ELi5: Why is the speed of electricity not the speed of sound?


I understand that the electrons themselves in a material travel fairly slow, but I’m talking about actual signal propagation. The speed of an electrical signal traveling through copper can achieve very high fractions of c, but the speed of sound in copper is only around 5010m/s. What causes the disconnect?

In: Physics

Electricity isn’t sound?

Sound is moving the atoms of the media, the copper wire in your example. This takes time, as the atoms are held together with metallic bonds.

Electricity is a change in electric potential. It’s not an electron traveling the length of the wire, it’s just bumping into the first electron, who bumps into the next, … to the end. The “speed of electric field bumping”, is unrelated to the speed of sound as the atoms themselves don’t actually move.

The speed of sound in copper? Do you mean how fast a sound signal travels through a copper cable?

Electricity is not electrons moving as such. It is the force of the electrons that moves. Just like sound is not the coppar atoms moving but the force of these atoms moving. So when you are saying that the actual electrons move slowly through the coppar that is because they always bumb into other electrons and transfer their energy to these. Just like when you send sound through a coppar bar the atoms themselves takes quite a long time to move any distance but the force that causes them to move is able to go through the bar quite quickly.

So if you were to send one electron worth of current down a wire it would move close to the speed of light because electrons are light. But this first electron would crash into another electron and this other electron would be the one going almost the speed of light. At least for a brief moment as it would crash into yet another electron.

The speed of sound in a material relates to how quickly a part of it will move when a force is imparted on it. For example, if you had a many km long pole made of copper and then you pushed one end the other end would only move once that force had moved through the material and this moves at the speed of sound.

Electricity moving through a conductor is an entirely different process.

You’re getting a lot of wrong answers here. It’s not about electrons pushing each other. That *would* be sound. The signal is an electromagnetic wave, i.e. light. The wire is just guiding the path of the electromagnetic wave, but no part of it *is* the wave. The electrons move as a *result* of the EM field, not the other way around.

The analogy that many people think of, with electrons acting like water in a hose, is useful up to a point, but it is still very different from reality.

**An electrical signal travels at the speed of light**, which in copper is about 200000km/s.

If electrons were only pushing against each other either by bouncing or through their electrostatic charge, then the wave would be traveling at the speed of sound but this is not what happens in reality.

When you connect a voltage source to a cable, the difference in potential between the positive and negative sides produces an electric field. This field travels through the cable at the speed of light and is responsible of the movement of electrons.

To answer your question, the disconnect happens because electrons are not only pushed by other electrons but also by the electric field traveling through the cable, which acts as a waveguide.

For instance, the average speed of electrons is very low and it’s called “drift velocity”.

EDIT: see the answer below, by u/ChaosSlave51 for a more physically accurate explanation.