Why Don't Wires in Our Homes Attract or Repel Each Other Despite AC Currents?

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Discussion Overview

The discussion explores why wires in homes do not attract or repel each other despite the presence of alternating current (AC) which creates changing magnetic fields. Participants examine the implications of current flow in household and power line wiring, considering both typical and extreme conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that a changing current in a wire generates a magnetic field, questioning why nearby wires do not exhibit noticeable attraction or repulsion.
  • One participant asserts that while wires do exert forces on each other, these forces are too small to be observed in everyday situations.
  • Another participant raises the question of how larger currents in power lines might affect the forces between them compared to household wiring.
  • It is suggested that the relationship between voltage and current in distribution lines may lead to different current levels than expected, prompting a potential calculation of forces based on typical spacing and current values.
  • Some participants highlight that typical appliance cords and coaxial cables have two wires carrying currents in opposite directions, which may influence the net magnetic effects.
  • One participant mentions that significant forces in power lines can occur during short circuits when currents are much larger than normal operating conditions.

Areas of Agreement / Disagreement

Participants generally agree that wires do create magnetic fields and exert forces on each other, but there is no consensus on the significance of these forces under normal conditions versus during short circuits. The discussion remains unresolved regarding the specific calculations and implications for power lines versus household wiring.

Contextual Notes

Participants express uncertainty about the typical currents in power lines and the assumptions underlying the formulas for magnetic fields and forces, particularly in the context of AC systems. There is also a lack of detailed calculations to support claims about the forces involved.

PhysicsInNJ
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Apologies if this is being posted in the wrong section! this is just a side thought I came across, it is not HW.

In learning about magnetism, I know that a changing current in a wire creates a magnetic field. So why are wires in our homes/ anything with wires close together not constantly attracting or repulsing each other?
At first I thought this was because they are not changing except when turned on/off which is such a small amount of time that the B field is negligible. But then I remembered that most homes have AC which is constantly changing anyways.
 
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PhysicsInNJ said:
a changing current in a wire creates a magnetic field.

Actually, any current, whether constant or changing, creates a magnetic field.

So why are wires in our homes/ anything with wires close together not constantly attracting or repulsing each other?

They are! However, these forces are too small to be noticeable under everyday conditions.
 
Ok, can see why household wires have negligible effects. But what about wires carrying large currrents, like power lines?
 
Distribution lines carry larger voltages and therefore smaller currents for equal amounts of power. The lines running along your street probably carry more current than a typical circuit in your house, but not by as much as you would expect if the voltages were equal. I haven't actually worked out the numbers myself, but it would be a nice exercise to find out the typical currents in those lines, and the typical spacing between them (1 m for neighborhood distribution lines?), and calculate the force per length of wire.

It also might make a difference that the usual formulas for magnetic fields and forces assume constant currents, whereas electical power systems are AC. The average force between two parallel wires probably depends on the relative phase of the currents in them.
 
Typical appliance cords and coaxial cables have two wires flowing in opposite directions.
 
The forces in power lines become significant during short circuits when the currents are many times larger than normal.
 

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