Main Difference – Eddy Current vs. Induced Current
Eddy current and induced current refer to currents that form on a conductor as a result of a changing magnetic field across it. The main difference between eddy current and induced current is that induced current refers to currents flowing in coils of wire in a closed circuit whereas eddy current refers to loops of currents flowing within pieces of larger conductors due to electromagnetic induction.
What is Induced Current
According to Faraday’s Law, whenever the magnetic flux through a conductor changes, an EMF is induced in the conductor. According to Lenz’s Law, the direction of the induced emf opposes the change in magnetic flux that causes it. If the magnetic flux is given by , then, according to Faraday’s Law the EMF induced is given by
The induced EMF is equal to the rate of change of magnetic flux. The negative sign in the formula simply indicates that this EMF opposes the change in flux that caused it. This is the mechanism that produces both so-called induced currents and eddy currents in conductors. So in this sense, they are both “induced” currents. However, the terminology is often used to differentiate between useful current generated in a coil (this is called the induced current) and the current generated in larger metals such as in the “core” of an electromagnet/ body of a metal (this is called eddy current). For example, we will look at the difference between eddy current and induced current in a transformer.
The image below shows a transformer. The coil on the left is provided with an alternating current. The current produces a magnetic field inside the coil, and since the current is constantly reversing direction, the magnetic flux inside the coil is always changing as well. The “transformer core” is a conductor whose function is to conduct the magnetic field to the coil on the right. The core is not directly connected to the power supply. There is a change in magnetic flux across this coil and, according to Faraday’s Law, a current gets induced in this coil as well. We can connect this current to a circuit and use this current to do work. It is this current, that gets induced in the second coil, that is called the “induced current”.
Note that there is a change in magnetic flux through the transformer core as well. Since the core is made of a conductor, a current gets induced in the core as well. This current flows in “loops” as shown below, and so they are named “eddy currents”. We cannot make use of this current, and this current takes away some of the energy from the original current and dissipates that energy in the form of heat. Therefore, transformer cores are usually “laminated“—i.e. sectioned off by adding layers of insulators—in order to reduce the eddy currents. This is also shown on the image below:
What is Eddy Current
As mentioned before, eddy currents refer to loops of currents induced within the bodies of large conductors. In the example of a transformer, eddy currents dissipate energy in the form of heat so they are undesirable. However, there are situations where eddy currents are useful as well. We will look at a few examples of eddy current’s use below.
Metal detectors: In metal detectors, an alternating current flowing in a coil in the detector produces a magnetic field with a changing magnetic flux. If the metal detector is hovered over a piece of metal, eddy currents start flowing in the metal. These eddy currents create a magnetic field of their own, and the metal detector can detect this magnetic field.
Induction heaters: Eddy currents can dissipate energy in the form of heat. In induction heaters, the energy dissipated is used to heat up substances. Induction cookers also use thew same principle. The video below shows how an induction heater is used to heat a bar of iron:
Difference Between Eddy current and Induced Current
Eddy currents refer to loops currents induced within large bodies of conductors, as a result of a changing magnetic field across it.
Induced currents typically refer to currents induced in coils connected to a closed circuit.
Induced currents are useful in transformers.
Eddy currents are undesirable as they dissipate energy in the form of heat. However, they are useful in some situations like in metal detectors and induction heaters.
“Idealised single-phase tranformer also showing the path of magnetic flux through the core.” by BillC at en.wikipedia (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Laminering av transformatorkärna” by Svjo (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Hopeful/Patient” by PROMichael Coghlan (Own work) [CC BY-SA 2.0], via flickr