Physics · Electromagnetic Induction · NEET
Both, depending on the device. In a transformer or motor core, eddy currents are HARMFUL because they waste energy as heat, so we reduce them with a laminated core. But in a braking system, furnace, or metal detector, the same effect is USEFUL and we design the device to make eddy currents strong. So the honest answer is: eddy currents themselves are neither good nor bad — it depends on whether you want their force, their heat, or their signal.
A strong electromagnet is placed near the spinning metal wheel or rail. When the metal moves through the field, the flux through it changes, so eddy currents form inside the metal. By Lenz's law these currents always oppose the motion that made them, so they create a retarding (backward) force that slows the metal down. There is no physical touching, so no brake pads wear out. Note: the force gets weaker as the train slows (less relative speed means less change of flux), so magnetic braking is smooth but cannot fully stop the train — friction brakes finish the job.
A high-frequency alternating current is passed through a coil surrounding the metal. This makes a fast-changing magnetic field, which induces strong eddy currents inside the metal piece. These eddy currents heat the metal by Joule heating (I squared times R heating) until it melts. The metal is heated directly by currents induced inside it, not by an outside flame, which is why furnaces can reach very high temperatures cleanly.
A metal detector has a coil carrying alternating current, making a changing magnetic field. When metal comes close, eddy currents form inside that metal. These eddy currents make their own magnetic field that pushes back on the detector's coil and changes the coil's inductance and signal. The circuit senses this change and sounds an alarm. No metal means no eddy currents means no change means no beep.
An ordinary electric heater heats by ordinary resistive (Joule) heating: current is pushed through a nichrome coil by a battery or mains, and the wire's resistance makes it glow hot. This current is driven by an applied voltage in a normal circuit — it is not induced by a changing magnetic flux inside a solid block. Eddy currents specifically mean induced loops from changing flux. That is why the NEET 2019 answer is the electric heater — it is the odd one out.
In which of the following devices, the eddy current effect is not used?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Magnetic braking (their opposing force slows trains and vehicles), induction furnace (their heat melts metal), and metal detectors (they change the sensing coil's signal). Remember them as Force, Heat, Signal.
The braking force depends on how fast the flux changes, which depends on speed. As the train slows, the relative speed drops, so the eddy currents and the force get weaker. Near zero speed the force is almost zero, so friction brakes are still needed to fully stop.
No. An induction furnace heats a metal by eddy currents induced by a changing magnetic field, with no direct electrical contact. An electric heater heats a wire by ordinary resistive (Joule) heating from a current driven directly through it.
Yes. Eddy currents are induced currents, so they always flow in the direction that opposes the change causing them. That is exactly why they create a retarding force in magnetic braking.
Transformers, electric motors, and generators. In these the eddy-current heat is a waste of energy, so the iron core is laminated (thin insulated sheets) to break up the current loops and cut the loss.
