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  • Electric cable

    Does electric cable become stiffer or heavier when electricity passes through it? After all water hose pipes become stiffer and heavier when water pases through them.
    MBB.

  • #2
    No, under normal circumstances the conductor will never get heavier and will not get stiffer. It may get warmer if the ampacity is high enough but that will only make the conductor more supple.
    Under fault conditions conductors can whip around quite violently as the fault current passes through them and can cause allot of collateral damage but this is only really a concern with high voltage power conductors.
    Cheers,
    Jon

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    • #3
      Many years ago at the old Army SEDRC (Special Electrical Devices Repair School) we used to have two students hold the power cables to the 60" searchlight up off of the ground and then turn on the arc light. The cables would separate a couple feet in the middle showing the force developed by the electric field.

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      • #4
        After all water hose pipes become stiffer and heavier when water pases through them.
        Your hose only becomes heavier and stiffer if it was empty before you turned on the faucet. Once the hose is full, it's weight will not change appreciably when you open the faucet wider, assuming the hose doesn't expand. Your electric cable is already "full" of electrons so when you push some in one end, the same amount come out the other. At least that is my simplistic view of current flow.

        Tom
        Tom - Spotsylvania, VA

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        • #5
          Originally posted by flathead4 View Post
          At least that is my simplistic view of current flow.

          Tom
          The usual water analogy, Flow is Current, Pressure is Voltage.
          Max.

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          • #6
            Originally posted by flathead4 View Post
            Your hose only becomes heavier and stiffer if it was empty before you turned on the faucet. Once the hose is full, it's weight will not change appreciably when you open the faucet wider, assuming the hose doesn't expand. Your electric cable is already "full" of electrons so when you push some in one end, the same amount come out the other. At least that is my simplistic view of current flow.

            Tom
            Actually a hose does become stiffer when it is under pressure. Look at footage of fire hoses that become stiff when the high pressure water is turned on. If the hose is completely full then on or off it should weigh the same, but in reality most hoses will collapse a bit when the pressure is turned off so the weight also will be a bit less. Even a fairly stiff garden hose will hold a bit more water when it expands under pressure.

            As for the water/electricity analogy, it is only that (an analogy) and it is limited at best. It was and is intended to provide a simple way for beginners to understand a bit about electricity and you can not draw too much about electric behavior beyond what was immediately intended from it.
            Last edited by Paul Alciatore; 09-27-2012, 12:24 AM.
            Paul A.
            SE Texas

            And if you look REAL close at an analog signal,
            You will find that it has discrete steps.

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            • #7
              Technically the wire might become stiffer, as it's generating a magnetic field that it then reacts with. There probably would be some stretching effect, which could make it stiffer.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                We used to tell some apprentices that an energized cable was heavier because it was full of electricity. The odd one even believed us!
                Location: Saskatoon, Saskatchewan, Canada

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                • #9
                  Originally posted by Arcane View Post
                  We used to tell some apprentices that an energized cable was heavier because it was full of electricity. The odd one even believed us!
                  That was an important fact to know, while the apprentice was on the way to get the wire stretcher, to hang from the skyhook, freshly painted with the plaid paint!

                  Cheers,
                  Jon

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                  • #10
                    It is heavier because it is full of electricity. Energy has mass along with everything else that isn't at absolute zero. Even photons have mass which is why they have momentum. They just don't have any rest mass which is why they cease to exist when brought to a stop.

                    The amount of extra mass due to energy is exceedingly small since the mass ratio of matter to energy is rather large being the speed of light Squared. However, small is not equal to zero.
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                    • #11
                      I guess in that context yes, it gains about as much mass as it does when you shine a light on the conductor... With my general resolution that is equivalent to 0 mass...

                      Cheers,
                      Jon

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                      • #12
                        Of course the practical answer is no, an energized wire isn't any heavier, but Evan brings up an interesting theoretical POV. If current is flowing in a wire, it is dissipating power (assuming it is not superconducting), which means there is a transfer of energy from the wire to the environment. I guess this would imply an energy content in the wire, which would add a mass equivalent.

                        If the wire were energised, but there is no current flowing, it looks like there would be no increase in mass. Does this seem right? the wire gets heavier only when current flows?
                        For just a little more, you can do it yourself!

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                        • #13
                          There is a potential difference in the wire to ground even if no current is flowing. It therefor does contain some energy in the relative frame of reference and therefor does have some extra mass. The same applies to molecules in respect of binding energy. The difference is called the "mass defect" since the binding energy represents a difference between the atomic mass and the energy mass.
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                          • #14
                            Hopefully Sheldon Cooper will chime in...........?
                            Max.

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                            • #15
                              Originally posted by Evan View Post
                              Even photons have mass which is why they have momentum. They just don't have any rest mass which is why they cease to exist when brought to a stop.
                              I object to this mutilation of terminology. Einstein and Lorentz both avoided using relativistic mass for a variety of reasons. It becomes unclear what role relativistic mass actually plays in the objective qualities of an object. In quantum field theory and general relativity, the word "mass" is ALWAYS understood to be "invariant mass" (i.e., "rest" mass). Unfortunately, relativistic mass is still widely taught in popular science books. It is a sin that everyone from Feynman to Hawking has committed in their popular science books.

                              In relativistic quantum field theory, relativistic mass is meaningless. In fact, even in relativistic quantum mechanics, relativistic mass is not used. The mass parameter is invariant (e.g. it's the rest mass that is characteristic of the system/particle). Instead, if we look at a simple example of trying to accelerate a massive particle to the speed of light, instead of interpreting the asymptotic behavior as increasing mass => smaller gains in velocity, we understand the situation as storing more and more energy in an internal degree of freedom.

                              Therefore, photons do not have mass and should never be said to have mass. They do have momentum, but momentum does not require mass. The basic Newtonian relationships are only approximations of a more fundamental physics. A convenient way to think about it is as follows: mass is a measure of matter and matter is that which has the freedom to explore the volume of the world cone. A photon (and the gluons and, presumably, the graviton) is confined to move only on the surface of the world cone and are not matter. Of course this distinction is one of semantics. This is a very practical definition and useful in the study of physics, but not everyone adheres to it.





                              Now that I have said all of that ... binding energy DOES lead to mass, although it's not entirely clear how this should be interpreted. Nevertheless, the energy of the three valence quarks "inside" of a proton is only a small fraction of the mass of the proton. The main contribution is from the "binding energy" of the quarks. Of course, you could just as well say that the mass comes from additional particles, rather than the "binding energy" (e.g., there is a whole host of virtual particles that pop in and out of existence and the parameters of this effect are controlled by the presence of three quarks in a color neutral state. Again, the concept of mass becomes blurred ... the proton looks like a mass eigentstate of an isospin symmetry and is relativistically invariant. So, increasing the energy of the proton doesn't increase the mass, even though we say the binding energy is responsible for its mass.

                              Funny how something as simple and intuitive like "mass" can become so complicated


                              Regarding the cable, you can also look at the energy stored in the electric and magnetic fields or the momentum flux density, represented by the Poynting vector. Furthermore, you might consider how magnetic fields are generated - it is actually a result of special relativity. When current is flowing, there are actually a few more electrons in the cable then there were when the cable was non-energized. In the water analogy, you might think of this as the compressibility of the water. Although it is very nearly zero, a change in pressure will result in a change in density. So you could make an argument that the cable gets stiffer and heavier when current is flowing, but for all intents and purposes, the answer is NO.

                              (ed_h - As I alluded to above, you don't need joule heating to indicate the presence of energy. Energy is stored in an electric and/or magnetic field. When current flows, some of this energy is converted into heat and released to the environment, but the energy exists in the fields whether or not current flows. Interestingly, this becomes a problem for particle accelerators. We generate an electric field and then expose a charged particle to the field. The particle "sucks" some of the energy out of the field. This can also affect the characteristics of the field (e.g. the "shape" of the field) so there is always a balancing act of how much energy we transfer to the particle and how much must remain in the fields)

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