Why are electrical wires made up of loads of tiny copper fibres instead of one fat copper wire?

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Title essentially. What advantage does having loads of fibres have over one solid piece? Flexibility?

In: Physics
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In general, the electrical power that is supplied to households and for most industrial uses is “Alternating Current’ or AC.

One of the properties of AC is that it has tendency to flow on the outer surface of any conductor. This phenomenon is called the ‘skin effect’.

That means that the flow of AC is most concentrated near the outer circumference of a wire, instead of being evenly distributed throughout the whole cross-section of the wire. This tendency increases with the frequency of AC.

To take advantage of the skin effect, a cable will be made out of numerous fine wires instead of one thick wire, in order to increase the surface area of the conductor for the same amount of the material.

A multi-strand wire also offers more mechanical strength then a single-strand wire. Is more flexible and resilient to damage.

You can get both types. Generally speaking the thicker the wire, the more current it will be able to safely carry, but yes it’s less flexible. By having many strands the idea is that you get the best of both (with individual strands breaking not leading to a total failure). As a result the wire within the walls is usually thick, single core (because it never has to move) with only the cabling that gets manipulated by people needing the flexibility that many strands provide.

Exactly right, the smaller wire-strands and jacket type make that kind of wiring much easier to work with. Think about the power cable for your TV or charging cable for your phone being difficult to bend.

Some wiring (like in your walls) is a solid strand of copper, but it doesn’t need to be flexible and doesn’t move after being installed.

A solid stand would get more stress being constantly bent over and over, and would break quite quickly.

You’ll see there are many types of wire. You’re referring to stranded wire. (See Under “Forms of wire” https://en.m.wikipedia.org/wiki/Wire).
In terms of mechanical properties stranded wire is more flexible and pliable making it easier to handle. It’s not so rigid. In terms of electrical properties it seems to be better for high frequencies

Solid wires are difficult to bend and limits how tight of bend you can make before damaging the wire. Solid wires also are heavier since they have metal filling the spaces between the bundle of thin wires.

With that said, for high current applications, you still need solid wires to transfer the power without excessive losses.

Flexibility and durability. You should always find the wiring in your vehicle with multi-strand cores. Solid core wires break a lot when placed in physically stressing environment, like a vehicle where the frame is vibrating a lot and subjected to a bunch of stretching aand warping as the vehicle shifts during turns, acceleration, and braking. Most applications that have a lot of vibrations will use multi-strand core wires as well.

Homes tend to have solid core wiring since they are generally stable and don’t move much.

Solid wires are not flexible and will break when you try to bend them. So there is an upper limit to the size a cable can be with a single wire before it cannot be used save for very straight run applications.