Question
How do generators, transformers, and eddy current devices work based on electromagnetic induction? What is the principle behind each, and how does Faraday’s law apply?
(CBSE 12 boards ask generator and transformer working for 5 marks; NEET tests eddy current applications)
Solution — Step by Step
A rectangular coil rotates in a uniform magnetic field. As the angle between the coil area vector and changes, the magnetic flux through the coil changes, inducing an EMF.
where = number of turns, = field strength, = coil area, = angular velocity. The output is alternating (sinusoidal).
Two coils (primary and secondary) wound on the same soft iron core. An AC in the primary creates a changing magnetic flux in the core, which induces an EMF in the secondary.
Step-up (): increases voltage, decreases current. Step-down (N_s < N_p): decreases voltage, increases current. Power is conserved: (for ideal transformer).
When a conductor moves through a magnetic field (or is placed in a changing field), circular currents are induced within the conductor itself. These are eddy currents.
Applications: electromagnetic braking (trains, roller coasters), induction heating (induction cooktops), metal detectors, and speedometers.
Minimising eddy currents: use laminated cores (thin insulated sheets) in transformers to reduce energy loss.
flowchart TD
A["Faraday's Law<br/>EMF = −dΦ/dt"] --> B["AC Generator"]
A --> C["Transformer"]
A --> D["Eddy Currents"]
B --> B1["Rotating coil in B field<br/>→ Sinusoidal AC output"]
C --> C1["Changing flux in core<br/>→ Voltage stepping"]
D --> D1["Currents in bulk conductor<br/>→ Braking, heating"]
style A fill:#ffeb3b,stroke:#333Why This Works
All three devices are applications of Faraday’s law: a changing magnetic flux induces an EMF. The generator creates the flux change by rotating a coil. The transformer creates it by feeding AC into the primary. Eddy currents arise from any conductor experiencing changing flux.
Lenz’s law determines the direction: the induced current always opposes the change that caused it. In a generator, this opposition creates a back-EMF. In electromagnetic braking, the eddy currents create a force opposing the motion of the conductor.
Alternative Method
For transformer problems, remember the ideal transformer equation and add efficiency for real transformers: . CBSE problems typically assume ideal (100% efficiency) unless stated otherwise. JEE may give efficiency and ask you to find the actual secondary current.
Common Mistake
Students write that a transformer works with DC. It does NOT. A transformer requires changing flux, which means the input must be AC. A steady DC current produces a constant magnetic field — no changing flux, no induced EMF. If you connect DC to a transformer, you get a momentary pulse when it is switched on/off (flux changes briefly) but no sustained output.