Physics · Electromagnetic Induction · NEET
The cause is the same (Faraday's law, changing flux). The difference is the shape of the conductor. In a coil the current is forced to flow along a thin wire in one clear loop. In a solid metal block the current has no wire to follow, so it spreads out into many closed whirlpool-like loops inside the metal. Those spread-out loops are what we call eddy currents.
Eddy currents need a continuous body of metal where current can circulate freely in 2D or 3D. A thin wire only gives one narrow path, so the swirling loops cannot form. A thick solid plate offers a big cross-section, so many circulating loops appear. This is exactly why transformer cores are made from thin sheets (laminations) instead of one solid block, to break up these loops.
They heat it. Eddy currents are real currents flowing through the resistance of the metal, so by Joule heating (H = I squared R t) they turn electrical energy into heat. This is wasteful in transformers (energy lost as heat) but useful in an induction furnace, where the goal is to melt the metal by heating it.
Their direction is set by Lenz's law: they flow so as to oppose the change in flux that created them. If flux through the metal is increasing, the eddy currents make a magnetic field that pushes back against that increase; if flux is decreasing, they try to keep it. This opposition is what produces the braking force on a moving metal plate.
Same physics, different goal. In a transformer we want energy to pass through, so eddy-current heat is pure loss and we minimise it with laminations. In a magnetic brake or induction furnace we WANT the effect: the opposing force stops a train smoothly, or the heat melts metal. So eddy currents are not 'good' or 'bad' by nature, only relative to what the device is trying to do.
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.
A changing magnetic flux through a solid piece of metal. By Faraday's law, changing flux induces an EMF, and because the metal is a good conductor, this EMF drives circulating currents (eddy currents) inside the body of the metal.
Heating effect (Joule heating H = I^2 R t warms or melts the metal) and mechanical/braking effect (by Lenz's law the currents oppose the motion or the flux change, creating a retarding force).
By using laminated cores, thin sheets of metal separated by an insulating varnish. This breaks the large swirling loops into many small ones with high resistance, so the eddy current heating drops sharply. This is covered in the next concept, laminated cores.
Both, depending on the device. Harmful (energy loss as heat) in transformers and motors; useful in induction furnaces, magnetic brakes, induction cooktops, and metal detectors.
Yes. Their direction is always such that they oppose the change in flux that produces them. This is why a magnet dropped through a copper pipe falls slowly, the eddy currents in the pipe resist the magnet's motion.
