Why Do Eddy Currents Form Circular Loops?

[Pranav Jha]

i know i haven't understood the concept of eddy current and that is why i am posting this question:
why are EDDY currents CIRCULAR (why form a loop in circular form)?

please also explain this answer that statement that i found on : http://wiki.answers.com/Q/What_is_an_eddy_current
"When time-varying magnetic field is applied to electrical machines like transformers, a time-varying emf is induced in the transformer cores(i get this). A SHORT CIRCUIT OCCURS AT THE MOLECULAR LEVEL IN THE CORE(this, i don't quiet understand)"

 

[Dadface]

The actual shape of the eddy currents depends on the structure and geometry of the system that is generating them,and can be found by the application of Lenz's law and or by applying the Lorentz force equation.

 

[Pranav Jha]

The actual shape of the eddy currents depends on the structure and geometry of the system that is generating them,and can be found by the application of Lenz's law and or by applying the Lorentz force equation.

what confuses me is most sources state the eddy currents being circular.

 

[Dadface]

what confuses me is most sources state the eddy currents being circular.

I have a vague memory of hearing that sort of statement before and I think it was in connection with the eddy currents that would be generated in the iron core of a transformer if it were solid rather than laminated.Hopefully someone will come in and elaborate on these circular currents but in the meantime a little bit of research is needed.

 

[Dadface]

Pranav,I now think that the relevant word I recall is "circulating".Although this can mean moving in a circle it can also mean,in the context of your question,moving through a circuit and returning to the starting point.Having thought about this briefly my first impression is that the eddy currents in an unlaminated core have a complex shape which spirals.

 

[Bob S]

i know i haven't understood the concept of eddy current and that is why i am posting this question:
why are EDDY currents CIRCULAR (why form a loop in circular form)?

please also explain this answer that statement that i found on : http://wiki.answers.com/Q/What_is_an_eddy_current
"When time-varying magnetic field is applied to electrical machines like transformers, a time-varying emf is induced in the transformer cores(i get this). A SHORT CIRCUIT OCCURS AT THE MOLECULAR LEVEL IN THE CORE(this, i don't quiet understand)"

To adequately explain this, and the need for laminations, we have to go back to Faraday's Law, in integral form:

$$\oint_C\bold{E\cdot}d\bold{\ell} = -\frac{\partial}{\partial t}\int_S \bold{B\cdot n}\;\;dS$$

The path on the left side is the perimeter of the surface (area) on the right side. The area need not be a circle. The magnetic flux in the area on the right creates a voltage in the circumference on the left. Let's consider a square area. The area, or the induced voltage, is proportional to L², where L is the length of a side. The perimeter of the square is 4L. So the voltage per unit path length (E or electric field) is 4L/L², or 4/L. So the induced electric field, and the resultant eddy current at the molecular level is negligible compared to macroscopic areas.

A standard 60 Hz transformer gets about 0.25 volts per square inch of iron area. The electric field around the perimeter is about 0.25/4 = 0.06 volts per inch. If the area was a micro-inch on a side, the electric field would be 60 nano-volts per inch. The voltages, electric fields, and eddy currents on a molecular scale are negligible.

Bob S