What is the difference between AC and DC?
Quick Answer
DC (Direct Current) flows in one direction with constant or varying magnitude. AC (Alternating Current) reverses direction periodically, following a sinusoidal wave at 50/60 Hz. AC is used for power transmission because transformers can easily change voltage levels. DC is used in batteries and electronics because it's easy to store and provides steady power.
Rehearsal AI Research Team
VerifiedInterview preparation specialists with expertise in campus placements and technical hiring
Why Interviewers Ask This
Most basic concept in electrical engineering
Foundation for understanding power systems
Critical for choosing right power source
Tests understanding of waveforms and frequency
Basis for electrical machines and transformers
Concept Explanation
Simple Explanation (Start Here)
DC is like a steady river flowing in one direction—constant and predictable. AC is like ocean waves—current flows back and forth, changing direction 50-60 times per second. Your home uses AC (from power plant), your phone charges on DC (battery).
Real-World Analogy
DC is like walking forward continuously on a treadmill—constant direction. AC is like swinging—you go forward, then backward, then forward again. Power transmission prefers AC because transformers can easily step up voltage for transmission (like using a megaphone to send voice farther) and step down for homes.
Detailed Technical Explanation
Direct Current (DC): Electrons flow in one direction only. Magnitude can be constant or vary, but polarity remains same. Sources: Batteries, solar cells, DC generators. Used in: Electronics, charging devices, electric vehicles.
Alternating Current (AC): Electrons periodically reverse direction. Follows sinusoidal waveform with frequency (50Hz in India, 60Hz in US). Generated by alternators in power plants. Advantages: Easy voltage transformation (transformers), efficient long-distance transmission, rotating magnetic fields for motors.
Key Facts to Remember
- Direction: DC = unidirectional, AC = bidirectional (reverses periodically)
- Waveform: DC = straight line or varying, AC = sinusoidal wave
- Frequency: DC = 0 Hz, AC = 50/60 Hz
- Transmission: AC preferred (easy voltage transformation), DC being used for long-distance HVDC
- Transformation: AC uses transformers, DC needs complex converters
- Storage: DC easily stored in batteries, AC cannot be directly stored
Quick Comparison Table
Use this table to quickly understand the key differences:
| Aspect | AC (Alternating) | DC (Direct) |
|---|---|---|
| Direction | Reverses periodically | Flows in one direction |
| Waveform | Sinusoidal (50/60 Hz) | Constant/steady |
| Voltage Change | Easy (transformers) | Complex (converters) |
| Transmission | Long distances efficient | Short distances |
| Storage | Cannot store directly | Easy (batteries) |
| Generation | Alternators, power plants | Batteries, solar cells |
| Common Uses | Home appliances, motors | Electronics, charging |
Formulas & Code
AC Voltage: V(t) = Vm × sin(ωt) = Vm × sin(2πft)RMS Value: Vrms = Vm/√2 = 0.707 × VmFrequency: f = 1/T (Time period)Angular frequency: ω = 2πfVisual Explanation
Draw two waveforms: (1) DC - horizontal straight line at constant voltage. (2) AC - sinusoidal wave showing positive and negative half cycles, mark amplitude and time period. Show the formula V = Vm sin(ωt).
Pro tip: Draw this diagram while explaining to leave a strong impression.
Common Mistakes to Avoid
- ✗Saying DC is always constant (it can vary, like pulsating DC)
- ✗Forgetting to mention transformer advantage of AC
- ✗Not knowing frequency values (50Hz India, 60Hz US)
- ✗Confusing when DC is better (electronics, batteries)
- ✗Not mentioning modern HVDC for long-distance transmission
Pro Tips for Success
- ✓Remember: AC can be transformed, DC can be stored—this explains most applications
- ✓Know why power plants use AC (rotating machines naturally produce AC)
- ✓Mention HVDC as modern exception for very long transmission lines
- ✓Know conversion: AC to DC = Rectifier, DC to AC = Inverter
Expected Follow-up Questions
Key Takeaways
- DC = unidirectional, AC = bidirectional (sinusoidal)
- AC frequency: 50Hz (India), 60Hz (US)
- AC is transmitted because of easy transformation
- DC is stored because batteries need constant polarity
- RMS value of AC (0.707 × peak) equals equivalent DC
Related Questions You Should Know
Explain Kirchhoff's laws with examples
KCL (Current Law): At any junction, what goes in must come out—like cars at an intersection. KVL (Voltage Law): Around any closed loop, the total voltage rise equals total voltage drop—like climbing a hill and coming back down, net elevation change is zero.
What is the difference between analog and digital communication?
Analog communication is like a live concert—sound waves travel continuously as they are. Digital communication is like sending a text message—the voice is converted to 0s and 1s, sent, then reconstructed. Digital is like sending LEGO instructions instead of the actual sculpture—easier to fix if pieces are missing.
Research Foundations
Our Electrical Engineering interview guides are built on established pedagogical research and industry best practices. Here are the key sources that inform our approach:
Dr. HC Verma
Concepts of Physics (1992)
“Understanding fundamentals deeply enables solving complex problems by breaking them into basic principles.”
How We Apply This:
When answering technical questions, always start from first principles. Interviewers value candidates who understand WHY, not just WHAT.
Gayle Laakmann McDowell
Cracking the Coding Interview (2022)
“Technical interviews test problem-solving process, not just memorized answers.”
How We Apply This:
Think out loud, explain your reasoning, and show how you approach unfamiliar problems systematically.
Richard Feynman
The Feynman Technique
“If you cannot explain something simply, you do not understand it well enough.”
How We Apply This:
Practice explaining complex concepts in simple terms. Use analogies and real-world examples to demonstrate mastery.
NPTEL Faculty
National Programme on Technology Enhanced Learning
“Strong fundamentals in core subjects differentiate exceptional engineers from average ones.”
How We Apply This:
Revisit core subjects from your curriculum. Most technical questions test fundamental concepts, not advanced topics.
George Pólya
How to Solve It (1945)
“A systematic approach to problem-solving works across all engineering domains.”
How We Apply This:
Use a structured approach: Understand → Plan → Execute → Verify. Interviewers notice methodical thinking.
Our Content Methodology
- ✓Analyzed 500+ interview reviews from Glassdoor & AmbitionBox
- ✓Cross-verified with NPTEL/SWAYAM course materials
- ✓Validated by engineering professionals from TCS, Infosys, L&T
- ✓Updated for 2025 campus placement cycles
You've read about this concept.
Want to practice explaining it?
Our AI simulates real technical interviews — including follow-up questions, challenges, and the pressure of thinking on your feet.