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# Re: [femm] Current

- To: femm@yahoogroups.com
- Subject: Re: [femm] Current
- From: Keith Gregory <k.gregory@lboro.ac.uk>
- Date: Mon, 22 Jul 2002 08:37:32 +0100
- In-reply-to: <ah6h3h+oi74@eGroups.com>

`As I understand it (and I have no doubt someone will correct me!) if you define a current density in the materials property you are in effect defining a total "impressed" current in the region. The actual current would be the current density in millions of amps per square metre (that's the same number as amps per square millimetre) multiplied by the cross-sectional area of the region in square metres.`
`Skin and proximity effects change the distribution of current in a conductor or a coil not the overall total current. In a conductor carrying alternating current emfs are induced in the conductor by its own magnetic field which tend to make the current move towards the surface of the conductor - this is skin effect. It can be thought of as being caused by internal circulating currents - eddy currents - which add to the impressed current in parts of the conductor but oppose it in others producing larger current density near the outside of a conductor than in the centre. The average current density (i.e. the total impressed current divided by the total conductor area) is unaffected but local current density in parts of a conductor can be very different.`
`If you have a conductor or a coil in which you are forcing a certain current to flow then that same current will flow whatever its distribution in the conductor. However, the apparent impedance of a conductor or coil is affected by the distribution so as frequency increases and current is crowded more and more towards the outside regions of a conductor more and more voltage would have to be applied to make the current flow.`
`Keith.`
`At 13:50 18/07/2002 +0000, you wrote:`

>     Hi, I'm new here, so be patient if I've missed anything.
>     I have started to use FEMM to look at current distribution within the
>     turns of a coil. Skin effect and Proximity effect. I have some
>     problem reconciling the current density specified in the original
>     materials property (for the copper) and the total current calculated
>     by integrating over the area of the wire cross section in the result.
>     Can someone enlighten me please?
>     Brian Hawes

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- References:
  - Current
    - From: "brian_hawes" <b.hawes1@physics.ox.ac.uk>

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