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If electricity is the flow of electrons, how come it can flow at the speed of light? Shouldn't how fast it moves be limited to a speed lower than the speed of light because it has mass?

Shristeerupa
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  • Possible duplicate: https://physics.stackexchange.com/q/358894/365939. – VaibhavK Sep 29 '23 at 14:57
  • The electrons don't travel at speed of light (they travel at a drift velocity) – Razz Sep 29 '23 at 15:00
  • FYI: "Electricity" is kind of a broad topic. It covers phenomena as diverse as how electric motors work and what causes the Northern Lights, and why your socks stick together when you take them out of the dryer. When you are asking questions about the movement of charged particles (e.g., the movement of valence electrons in metal wires) then you are asking specifically about electric current. – Solomon Slow Sep 29 '23 at 15:13
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    Electricity is not the flow of electrons. Electric current is the flow of charge, but that charge is often not composed of electrons. – John Doty Sep 29 '23 at 16:13

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Electricity does not move at the speed of light. In a conductor the individual electron drift velocity is often surprisingly low, while the signals and energy are transmitted far faster. Electromagnetic waves move near the speed of light... but if the medium has an index of refraction $n$ the speed will be $c/n$. Similarly in a cable the capacitance and inductance produce a signal speed that is lower than in free space.

Anders Sandberg
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    Might be worthwhile to say something about what happens when a switch in the circuit is opened or closed... Something about how the news that the switch has been opened or closed propagates from the switch, in both directions, all the way around the circuit to tell all of those electrons when it's time to stop or start "drifting." I wish I could say something about it myself, but I don't know whether it can be said without diving straight into Maxwell's theory, and if there's no way to avoid that, then I'm not competent to explain it. – Solomon Slow Sep 29 '23 at 15:26
  • In RG-58 coax, the speed of a signal is about 1.5 times slower than the speed of light in free space. Of course, the cable velocity factor is a function of frequency for any given cable. – Jon Custer Sep 29 '23 at 17:11
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What happens is exactly the same as what happens in a garden hose that contains water heated by the sun. When you open the valve, water comes out immediately. It takes a while for cold water to come out of the open end. The start of the joint movement of the water molecules when the tap is opened is incredibly fast, but the individual molecules have a much lower flow velocity.

Exactly the same happens with an electric current. The information about changes in the current reaches the other end of the line almost at the speed of light, whereas the individual electrons move forward rather very sluggishly.

HolgerFiedler
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Consider a Newton's cradle. You lift up and release the ball at one end. Just before it strikes the next ball, it has a certain velocity $v$, which depends on how for you initially raised the ball. Very soon after the collision, the ball at the other end starts to move. The "motion" is propagated from the ball on one end to the one at the other at about the speed of sound $v_s$ in the material the balls are made of, which is much greater than $v$. This is in spite of the fact that at no point during this process is any one ball moving faster than $v$: there is just not enough energy in the system for this to happen.

This is largely analogous to the propagation of electric signals in wires. The electron drift velocity is like $v$, and the speed of light is like $v_s$.

Puk
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