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We know that when currents in two wires move parallel to each other, they attract each other and if they are moving anti-parallel to each other, they repel each other but we cannot observe this in our daily routine; why?

I know this experiment is based on DC circuit and we cannot observe this in electric transmission lines because in our homes we usually use AC.

But what is the actual reason behind that we cannot observe an attractive or repulsive force in daily circuits? If actually it is because of AC currents, then why does AC current not show magnetic effect?

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  • $\begingroup$ According to this, one can calculate: the force per unit length of a cable carrying $20A$ with $1cm$ appart, is $8$ milinewtons/meter. Not that great.... $\endgroup$ – Physicist137 Nov 15 '16 at 15:30
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    $\begingroup$ @Physicist137 Yes, but if you have e.g. a coil of wire in a transformer, the wires are under a millimeter apart, the field is significantly magnified by the ferromagnetic core, and you have many coils acting. The coils won't move, because they're glued in place, but they do 'breathe' as the current oscillates, and this is what causes the hum of a transformer at 50-60 Hz. $\endgroup$ – Emilio Pisanty Nov 15 '16 at 19:08
  • $\begingroup$ (ahem, scratch that: at 100-120 Hz, since the direction of the current doesn't matter, or most probably one of its harmonics, at 400-480 Hz, which is deeper in the human hearing range.) $\endgroup$ – Emilio Pisanty Nov 15 '16 at 21:43
  • $\begingroup$ You’ve never seen speaker wires jerk due to EMF? $\endgroup$ – JDługosz Nov 15 '16 at 23:06
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We absolutely observe this in daily life. Every time you see a motor running, you see the effect of the force of currents on each other. I recommend the following experiment:

Create an apparatus with two parallel conductors (fairly close together) where you can adjust the tension in the wires (like a guitar - but make sure you take care of insulation). "Tune" the tension in the wires to twice the frequency of the mains power in your country (50 Hz or 60 Hz). Then make the wires part of an electrical circuit. You will see them starting to vibrate - confirming that there is a force (with a frequency equal to the frequency of the mains power) between the wires. Why twice? Because if the currents in the two wires are in phase, the direction of force will be the same twice per cycle - so a 50 Hz mains frequency will excite a string tuned to 100 Hz.

Usually, conductors run in pairs, with a fixed distance (insulator) between them. Such a configuration prevents you from noticing the force between them. Note also that the force is not large - $2\times 10^{-7}~\rm{N}$ for two wires that are 1 m apart and carrying 1 A of current. Which makes it hard to notice unless the currents are very large, or you set up a sensitive experiment.

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    $\begingroup$ One nice thing: the force of two wires 1m carrying 1A, is the very definition of the ampere itself. =). $\endgroup$ – Physicist137 Nov 15 '16 at 15:41
  • $\begingroup$ @Physicist137 Not for long. $\endgroup$ – Emilio Pisanty Nov 15 '16 at 19:02
  • $\begingroup$ Cool YouTube experiment video time? $\endgroup$ – dbanet Nov 15 '16 at 20:52
  • $\begingroup$ @dbanet - the thought crossed my mind... I found this one which does something similar (but uses a permanent magnet and a single string). Easy to see how you could adapt it. $\endgroup$ – Floris Nov 15 '16 at 20:55

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