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Does low voltage dc travel on surface

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  • J Tiers
    replied
    Originally posted by philbur
    Do you really mean "on the surface" or "close to the surface".

    Phil
    Depending on what frequency is involved, it can be a distinction without a difference.

    "At" the surface is a good compromise description.

    Leave a comment:


  • philbur
    replied
    Do you really mean "on the surface" or "close to the surface".

    Phil

    Originally posted by J Tiers
    Even DC travels only on the surface for the first short time after connection.... then the conduction penetrates deeper and deeper. But that would never be noticeable to you.

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  • J Tiers
    replied
    AC travels in a depth related to the frequency of the AC AND the resistance of the material.

    The lower the frequency and/or the higher the resistance, the more it can penetrate. For 60 Hz or 50 Hz, the depth of conduction is quite substantial, for microwave frequencies, it may be very shallow indeed.

    Even DC travels only on the surface for the first short time after connection.... then the conduction penetrates deeper and deeper. But that would never be noticeable to you.

    I suspect a coincidence...... cleaning the surface also improved connections, etc.

    Leave a comment:


  • rohart
    replied
    Mac, if all AC travels on the surface, why do old household cables consist of multiple stands, but these are replaced when modernised by single thicker strands to make up the same cross section for the capacity ? Surely you'd need the same circumference for the capacity to get the same surface ? I'll go with Black M.

    And Lew, taking something off the surface, which might or might not conduct much, cannot possibly reduce the resistance with DC. If the muck - corroded salts possibly - on the surface has a high resistance and can conduct only a small proportion of the current, it is contributing to the conductivity of the whole.

    Mark, you've deluded yourself somehow. But as the great and late electronics guru Bob Pease (not the member here) would have said : What's all this conductivity stuff, anyhow ?

    Leave a comment:


  • macona
    replied
    Originally posted by Black_Moons
    No, DC travels through the entire depth of the conductor, its high *freqency* AC that travels on the skin.

    What could of been happening is the connection to the terminal was failing and needed to be cleaned, as well as the connection from terminal to terminal.
    Actually, all AC travels on the surface, the skin depth is relative to the frequency. Thats why conductors at sub stations and power plants are either aluminum pipe or flat bar. Even at 60H the skin depth is 1/3".

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  • danlb
    replied
    Originally posted by Forrest Addy
    Nit pick point. Electriicity in a circuit proportions its current according to the resistance of multiple circuits. There's even a formula R = 1 / (1/R1 + 1 /R2 + 1/R3 ...) If you happen to be numbered among the R's then you can expect to be zapped according to the voltage present and your contact resistance to the electrical circuit. It doessn't take very many milliamps for the shock the be severe.

    Here's someting interesting that prompted this nearly off topic response:

    http://en.wikipedia.org/wiki/Electric_shock

    that seems to be about the post by high country, not mine.
    Originally posted by highcountry
    Electricity does not care what or where the conductor is, it will always find the path of least resistance.....that may not always be one single path.
    Dan

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  • Forrest Addy
    replied
    Originally posted by danlb
    DC leaks when there is dirt and moisture to provide a path to ground. I usually only see power lines cleaned near the insulators at the poles.

    The same applies to a battery pack. The right combination of dirt will cause a high resistance short that eventually will run the battery down.

    Dan
    Nit pick point. Electriicity in a circuit proportions its current according to the resistance of multiple circuits. There's even a formula R = 1 / (1/R1 + 1 /R2 + 1/R3 ...) If you happen to be numbered among the R's then you can expect to be zapped according to the voltage present and your contact resistance to the electrical circuit. It doessn't take very many milliamps for the shock the be severe.

    Here's someting interesting that prompted this nearly off topic response:

    http://en.wikipedia.org/wiki/Electric_shock
    Last edited by Forrest Addy; 05-12-2012, 05:20 PM.

    Leave a comment:


  • MaxHeadRoom
    replied
    Originally posted by danlb
    I've never seen DC transmission lines (as in cross country). The ones I'm familiar with are always AC so that it's easier to step the voltage up and down.

    Dan
    It is commonly done now AC converted to DC for long distance transmission and back to AC at the end use point.
    Done to lower losses.
    http://www.energy.siemens.com/hq/en/...smission/hvdc/
    Max.
    Last edited by MaxHeadRoom; 05-12-2012, 02:57 PM.

    Leave a comment:


  • high country
    replied
    Electricity does not care what or where the conductor is, it will always find the path of least resistance.....that may not always be one single path.

    Leave a comment:


  • danlb
    replied
    Originally posted by high country
    If dc doesn't creep, why do they clean dc transmission lines?

    DC leaks when there is dirt and moisture to provide a path to ground. I usually only see power lines cleaned near the insulators at the poles.

    The same applies to a battery pack. The right combination of dirt will cause a high resistance short that eventually will run the battery down.

    Dan

    Leave a comment:


  • danlb
    replied
    I've never seen DC transmission lines (as in cross country). The ones I'm familiar with are always AC so that it's easier to step the voltage up and down.

    To the original post...

    If DC traveled only on the surface, then the cables used to transport it would be hollow pipes. Twice the surface area with a tenth of the materials.

    I've seen some instances where I cleaned the "contact area" without realizing that as I install the battery the metal tab bends and the area of contact moves one way or another. I was actually cleaning the wrong area.

    Is it possible that in cleaning the whole area you are also cleaning other contacts or joints?

    Dan

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  • high country
    replied
    If dc doesn't creep, why do they clean dc transmission lines? I have measured small amounts of resistance on top of a dirty battery. Dirt is conductive....just not highly conductive.

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  • Rosco-P
    replied
    If the OP's assertion were true, the following statement would also be true.
    If I clean the post of my truck's battery and only the inside of the battery cable clamp, the truck should not start.

    I found the above statement to be false.
    Last edited by Rosco-P; 05-12-2012, 11:23 AM.

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  • Lew Hartswick
    replied
    Originally posted by aboard_epsilon
    YES, THAT'S EXACTLY WHAT I READ ON THE WEB ..

    but my "short" experience of this tells me something else.
    Well your "short experience" is wrong. It's possible that the material
    under the surface has a higher resistance than the surface, ie. a steel
    spring with some plating on the surface, and this is what is causing
    your phenom.
    ...Lew...

    Leave a comment:


  • aboard_epsilon
    replied
    Originally posted by Black_Moons
    No, DC travels through the entire depth of the conductor, its high *freqency* AC that travels on the skin.

    What could of been happening is the connection to the terminal was failing and needed to be cleaned, as well as the connection from terminal to terminal.
    YES, THAT'S EXACTLY WHAT I READ ON THE WEB ..

    but my "short" experience of this tells me something else.

    i made very good jobs of either end ..but not until there was a clear path on the surface from one end to the other would they conduct .

    perhaps I need to find more dysfunctional makita drills ..to prove my point ..as two isn't conclusive proof.

    all the best.markj

    Leave a comment:

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