Pergerakan elektron, cepat atau lambat?

Berdasarkan pengamatan dan pengalaman sehari-hari, mudah bagi kita untuk menganggap listrik itu cepat. Misalnya, saat kita memanipulasi (menekan) sakelar (switch) maka, dalam persepsi kita, saat itu juga lampu bohlam menyala (atau padam, tergantung keadaan awal). Saat ada orang yang menyentuh bagian peralatan atau konduktor lainnya yang teraliri listrik (hot), saat itu juga biasanya orang tersebut beraksi. Tanpa pelindung, orang itu mengalami fenomena yang kita sebut kesetrum (tersengat aliran listrik, electric shock) dan dapat mengakibatkan kematian (electrocuted).

Akan janggal kalau kita mendengar bahwa aliran elektron itu biasanya justru lambat. Listrik itu identik dengan elektron, kalau listrik cepat, bagaimana mungkin elektron bergerak lambat?

Ya, jawaban cepatnya memang elektron itu bergerak lambat, tidak seperti aliran energi listrik dari muatan yang dibawanya. Salah satu sumber (yang nanti bisa anda baca sendiri) menyatakan bahwa pergerakan elektron itu sangat lambat, pergeserannya hanya berkisar pada satu meter per jam. Bagaimana mungkin?

Seperti banyak halaman lain yang coba saya susun, untuk hal-hal seperti ini ada bagian yang saya mengajak pembaca untuk menyimpulkan sendiri dari sumber-sumber bacaan yang saya sediakan (rujuk). Juga dari mendengar dan menonton sumber-sumber belajar yang relevan. Mengapa begitu? Karena biasanya setiap saya menulis yang pertama saya bayangkan adalah para pelajar (mahasiswa atau lainnya). Untuk merekalah (semua yang sebangsa dan setenah air di manapun berada) saya mencoba berbagi. Kalau sekedar kumpulan fakta, sudah banyak sumber di Internet (seperti yang saya rujuk), persoalannya tinggal melakukan penapisan (filtering).

Pertama untuk pertanyaan atau keingintahuan seperti ini, ada baiknya untuk mencari di situs tanya jawab atau diskusi umum seperti Quora. Jika dapat akses yang baik, bisa juga mencari di Reddit. Baru kemudian ke situs seperti Stackexchange. Wikipedia juga bagus dijadikan sebagai awalan untuk memahami lingkup persoalan / bahasan.

The speed of light, often called “c”, in the lower case, commonly refers to the speed of light in a vacuum, which is the fastest it will go. Its about 300 million meters per second.

The propogation of electric field, or electrical signalling using electrical signals in a wire is a bit slower. It can be anywhere from about 50% of c to 99% of c, depending upon the wire and insulation composition and construction.  See the wiki article Velocity factor.

There’s another speed which is the drift velocity. This can be considered like tracking an individual electron and its very slow, on the order of fractions of meters per second. You can not picture electrical signal propagation as s single electron or a batch of electrons travelling in the wire from one end to the other at 75% of the speed of light… you must consider that the electrons bump into each other and trade places with a few holes to propagate the field, much as when you hear a sound at a distance of 1000 feet, the sound presure on your ear is not the molecules of air originally moved a thousand feet away but a compression wave moving from molecule to molecule over that distance.  See wiki article Drift velocity

~ Loring Chien quora16

How fast does electricity flow? This is a good question, because it seems like a simple enough question, but usually it indicates some underlying misconceptions. The first difficulty in answering the question is knowing, what is electricity? Do you mean:

1. How fast do changes in electrical fields propagate? or…
2. How fast do electrical charge carriers move?

Usually, people asking this question actually care about the former, but are thinking about the latter. However, not having a clear understanding of the difference, their underlying concern actually can’t be addressed without stepping back and addressing the underlying misconceptions which lead to the question.

Understand is this: there are forces, and there are things that transmit forces, and they are not the same thing. Here’s an example: I’m holding one end of a rope, and you are holding the other end. When I want to get your attention, I tug the rope. There is the rope, and there is the tug. The tug travels as a wave of force down the rope at the speed of sound in the rope. The rope itself will move at some other speed.

Say I have two lookout towers, and when I see the approaching invaders, I shout to the other tower. Sound will travel as waves in the air at the speed of sound. How fast are the molecules in air moving? Do you care?

Some people won’t let this go until the motion of the molecules is actually explained, even though it’s usually not relevant to their concerns. So here’s the answer: the molecules are flying around in all random directions, all the time. They fly around because they have non-zero temperature. Some are very fast. Some are very slow. They bump into each other all the time. It’s very random.

When you shout, your vocal tract compresses (and rarefies, as your vocal cords vibrate) some of the air. The molecules in this compressed region want to move to a region with less pressure, so they do. But now this nearby region has too much air, and is a little more compressed than the air around it, so the compressed region expands outward a little more. This wave of compression moves through the air at the speed of sound.

All of this happens superimposed on the random motion of the molecules previously mentioned. It’s unlikely that the same molecules that were in your vocal tract will be the ones that vibrate in the listener’s ear. If you watch individual molecules, you will observe them going in all directions. Only if you observe a lot of them will you notice that slightly more went in one direction versus another. It is true for all things we would call “sound” that the random motion of the molecules due to thermal noise is much more than their motion due to sound. When the “sound” becomes the more relevant motion, we tend to call it not “sound” but rather an “explosion”.

The situation with electricity is not much different. A metal conductor is full of electrons that are free to wander around the entire circuit in random directions, and they do, simply because they are warm. Things in our circuits make waves in this sea of electrons, and these waves propagate at the speed of light1. At the currents we typically encounter in circuits, most of the electron motion is due to thermal noise.

So now we can answer the questions:

How fast do changes in electrical fields propagate? At the speed of light in the medium in which they are propagating. For most cables, this is in the neighborhood of 60% to 90% of the speed of light in a vacuum.

How fast do electrical charge carriers move? The velocities of individual charge carriers are random. If you take the average of all these velocities, you can get some velocity that depends on the charge carrier density, and the current, and the conductor’s cross-sectional area, and it’s typically less than a few millimetres per second in a copper wire. Above that, resistive losses become high in ordinary metals and people tend to make the wires bigger instead of forcing the charges to move faster.

Further reading: Speed of Electricity Flow by Bill Beaty

1: The speed of light depends on the material in which the light is propagating, just as with sound. See Wave propagation speed.

~ Phil Frost 

The individual electron velocity in a metal wire is typically millions of kilometers per hour. In contrast, the drift velocity is typically only a few meters per hour while the signal velocity is a hundred million to a trillion kilometers per hour. In general, the signal velocity is somewhat close to the speed of light in vacuum, the individual electron speed is about 100 times slower than the signal velocity, and the electron drift speed is as slow as a snail.

Science Questions with Surprising Answers

Untuk mempelajari lebih lengkap tentang bahasan ini dengan membaca dan membandingkan antara satu sumber informasi dengan sumber informasi lainnya, saya sudah mencoba mengumpulkan sumber-sumber belajar yang menarik. Sumber-sumber ini telah saya pilih dari sejumlah sumber yang dapat ditemukan dengan mesin pencari. Hasilnya dapat diakses melalui link pada kumpulan RSS berikut (situs eksternal, Bundlr)

Salah satu cara yang menarik dan mempermudah proses belajar di era informasi ini adalah dengan menyaksikan tayangan audiovisual. Dengan menonton video yang sering tidak hanya berisi ceramah atau narasi tetapi juga visualisasi menarik seperti animasi, proses untuk memahami sesuatu yang bahkan bersifat abstrak dapat berlangsung dengan lebih efektif.

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