What determines the size of an Eddy Current?

Submitted by Gopher_Lad t3_11bp6ii in askscience

I understand the idea of magnetic braking, and that a Eddy Current will be induced in a solid conductor as it's magnetic flux changes.

Why does perforating the conductor with slits prevent these currents? Wouldn't smaller loops form and still have the same net braking force.

I'm using this MIT pendulum example to show what I mean. The pendulum with slits doesn't brake.

Do Eddy currents have a minimum size? If so what determines the size?

https://i.ytimg.com/vi/P8CMxUqCfmA/mqdefault.jpg

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Comments

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JonJackjon t1_j9zbbfe wrote

Currents induced in a conductive material are a function of the magnetic field being imposed on the material and its electrical resistance.

Putting holes in a subject material increases the electrical resistance. Hence the resulting field will be weaker.

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Gopher_Lad OP t1_j9zgkw2 wrote

You aren't physically changing the coppers properties by cutting holes into it though. Why doesn't the field induce many smaller loops of current?

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wtfomg01 t1_ja16tt1 wrote

You're changing its structure and the field is both a function of its properties and its structure.

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kilotesla t1_ja0lh1v wrote

Yes, smaller loops of current do form. The current in a given loop is proportional to the rate of change of magnetic flux through it, and is inversely proportional to the resistance around the loop. With the slits as shown, the flux through each is much smaller than the total flux, and the resistance around the loop starts going up once the copper is divided into strips: The path length is roughly constant at double the length of a strip, and the conduction area gets smaller as it gets divided more. So the current per loop goes down as the square of the number of slits, and the effect become negligible.

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