What are eddy currents and how do they form in metal objects?
If the wire is heated (as most metals are), this changing current causes the charge to move as well. A hot copper wire can become electrically conductive even at room temperature, so it’s not uncommon to see a water pipe that has a continuous stream of water flowing up through it’s length. Such an object is called a ‘water pipe heater’. This phenomenon is known as ‘eddy current heating’ or ‘electrical resistance heating’.
Eddy currents form when a conductor (metal) is set into a magnetic field. Loosely speaking though, eddy currents are regular density and speed of charge movement. A wire in a magnetic field is subject to the Lorentz force which results in the conductor carrying an electric current. This current is perpendicular to both the wire and the magnetic field.
Eddy currents in turn result in Joule heating , where the eddy current results in a distribution of electrical charge along the surface that is proportional to the strength of the magnetic field, and arises because of resistivity and conductivity properties of conductors.
How do the properties of different metals affect the strength of eddy currents?
The further away from the center of the charge, the more ‘dissipative’ (ie weak) it becomes. When something is very hot (such as a welding torch) the heat causes the electrons to become coupled together, which means that they no longer behave individually and can hardly be considered to be independent. So in a metal object under such conditions you will find that very weak currents are formed. Weak eddy currents have been used for experimental reasons for some time now, but this has only been with small objects like wires and strips of foil. But that is not the case with your welding torch; the very strong currents under such conditions can easily destroy the object being welded.
How do eddy currents get started in a metal object? The first thing to consider is, how does heat affect eddy currents? In a normal metal object you will find that at room temperature (20-23 degrees celcius) no currents exist. But if the metal object is heated, you will find that when it gets hotter the resistance of the metal decreases and more eddy currents form.
A piece of metal with a certain width, length and thickness is put into a special container. The temperature inside the container is increased slowly until there are sufficient heat to start induction currents. When this happens, the current can be tested by using a voltmeter (instrument) to measure how much power the current in use has. A power of 1 watt means that the current works at 1 joule per second. A good conductor will have a low resistance and so is able to carry more energy than an insulator. This means that, when the heat increases, the voltage in the coils of a power outlet will increase.
What practical applications can eddy currents have in industry and everyday life situations?
The use of weak currents for experimental purposes has benefitted technology greatly. It is also a useful method to determine the distribution of charge within a substance, which is a typical property in electronics. Furthermore, eddy currents are good conductors and therefore can be used as material to transmit electric energy. Examples are in transformers and electric motors. They are used to efficiently transfer heat and they give some materials magnetic properties, or prevent substances from sticking to certain surfaces.
Eddy currents are currents that are caused by a change in the magnetic field. In metals, eddy currents can be easily generated by mechanical force. This force is then converted into heat energy and this is the reason why the metal becomes hot.
Eddy currents can be used in industries to carry out their processes or to check the quality of materials used in production. It can be used to check the quality of metal sheets of various industries and when working with steel, checking for places where the metal is weak or fractured. However, this method has only been used in small scale industries due to the high cost of equipment required to generate eddy currents.
Can you think of any other ways that eddy currents might be used or harnessed to our advantage?”
The electrical function of the current itself, combined with its ability to heat objects (as would happen in a welding torch or an electric motor) make it an interesting tool in industry. These properties combined with the fact that metals are conductive makes them good conductors. So using them in transformers and motors is a perfect example of the application of eddy currents to industry. Finally, they can be used as material to transmit electricity or heat energy, or can be used as a magnetic field generator. So, the usefulness of the eddy current in industry is worthy of further study.
Harnessing The Power Of Eddy Currents
By Andrew M. Sanchez
Can you think of any other ways that eddy currents might be used or harnessed to our advantage? Maybe as a generator, where we could extract usable current from the magnetic fields that can be generated by an electromagnet?” Cindy asked.
“Sure,” I replied, “A generator is just a device that is able to generate eddy currents in metals. Basically, a generator is a device that will rotate when current flows through it.”
“What about motors? Are they the same thing as generators?” asked Cindy.