Eddy currents are circulating currents induced inside a conductor by a changing magnetic field. In Honors Physics, they show up in electromagnetic induction, magnetic braking, and unwanted energy loss in transformers and motors.
In Honors Physics, eddy currents are loops of electric current that form inside a metal when the magnetic flux through it changes. They are not wired into a circuit. Instead, the current is induced within the body of the conductor itself, usually as many tiny circular paths spread through the material.
The reason they appear is electromagnetic induction. When a conductor experiences a changing magnetic field, charges in the metal feel a push and start moving. Because the charges are free to move through the material, they keep circulating, which creates those swirling current patterns. A solid sheet of metal can develop many overlapping loops, which is why the term "eddy" fits the image of water swirling in a current.
The direction of an eddy current is set by Lenz's law. The induced current creates its own magnetic field that opposes the change that caused it. If the magnetic flux through the conductor is increasing, the eddy current produces a field that tries to reduce that increase. If the flux is decreasing, the induced field tries to keep it from dropping too fast. That opposition is why eddy currents can slow motion or resist changes in a magnetic system.
You usually see eddy currents when a metal moves through a magnetic field, when a magnet moves near a conductor, or when the magnetic field around the conductor changes with time. A classic lab demonstration is dropping a magnet through a copper tube. The falling magnet changes the field in the tube, which induces eddy currents in the copper. Those currents create a magnetic field that pushes back on the magnet, so it falls much more slowly than it would in open air.
That same effect can be useful or wasteful. In a brake, the opposing force can smooth and slow motion without physical contact. In a transformer or motor, though, eddy currents waste energy as heat because the induced currents run through the metal and meet resistance. That is why real devices often use laminated cores, which break up the conductive path and reduce the size of the circulating currents.
Eddy currents connect the big ideas of electromagnetic induction, Lenz's law, and real device design in Honors Physics. They are one of the clearest examples of how a changing magnetic field does not just "make electricity" in a wire, it can also make current inside the metal itself.
This matters because it explains both the useful and unwanted side of induction. In magnetic braking, eddy currents turn motion into heat and create smooth resistance without frictional contact. In transformers, generators, and motors, the same induced currents can drain energy from the system and warm up the core. When you see laminated metal cores, that design choice is really a response to eddy current losses.
Eddy currents also give you a concrete way to reason through direction. If the field changes, ask what the induced current must do to oppose that change. That habit shows up in lab observations, diagrams of moving magnets, and questions about why some metals slow down nearby magnets while others do not. Once you can picture the circulating current loops, the rest of induction problems become much easier to trace.
Keep studying Honors Physics Unit 20
Visual cheatsheet
view galleryElectromagnetic Induction
Eddy currents are one form of electromagnetic induction. The same changing magnetic flux that can induce current in a wire loop can also induce circulating currents inside a solid conductor. If you know the induction idea first, eddy currents make sense as the "inside the metal" version of it.
Lenz's Law
Lenz's law tells you the direction of the eddy currents. The induced currents always produce a magnetic field that opposes the change in flux, which is why they can resist motion or slow a magnet. If you get the direction wrong, the force picture for braking and damping will be backwards.
Magnetic Braking
Magnetic braking is one of the cleanest applications of eddy currents. A moving conductor or magnet induces currents that create a resistive magnetic force, which slows the object without direct contact. This is the same effect used in some trains, exercise equipment, and lab demos with magnets and metal plates.
Distribution Transformers
Transformers can lose energy because eddy currents form in the iron core when the magnetic field changes. That loss shows up as unwanted heating, so transformer cores are often laminated to interrupt the current paths. If a question asks why a core is layered instead of solid, eddy currents are the reason.
A quiz question might show a magnet moving toward a copper ring, a falling magnet through a tube, or a transformer core that is getting hot, and ask you to identify the cause. Your job is to spot the changing magnetic field, name the induced circulating current, and explain the direction using Lenz's law. If the prompt asks about energy, connect the induced current to heating and resistance. If it asks about motion, connect the current to magnetic drag or damping. In a lab write-up, you may also need to explain why a solid metal core produces more loss than a laminated one.
Electromagnetic induction is the broader process where a changing magnetic field produces an emf or current. Eddy currents are a specific outcome of that process inside a conductor. So induction is the general rule, while eddy currents are one common pattern you see when the conductor is a solid piece of metal.
Eddy currents are circulating currents induced inside a conductor when the magnetic field through it changes.
Their direction follows Lenz's law, so they oppose the change in magnetic flux that created them.
They can slow motion in magnetic braking, which makes them useful in contactless braking systems and lab demos.
They can also waste energy as heat in transformers, motors, and generators, which is why laminated cores are used.
If a moving magnet or changing field is involved, ask whether eddy currents are being created and what they are doing to the motion or heat.
Eddy currents are loops of induced current that form inside a conductor when the magnetic field through it changes. They are not part of a wire circuit, they circulate within the metal itself. In Honors Physics, they come up in induction, braking, and energy loss.
They oppose motion because the changing magnetic field that creates them also determines their direction. By Lenz's law, the induced current makes a magnetic field that resists the change in flux. That resistance can show up as a drag force on a moving magnet or conductor.
No. They are useful in magnetic braking and in some heating applications, like induction heating. They become a problem when they waste energy as heat inside devices that are supposed to transfer or convert energy efficiently, like transformers and motors.
Laminations break up the path for circulating currents, so eddy currents stay smaller. With less current flowing in the core, there is less resistive heating and less energy loss. That makes the transformer run more efficiently.