by If you’re studying Alternating Currents in Class 12 Physics or preparing for AP Physics, understanding the transformer working principle is essential. Transformers are everywhere—from phone chargers to power grids—and they’re a common topic in board exams, JEE, NEET, and AP Physics tests.
In this guide, you’ll learn:
- What is a transformer?
- How does a transformer work?
- Transformer formula and turns ratio
- Step-up vs step-down transformer
- Energy losses and efficiency
- Real-world applications
- Solved numerical example
What Is a Transformer?
A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It works only with alternating current (AC)—not DC—because it relies on a changing magnetic flux.
Working Principle of a Transformer
The transformer operates on the principle of mutual induction—when an alternating current flows through the primary coil, it creates a changing magnetic field in the iron core. This changing flux links with the secondary coil, inducing an EMF (voltage) across it.
Key Conditions:
- Input must be AC (to produce changing flux)
- Coils are electrically isolated but magnetically coupled
- Core is made of laminated soft iron to reduce eddy current losses
Transformer Formula & Turns Ratio
The voltage transformation depends on the number of turns in the primary and secondary coils:
Transformer Equation:
Vs / Vp = Ns / Np = Ip / Is
Where:
• Vp, Vs = Primary and secondary voltages
• Np, Ns = Number of turns in primary and secondary coils
• Ip, Is = Primary and secondary currents
Step-Up vs Step-Down Transformer
| Type | Ns vs Np | Voltage | Use Case |
|---|---|---|---|
| Step-Up | Ns > Np | Vs > Vp | Power transmission (e.g., 11kV → 132kV) |
| Step-Down | Ns < Np | Vs < Vp | Phone chargers, household appliances (e.g., 230V → 12V) |
Energy Losses in a Transformer
Ideal transformers are 100% efficient, but real ones suffer from:
- Copper losses (I²R heating in windings)
- Iron losses:
- Eddy current loss → reduced by laminating the core
- Hysteresis loss → minimized using soft iron
- Flux leakage (not all flux links secondary coil)
- Magnetostriction (humming noise)
Applications of Transformers
- Electric power transmission over long distances
- Voltage adapters for electronics
- Isolation transformers in medical equipment
- Impedance matching in audio systems
Solved Example: Transformer Numerical (Class 12)
Problem: A step-down transformer converts 220 V AC to 22 V AC. If the secondary coil has 100 turns, how many turns are in the primary coil? Assume ideal conditions.
Solution:
Using: Vs/Vp = Ns/Np
22 / 220 = 100 / Np
1/10 = 100 / Np
Np = 100 × 10 = 1000 turns
FAQs About Transformers (Class 12 Physics)
Q: Why doesn’t a transformer work with DC?
A: DC produces a constant magnetic field (no change in flux), so no EMF is induced in the secondary coil.
Q: Can a step-up transformer increase power?
A: No. It increases voltage but decreases current—power (ideally) remains constant: Pin = Pout.
Q: What is the efficiency of a real transformer?
A: Typically 95–99% for large power transformers.
Conclusion
Mastering the transformer working principle not only helps you ace Class 12 Physics exams but also builds a foundation for understanding modern power systems. Focus on the relationship between turns ratio, voltage, and current—and always remember: transformers need AC to work!
