example: In a sound wave, the amplitude is related to the loudness of the sound, and in a light wave, it relates to the brightness.

ii) Frequency:

  • Definition: Frequency is the number of complete cycles (oscillations) a wave undergoes per unit time. It is measured in Hertz (Hz).
  • Formula: f = 1 / T
    Where f is frequency and T is the time period of the wave.

iii) Wavelength:

  • Definition: Wavelength is the distance between two consecutive points in phase on a wave, such as two crests or two troughs. It is typically denoted by λ.
  • Unit: Meter (m)
  • Formula: λ = v / f
    Where λ is the wavelength, v is the wave velocity, and f is the frequency of the wave.

5. Define Node, Branch, and Loop

i) Node:

  • Definition: A node is a point in an electrical circuit where two or more components are connected. It is a junction where current can split or combine.
  • Example: In a simple resistor network, the points where resistors connect are nodes.

ii) Branch:

  • Definition: A branch is a single element or a combination of elements (like resistors, capacitors, or inductors) that are connected between two nodes in an electrical circuit.
  • Example: A wire connecting two resistors is a branch.

iii) Loop:

  • Definition: A loop is any closed path in an electrical circuit where current can flow, and it is formed by connecting a sequence of branches without any junctions.
  • Example: In a simple circuit, a path from the power supply, through a resistor, and back to the power supply forms a loop.

6. Explain Factors Affecting Resistance Value

The value of resistance is affected by:

  • Material: Different materials have different resistivity (e.g., copper has low resistivity, rubber has high).
  • Length: Resistance increases with the length of the conductor.
  • Cross-sectional Area: Resistance decreases with a larger cross-sectional area.
  • Temperature: Resistance increases with temperature for most materials.
  • Physical State: Resistance can vary depending on whether the material is solid, liquid, or gas.

7. What is Phase and Phase Difference?

Phase:

  • Definition: Phase refers to the position of a point within one complete cycle of a periodic wave. It describes the state of the wave at any given moment in time, usually measured in degrees or radians.
  • Example: In a sine wave, the phase can indicate whether the wave is at its maximum, minimum, or zero crossing.

Phase Difference:

  • Definition: Phase difference is the difference in the phase of two waves at a given point in time. It shows how much one wave is leading or lagging behind another.
  • Unit: Measured in degrees or radians.
  • Formula: If two waves have phases φ1 and φ2, then the phase difference is Δφ = φ1 – φ2.

8. Draw Different Types of Signals

[Note: Diagrams are required to answer this question fully. Common signal types include Sine wave, Square wave, Triangle wave, Sawtooth wave, DC signal.]

9. Define Power Factor

Power factor is the ratio of real power (used to do work) to apparent power (total power supplied). It indicates how efficiently electrical power is being used. A power factor of 1 (or 100%) means all the energy supplied is used effectively.

10. Can DC Power be Applied to Transformers?

No, transformers only work with alternating current (AC). DC power cannot be applied to transformers because they rely on a changing magnetic field to induce voltage, which only occurs with AC.

11. What are Step-Up and Step-Down Transformers?

  • Step-Up Transformers: Increase voltage while decreasing current. They have more turns of wire on the secondary coil than the primary coil.
  • Step-Down Transformers: Decrease voltage while increasing current. They have fewer turns of wire on the secondary coil than the primary coil.

12. Calculate Transformer Turns Ratio

What is the turns ratio of a transformer with 500 turns in the primary winding and 1000 turns in the secondary winding?

The turns ratio of a transformer is the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.

For this transformer, the turns ratio is:

Turns Ratio = Secondary Turns / Primary Turns = 1000 / 500 = 2

So, the turns ratio is 2:1.

13. Write Classification of Transformers

Transformers can be classified based on various factors. Here are the main classifications:

  • Based on Voltage Level:
    • Step-up Transformer: Increases voltage.
    • Step-down Transformer: Decreases voltage.
  • Based on Construction:
    • Core Type Transformer: Windings are wound around a central core.
    • Shell Type Transformer: Windings are placed around the core.
  • Based on Application:
    • Power Transformer: Used in power generation and transmission.
    • Distribution Transformer: Used for distributing electricity to consumers.
    • Isolation Transformer: Used for isolating circuits to prevent electrical shock.
    • Autotransformer: Has a single winding that acts as both primary and secondary.
  • Based on Phases:
    • Single-phase Transformer: Used in single-phase electrical systems.
    • Three-phase Transformer: Used in three-phase electrical systems.
  • Based on Cooling Method:
    • Oil-immersed Transformer: Cooled by oil.
    • Air-cooled Transformer: Cooled by air.

Each classification helps in selecting transformers based on specific needs and applications.

Medium Questions

14. Define and Explain Ohm’s Law

Ohm’s Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance of the conductor. Mathematically, it is expressed as:

V = I × R

Where:

  • V = Voltage (in volts)
  • I = Current (in amperes)
  • R = Resistance (in ohms)

Explanation:

  • Direct Proportionality: If the voltage across a conductor increases, the current increases, assuming the resistance is constant.
  • Inverse Proportionality: If the resistance increases, the current decreases, assuming the voltage is constant.

Applications of Ohm’s Law:

  • Electrical Circuit Design: Helps in determining the correct values for components like resistors to control the current and voltage.
  • Power Calculation: Can be used to calculate the power consumed in electrical devices using the formula P = V × I.
  • Troubleshooting: Helps identify faults in circuits by relating voltage, current, and resistance values.
  • Electronic Devices: Used in the design of transistors, amplifiers, and other electronic components.

Limitations of Ohm’s Law:

  • Non-Linear Materials: Ohm’s Law does not apply to materials where the relationship between current and voltage is non-linear, such as semiconductors, diodes, and transistors.
  • Temperature Variations: In some materials, resistance changes with temperature, which can affect the current and voltage relationship.
  • High Frequencies: At high frequencies, factors like inductance and capacitance affect the current, making Ohm’s Law less accurate.

15. Explain Types of Circuits

Types of Circuits:

Open Circuit:

  • Definition: An open circuit occurs when the current path is broken or disconnected, preventing current from flowing.
  • Cause: This can happen if there is a switch turned off, a broken wire, or a disconnected component.
  • Example: A light switch turned off, or a broken wire in a circuit.
  • Effect: No current flows, and devices in the circuit do not work.

Closed Circuit:

  • Definition: A closed circuit is a complete path where current can flow freely from the power source through the components and back.
  • Cause: This happens when all connections are intact, and the switch is in the “on” position.
  • Example: A light bulb turned on in a circuit where the switch is closed, allowing current to flow.
  • Effect: Current flows, and the devices in the circuit operate normally.

Short Circuit:

  • Definition: A short circuit occurs when the current bypasses the normal path, typically due to a fault like a connection between two points with different potentials (e.g., a wire touching another wire or metal part).
  • Cause: This usually happens when insulation is damaged, or wires are improperly connected.
  • Example: A power wire touching a ground wire, creating a direct connection.
  • Effect: A short circuit causes a large current to flow, potentially damaging the circuit components and causing overheating, fires, or tripping circuit breakers.

16. Explain Active, Reactive, and Apparent Power

Active Power (Real Power):

  • Definition: Active power, also known as real power, is the actual power consumed by a load to perform work, such as lighting a bulb or running a motor.
  • Unit: Measured in watts (W).
  • Formula: P = V × I × cos(θ)
    Where θ is the phase angle between voltage and current.
  • Explanation: It is the portion of power that is converted into useful work. For purely resistive loads (e.g., heaters, light bulbs), active power is equal to the total power supplied.

Reactive Power:

  • Definition: Reactive power does not perform any useful work but is necessary to sustain the magnetic fields in inductive and capacitive components like motors and transformers.
  • Unit: Measured in volt-amperes reactive (VAR).
  • Formula: Q = V × I × sin(θ)
  • Explanation: It alternates between the source and the load, contributing to the total power but not actually being consumed. It’s present in circuits with inductive or capacitive elements (e.g., motors, inductors).

Apparent Power:

  • Definition: Apparent power is the total power supplied to the circuit, which combines both active and reactive power.
  • Unit: Measured in volt-amperes (VA).
  • Formula: S = V × I
  • Explanation: Apparent power represents the overall power in the circuit and is the combination of real power (active power) and reactive power. It is the total power that flows from the source to the load.

17. Compare Series and Parallel Connections

Series Connection:

  • Current: Same current flows through all components.
  • Voltage: Voltage is divided across the components.
  • Total Resistance: Rtotal = R1 + R2 + … + Rn.
  • Advantages: Simple to design, easy to connect.
  • Disadvantages: If one component fails, the whole circuit stops working.

Parallel Connection:

  • Current: Current is divided among the components.
  • Voltage: Voltage is the same across all components.
  • Total Resistance: 1 / Rtotal = 1 / R1 + 1 / R2 + … + 1 / Rn
  • Advantages: If one component fails, others continue working; easier to add components.
  • Disadvantages: More complex wiring, higher current demand.

18. Describe Working Principle of Transformer

Working Principle of a Transformer:

A transformer works on the principle of electromagnetic induction, specifically mutual induction. It consists of two coils, the primary coil and the secondary coil, wound around a magnetic core.

  • AC Supply to Primary Coil: An alternating current (AC) is supplied to the primary coil, which creates a changing magnetic field around it.
  • Magnetic Field: The changing magnetic field induces a varying magnetic flux in the core, which passes through the secondary coil.
  • Induced Voltage: According to Faraday’s Law of Induction, the changing magnetic flux induces a voltage in the secondary coil.
  • Voltage Transformation: The voltage induced in the secondary coil depends on the turns ratio between the primary and secondary coils. The relationship is given by:

Vp / Vs = Np / Ns

Where:

  • Vp = Voltage in primary coil
  • Vs = Voltage in secondary coil
  • Np = Number of turns in primary coil
  • Ns = Number of turns in secondary coil

If Np > Ns, the transformer is a step-down transformer (reduces voltage). If Np < Ns, it’s a step-up transformer (increases voltage).

Long Questions

19. Define and Explain Kirchhoff’s Laws

Define and Explain Kirchhoff’s Laws for solving Electrical Circuits.

1. Kirchhoff’s Current Law (KCL)

In any network, the algebraic sum of electric currents at any node is zero.

In other words, in a network, the sum of current flowing away from a node is equal to the sum of current flowing towards the node.

For this, the currents flowing towards a node are treated as positive and currents flowing away from the node are treated as negative.

[Note: A diagram showing node H with currents I1, I2, I3, I4, I5, I6 is referenced but missing.]

In a diagram, one node H of a network is shown. In this, I1, I4 and I5 are flowing towards the node, so these are assigned a positive sign, whereas currents I2, I3 and I6 are flowing away from the node, so these are assigned a negative sign.

&therefore; I1 − I2 − I3 + I4 + I5 + I6 = 0

&therefore; I1 + I4 + I5 = I2 + I3 + I6

Sum of currents flowing toward node = Sum of currents flowing away from the node.

2. Kirchhoff’s Voltage Law (KVL)

The algebraic sum of voltages in a closed loop is zero.

ΣV = 0

This may be voltage drop or voltage rise.

For example, if current flows through an element from higher potential to lower potential, the voltage is a drop. If current is assumed to flow from the positive terminal of a battery to the negative terminal of a battery, it is a voltage drop.

But if the current is assumed to flow from the negative terminal of the battery to the positive terminal, it is a voltage rise.

Voltage drop is taken as negative and voltage rise is taken as positive.

In any closed loop of a network, Sum of voltage drops = Sum of voltage rises.

20. Explain Active and Passive Components

Explain Active and Passive components with Examples.

Active Component:

An active component is one which is capable of delivering power to some external device.

E.g., voltage source or current source.

Voltage Source:

A voltage source may be a DC or an AC source.

A DC voltage source is a battery, and an AC voltage source is obtained from an AC supply.

Current Source:

  • A current source may be a DC current source or an AC current source.

Passive Component:

A passive component cannot deliver power or cannot process the electrical signal. Examples include resistors, inductors, and capacitors.

1.) Resistor:

The property of a material to oppose the flow of electric current through it is known as resistance.

Electric current flows due to the flow of electrons.

Resistance of a conductor is:

R = ρ * l / a

ρ = Specific resistance
l = Length of conductor
a = Area of cross section of conductor

Its unit is ohm and it is denoted by Ω.

When V volts are applied across a conductor having resistance of R ohm and current I ampere flows, I = V / R.

Power is dissipated in it due to current flow.

P = V * I = V * V / R = V2 / R = I2 * R

2) Capacitor:

When an insulating material is placed between two conducting plates, a capacitor is formed.

The ability of a capacitor to store electric charge is called capacitance, and its symbol is C. Its unit is farad (F).

C = Q / V

3) Inductor:

  • An inductor is a coil of wire which may have a core of air, iron, or ferrite (a brittle material made from iron). Its electrical property is called inductance, and the unit for this is the Henry (H).

L = N * Φ / I

21. Calculate Avg. and RMS Value for AC Signal

Calculate Average Value and RMS Value for AC sinusoidal signal.

1. Average Value (Vavg)

The average value over one complete cycle (0 to 2π) of a pure sinusoidal waveform is zero because the positive and negative halves cancel each other out.

However, for practical applications, we often calculate the average over a half-cycle (from 0 to π):

Vavg (half-cycle) = (2 / π) * Vpeak &approx; 0.637 * Vpeak

2. RMS Value (Vrms)

The RMS value of a sinusoidal waveform is calculated as the peak value divided by the square root of 2.

Vrms = Vpeak / √2 &approx; 0.707 * Vpeak

Similarly, for current:

Irms = Ipeak / √2 &approx; 0.707 * Ipeak

22. Derive Star-Delta Transformation

Derive expression for Star-Delta transformation.

Star to Delta Transformation

Let the resistances in the star network be:

  • R1 between nodes A and the common node O
  • R2 between nodes B and the common node O
  • R3 between nodes C and the common node O

Let the resistances in the equivalent delta network be:

  • RAB between nodes A and B
  • RBC between nodes B and C
  • RCA between nodes C and A

Condition for Equivalence

The resistance between any two terminals in the star network must equal the resistance between the same terminals in the delta network.

Between terminals A and B:

RAB,star = R1 + R2

RAB,delta = RAB || (RBC + RCA) = (RAB * (RBC + RCA)) / (RAB + RBC + RCA)

Between terminals B and C:

RBC,star = R2 + R3

RBC,delta = RBC || (RAB + RCA) = (RBC * (RAB + RCA)) / (RAB + RBC + RCA)

Between terminals C and A:

RCA,star = R3 + R1

RCA,delta = RCA || (RAB + RBC) = (RCA * (RAB + RBC)) / (RAB + RBC + RCA)

Derivation for Delta Resistances

By equating the terminal resistances and solving the system of equations, the relationships for the delta resistances in terms of star resistances are derived:

RAB = (R1R2 + R2R3 + R3R1) / R3

RBC = (R1R2 + R2R3 + R3R1) / R1

RCA = (R1R2 + R2R3 + R3R1) / R2

23. Derive Delta-Star Transformation

Derive expression for Delta-Star transformation.

Delta to Star Transformation

Let the resistances in the delta network be:

  • RAB between nodes A and B
  • RBC between nodes B and C
  • RCA between nodes C and A

Let the resistances in the equivalent star network be:

  • R1 between node A and the common node O
  • R2 between node B and the common node O
  • R3 between node C and the common node O

Condition for Equivalence

The resistance between any two terminals in the delta network must equal the resistance between the same terminals in the star network.

Between terminals A and B:

RAB,star = R1 + R2

RAB,delta = RAB || (RBC + RCA) = (RAB * (RBC + RCA)) / (RAB + RBC + RCA)

Between terminals B and C:

RBC,star = R2 + R3

RBC,delta = RBC || (RAB + RCA) = (RBC * (RAB + RCA)) / (RAB + RBC + RCA)

Between terminals C and A:

RCA,star = R3 + R1

RCA,delta = RCA || (RAB + RBC) = (RCA * (RAB + RBC)) / (RAB + RBC + RCA)

Derivation for Star Resistances

By solving these equations, we get the star resistances as:

R1 = (RAB * RCA) / (RAB + RBC + RCA)

R2 = (RAB * RBC) / (RAB + RBC + RCA)

R3 = (RBC * RCA) / (RAB + RBC + RCA)

24. Explain Construction of Transformer

Explain construction of transformer.

[Note: A detailed explanation of transformer construction, including core types (core and shell), windings (primary and secondary), insulation, tank, cooling system, etc., is expected here.]

25. Distinguish Between Core and Shell Type Transformer

Distinguish between core and shell type transformer.

[Note: A comparison highlighting differences in core shape, winding placement, magnetic path, natural cooling efficiency, and typical applications is expected here.]

26. Derive EMF Equation of Transformer

Derive emf equation of transformer.

[Note: The derivation involves Faraday’s Law of Induction, considering the sinusoidal flux and the number of turns in the primary and secondary windings, leading to E = 4.44 f N Φm.]

27. Use Nodal Analysis to Determine Current

Use Nodal analysis to determine current flowing through 10 Ohm resistor.

[Note: A circuit diagram is required to solve this problem.]

28. Determine Rab Using Star-Delta Transformation

Determine Rab using star-delta transformation.

[Note: A circuit diagram is required to solve this problem.]

29. Determine Rab Using Star-Delta Transformation

Determine Rab using star-delta transformation.

[Note: A circuit diagram is required to solve this problem.]

30. Determine Current Using Nodal Analysis

Determine current flowing through 5-Ohm resistor by the use of nodal analysis.

[Note: A circuit diagram is required to solve this problem.]

31. Series Resistors Calculation

Resistors of 2 Ohm, 20 Ohm, 70 Ohm, are connected in Series across 200-Volt supply. Find the equivalent resistance and the current through each resistance connected in the circuit.

[Note: Calculation steps are expected here.]

32. Parallel Capacitors Calculation

Capacitors of 1 μF, 2 μF, 4 μF, and 6 μF are connected in parallel across 100 volt d.c supply. Find the equivalent capacitance and the charge on each capacitor.

[Note: Calculation steps are expected here.]

33. Series Capacitors Voltage Calculation

Capacitors of 10 μF, 20 μF and 30 μF are connected in series and supply of 200 V d.c is given. Find the voltage across each capacitor.

[Note: Calculation steps are expected here.]

34. Determine R and Current

Determine R from above circuit & find value of current.

[Note: A circuit diagram is required to solve this problem.]

35. Use Nodal Analysis to Determine Current

Use Nodal analysis to determine current flowing through 5 Ohm resistor.

[Note: A circuit diagram is required to solve this problem.]

36. Determine Req and Source Current

Determine Req & Current supplied by source.

[Note: A circuit diagram is required to solve this problem.]

37. Transformer Voltage and Current Calculation

A transformer has 250 turns of the primary winding and 20 turns for secondary winding. Determine the secondary voltage if the secondary circuit is open and primary voltage is 100V. Determine current in the primary and secondary winding, given that the secondary winding is connected to resistance load 15ohm.

[Note: Calculation steps are expected here.]