1. What is Electrical Engineering?

Electrical Engineering is the branch of engineering that deals with the study and application of electricity, electronics, and electromagnetism. It involves designing, developing, testing, and supervising electrical equipment and systems.

Major domains include:

• Power generation, transmission, and distribution
• Electronics and microelectronics
• Control systems and automation
• Telecommunications and signal processing
• Instrumentation and measurements
• Computer hardware and embedded systems

2. State 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, provided the temperature remains constant.

Formula: V = I × R

Where:

• V = Voltage (Volts)
• I = Current (Amperes)
• R = Resistance (Ohms, Ω)

Derived forms: I = V/R and R = V/I

This fundamental law is the foundation for circuit analysis and is applicable to resistive circuits.

3. What are Kirchhoff's Laws?

Kirchhoff's Current Law (KCL): The algebraic sum of all currents entering and leaving a node (junction) in a circuit is zero. In other words, total current entering = total current leaving.

Formula: ΣI_in = ΣI_out or ΣI = 0

Kirchhoff's Voltage Law (KVL): The algebraic sum of all voltages around any closed loop in a circuit is zero. The sum of voltage rises equals the sum of voltage drops.

Formula: ΣV = 0 (around a closed loop)

These laws are fundamental for analyzing complex electrical circuits and are based on conservation of charge (KCL) and conservation of energy (KVL).

4. Difference between AC and DC

5. What is the difference between active and passive components?

Active Components: Can control current flow and are capable of power amplification. They require an external power source to operate and can introduce energy into the circuit.

Examples: Transistors, diodes, operational amplifiers, integrated circuits, SCRs

Passive Components: Cannot control current flow and cannot amplify power. They can only consume, store, or release energy. Do not require external power.

Examples: Resistors, capacitors, inductors, transformers

6. Explain series and parallel circuits

Series Circuit: Components connected end-to-end in a single path. Current is the same through all components; voltage divides.

• Same current: I_total = I₁ = I₂ = I₃
• Total voltage: V_total = V₁ + V₂ + V₃
• Total resistance: R_total = R₁ + R₂ + R₃

Parallel Circuit: Components connected across common voltage points. Voltage is the same across all components; current divides.

• Same voltage: V_total = V₁ = V₂ = V₃
• Total current: I_total = I₁ + I₂ + I₃
• Total resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃

7. What is power factor?

Power factor (PF) is the ratio of real power (active power) to apparent power in an AC circuit. It indicates how effectively electrical power is being converted into useful work.

Formula: Power Factor = Real Power (W) / Apparent Power (VA) = cos φ

Where φ is the phase angle between voltage and current.

Range: 0 to 1 (or 0% to 100%)

• Unity PF (1.0): All power is real power; ideal condition
• Leading PF: Capacitive load; current leads voltage
• Lagging PF: Inductive load; current lags voltage

Importance: Low power factor increases current for same power, causing higher losses and requiring larger equipment. Industries pay penalties for low PF.

8. What is resonance in electrical circuits?

Resonance occurs in AC circuits when the inductive reactance equals the capacitive reactance, causing them to cancel each other out. At resonance, the circuit behaves as purely resistive.

Series Resonance:

• Condition: X_L = X_C (where X_L = 2πfL and X_C = 1/(2πfC))
• Resonant frequency: f₀ = 1/(2π√LC)
• Impedance is minimum (equals R only)
• Current is maximum
• Used in: Radio tuning circuits, filters

Parallel Resonance:

• Same frequency formula
• Impedance is maximum
• Current is minimum
• Used in: RF oscillators, impedance matching

9. What is a transformer?

A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It changes AC voltage levels while keeping frequency constant.

Working principle: Based on Faraday's law of electromagnetic induction. When AC flows through the primary winding, it creates a changing magnetic flux in the core, which induces voltage in the secondary winding.

Voltage transformation: V_s/V_p = N_s/N_p

Current transformation: I_p/I_s = N_s/N_p

Where N is number of turns, V is voltage, I is current, p = primary, s = secondary

Types: Step-up (increases voltage), Step-down (decreases voltage), Isolation, Auto-transformer

10. Difference between DC motor and AC motor

11. What is back EMF in a motor?

Back EMF (Electromotive Force), also called counter EMF, is the voltage generated in a motor that opposes the applied voltage when the motor armature rotates in a magnetic field.

Why it occurs: When the motor runs, the conductors cut through the magnetic field, inducing a voltage (by Faraday's law) that opposes the applied voltage (by Lenz's law).

Formula: E_b = V - I_a R_a

Where E_b is back EMF, V is applied voltage, I_a is armature current, R_a is armature resistance

Importance:

• Limits the armature current (prevents excessive current)
• Increases with motor speed
• At standstill (starting), back EMF is zero → maximum current flows
• Self-regulating mechanism for motor operation

12. What is slip in an induction motor?

Slip is the difference between synchronous speed and actual rotor speed in an induction motor, expressed as a percentage of synchronous speed.

Formula: Slip (s) = (N_s - N_r) / N_s × 100%

Where:

• N_s = Synchronous speed = 120f/P (rpm)
• N_r = Rotor speed (rpm)
• f = Supply frequency (Hz)
• P = Number of poles

Typical values: 2-5% at full load for induction motors

Why slip is necessary: Slip must exist for torque production. If rotor rotated at synchronous speed, there would be no relative motion between rotor and stator field → no induced current → no torque.

13. What is a three-phase system?

A three-phase system is a method of AC power generation, transmission, and distribution using three separate AC voltages that are 120° out of phase with each other.

Advantages over single-phase:

• More economical for power transmission (less conductor material)
• Higher power capacity for same size conductors
• Constant power delivery (no pulsations)
• Smaller, more efficient motors
• Can produce rotating magnetic field directly

Configurations: Star (Y) connection and Delta (Δ) connection

Line voltage in Star: V_L = √3 × V_ph

Line current in Delta: I_L = √3 × I_ph

14. What is the difference between Star and Delta connections?

15. What is earthing/grounding and why is it important?

Earthing (grounding) is the process of connecting the metallic parts of electrical equipment to the earth (ground) through a conductor, providing a safe path for fault currents.

Purpose and importance:

• Safety: Protects people from electric shock by providing low-resistance path to ground
• Equipment protection: Prevents damage from lightning strikes and power surges
• Voltage stabilization: Maintains system voltage at safe levels
• Fire prevention: Prevents arcing and sparking that could cause fires
• Fault detection: Enables protective devices (fuses, circuit breakers) to operate

Types: Plate earthing, Pipe earthing, Rod earthing, Strip earthing

16. What is a diode and how does it work?

A diode is a two-terminal semiconductor device that conducts current in only one direction (from anode to cathode). It acts as a one-way valve for electric current.

Structure: P-N junction formed by joining P-type and N-type semiconductors

Working principle:

• Forward bias: Positive terminal connected to P-side, negative to N-side → conducts (low resistance)
• Reverse bias: Positive to N-side, negative to P-side → blocks current (high resistance)

Applications: Rectification (AC to DC conversion), voltage regulation, signal demodulation, clipping and clamping circuits

Types: Zener diode, LED, Schottky diode, Photodiode, Tunnel diode

17. Difference between NPN and PNP transistors

NPN Transistor: Has two N-type semiconductors separated by a thin P-type layer (Emitter-Base-Collector: N-P-N)

• Current flows from collector to emitter
• Base-emitter junction forward biased; base-collector reverse biased
• Conventional current flows into the base
• More common due to better electron mobility

PNP Transistor: Has two P-type semiconductors separated by a thin N-type layer (Emitter-Base-Collector: P-N-P)

• Current flows from emitter to collector
• Opposite biasing compared to NPN
• Conventional current flows out of the base
• Used in complementary circuits

Key point: Both perform amplification and switching; the difference is in current flow direction and biasing polarity.

18. What is an operational amplifier (Op-Amp)?

An operational amplifier (Op-Amp) is a high-gain DC-coupled voltage amplifier with differential inputs and typically a single output. It's one of the most versatile integrated circuits.

Characteristics:

• Very high open-loop gain (typically 10⁵ to 10⁶)
• Very high input impedance (ideally infinite)
• Very low output impedance (ideally zero)
• Wide bandwidth

Common configurations:

• Inverting amplifier: Output inverted and amplified
• Non-inverting amplifier: Output in-phase and amplified
• Voltage follower: Unity gain, buffer circuit
• Summing amplifier: Adds multiple inputs
• Integrator and Differentiator: Analog computation

19. What is the difference between analog and digital signals?

20. What is a control system?

A control system is an interconnection of components forming a system configuration that provides a desired system response. It manages, commands, directs, or regulates the behavior of other devices or systems.

Types:

1. Open-Loop Control System: Output has no effect on the control action. No feedback.

• Examples: Washing machine timer, automatic toaster, stepper motor
• Advantages: Simple, economical, easy to maintain
• Disadvantages: Inaccurate, affected by disturbances, no self-correction

2. Closed-Loop Control System: Output is fed back and compared with input. System adjusts to minimize error.

• Examples: Temperature control (thermostat), cruise control, autopilot
• Advantages: Accurate, less affected by disturbances, automatic error correction
• Disadvantages: Complex, expensive, stability issues possible

21. What is PID control?

PID (Proportional-Integral-Derivative) control is a feedback control algorithm widely used in industrial control systems to continuously calculate an error value and apply correction.

Three components:

• Proportional (P): Output proportional to current error. Provides immediate response but may leave steady-state error.
• Integral (I): Output based on accumulated past errors. Eliminates steady-state error but may cause overshoot.
• Derivative (D): Output based on rate of error change. Predicts future error, reduces overshoot and improves stability.

Control equation: u(t) = K_p × e(t) + K_i × ∫e(t)dt + K_d × de(t)/dt

Applications: Temperature control, motor speed control, process control in chemical plants

22. State Faraday's Law of Electromagnetic Induction

Faraday's Law states that a changing magnetic field through a coil induces an electromotive force (EMF) in the coil. The magnitude of induced EMF is proportional to the rate of change of magnetic flux.

Formula: ε = -N × dΦ/dt

Where:

• ε = Induced EMF (Volts)
• N = Number of turns in the coil
• dΦ/dt = Rate of change of magnetic flux
• Negative sign indicates direction (Lenz's law)

Applications: Transformers, electric generators, induction motors, wireless charging

23. What is Lenz's Law?

Lenz's Law states that the direction of induced current is such that it opposes the change in magnetic flux that produced it. This is the reason for the negative sign in Faraday's law.

Physical meaning: Nature opposes any change. When flux increases, induced current creates a field opposing the increase. When flux decreases, induced current tries to maintain the flux.

Applications: Explains back EMF in motors, eddy current braking, electromagnetic damping

24. What is skin effect?

Skin effect is the tendency of AC current to concentrate near the surface (skin) of a conductor rather than being uniformly distributed across the cross-section.

Why it occurs: At high frequencies, the changing magnetic field induces eddy currents that oppose current flow at the conductor center, forcing current toward the surface.

Consequences:

• Effective resistance increases with frequency
• Current-carrying capacity decreases
• Power losses increase

Solutions: Use stranded conductors (Litz wire), hollow conductors for high-frequency applications, larger conductor diameter

25. What is the difference between accuracy and precision?

Accuracy: How close a measured value is to the true or accepted value. Measures correctness.

Precision: How close multiple measurements are to each other. Measures consistency/repeatability.

Examples:

• High accuracy, high precision: Measurements are both correct and consistent (ideal)
• High accuracy, low precision: Average is correct but measurements vary widely
• Low accuracy, high precision: Measurements are consistent but systematically wrong (calibration error)
• Low accuracy, low precision: Measurements are both wrong and inconsistent (worst case)

26. What is a multimeter?

A multimeter is a versatile electronic measuring instrument that combines several measurement functions into one unit. Also called a VOM (Volt-Ohm-Milliammeter).

Functions:

• Voltmeter: Measures voltage (AC and DC)
• Ammeter: Measures current (AC and DC)
• Ohmmeter: Measures resistance
• Additional features: Continuity testing, diode testing, capacitance measurement, frequency measurement

Types: Analog multimeter (moving pointer), Digital multimeter (DMM) with LCD display

27. What is a rectifier?

A rectifier is a device that converts alternating current (AC) to direct current (DC) by allowing current to flow in only one direction.

Types:

• Half-wave rectifier: Uses one diode; converts only one half-cycle. Simple but inefficient (50% utilization).
• Full-wave rectifier: Uses two or four diodes; converts both half-cycles. More efficient.
• Bridge rectifier: Uses four diodes in bridge configuration. Most common; doesn't require center-tapped transformer.

Applications: Power supplies, battery chargers, DC motor drives, welding equipment

28. What is an inverter?

An inverter is a device that converts direct current (DC) to alternating current (AC). It performs the opposite function of a rectifier.

Working principle: Uses electronic switches (transistors, IGBTs, MOSFETs) to rapidly switch DC polarity, creating an AC waveform.

Types by waveform:

• Square wave: Simple, cheapest, poor quality
• Modified sine wave: Better than square, acceptable for most loads
• Pure sine wave: Best quality, suitable for all AC equipment, most expensive

Applications: UPS systems, solar power systems, variable frequency drives (VFD), electric vehicles

29. What is a thyristor (SCR)?

A thyristor (Silicon Controlled Rectifier - SCR) is a four-layer, three-terminal semiconductor device that acts as a switch, conducting current in one direction when triggered.

Terminals: Anode, Cathode, Gate

Operation:

• Normally OFF (non-conducting) even when forward biased
• Turns ON when gate pulse is applied
• Remains ON even after gate signal removed (latching)
• Turns OFF only when current falls below holding current

Applications: AC/DC motor speed control, light dimmers, voltage regulators, battery charging, welding equipment

30. Why are electrical cables color-coded?

Electrical cables are color-coded for safety and standardization to help identify the function of each wire and prevent dangerous connections.

Standard color codes (may vary by region):

• Live/Hot (Brown/Red/Black): Carries current from source
• Neutral (Blue/Black): Completes circuit, carries return current
• Ground/Earth (Green/Yellow, Green-Yellow striped): Safety connection to earth

Importance: Prevents accidental connections, aids troubleshooting, ensures compliance with standards, critical for safety during maintenance

31. What causes an electrical short circuit?

A short circuit occurs when electric current takes an unintended path with very low resistance, bypassing the normal load. This causes excessive current flow.

Common causes:

• Damaged insulation allowing conductors to touch
• Loose connections
• Moisture or water ingress
• Rodent or pest damage to wiring
• Manufacturing defects in equipment
• Overheating causing insulation breakdown

Consequences: High current → overheating → fire risk, equipment damage, electric shock hazard

Protection: Fuses, circuit breakers, RCDs (Residual Current Devices), proper wiring and installation

32. What is the purpose of a capacitor in a circuit?

A capacitor stores electrical energy in an electric field between its plates and has multiple important functions:

• Energy storage: Camera flash, backup power
• Filtering: Smoothing DC output in power supplies (removing AC ripple)
• Coupling: Passing AC signals while blocking DC between stages
• Decoupling/Bypass: Providing local power reservoir, reducing noise
• Timing: In oscillators and timer circuits with resistors
• Power factor correction: Compensating inductive loads in AC systems
• Tuning: In resonant circuits (radios, filters)

33. Why does a transformer hum?

Transformer humming is caused by magnetostriction — the physical phenomenon where magnetic materials slightly change dimensions when magnetized.

Mechanism:

• AC current creates alternating magnetic field in the core
• Core laminations expand and contract at twice the line frequency (100 Hz for 50 Hz supply, 120 Hz for 60 Hz)
• These vibrations produce audible sound

Other contributing factors: Loose laminations, poor mounting, load conditions, harmonic currents

Reduction methods: Better core material, tight clamping, vibration damping, sound enclosures

34. What is the difference between kW and kVA?

kW (Kilowatt): Measure of real power — the actual power consumed or produced that does useful work.

kVA (Kilovolt-Ampere): Measure of apparent power — the total power in the circuit (combination of real and reactive power).

Relationship: kW = kVA × Power Factor

Example: A motor rated 10 kVA with 0.8 power factor actually delivers 8 kW of useful power. The remaining 6 kVAR is reactive power.

Usage: kW for energy bills (what you pay for); kVA for sizing generators, transformers, and cables (what equipment must handle)

35. How do you test if a fuse is blown?

Method 1 — Visual Inspection:

• Remove fuse from holder
• Look for broken wire inside glass fuse
• Check for discoloration or blackening

Method 2 — Continuity Test with Multimeter:

• Set multimeter to continuity or resistance (Ω) mode
• Remove fuse from circuit
• Touch probes to both ends of fuse
• Good fuse: Beep sound (continuity) or ~0 Ω resistance
• Blown fuse: No beep, infinite (OL) resistance

Safety: Always disconnect power before testing! Never test fuse while circuit is energized.