Wikipedia:Reference desk/Archives/Science/2021 September 20

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September 20

Which reach first at the other end of the cell?

The conventional direction of flow of current is from the positive terminal of the cell to the negative terminal of the cell through the external circuit. But Electrons flow from the negative terminal to the positive through the circuit. So which reach faster at other end of the cell: free electrons or current? Rizosome (talk) 06:32, 20 September 2021 (UTC)

Conventional current is not a real physical flow. It is a mathematical trick traditionally used in circuit analysis that works because things like Kirchhoff's circuit laws work when thought of as electron flow or when we pretend something flows in the other direction. Nothing flows from the positive to the negative. 85.76.72.46 (talk) 07:26, 20 September 2021 (UTC)

That's pretty much the same. Be aware the electric current flow in a circuit is not like a fountain, where water appears and disappears as pumps start and stop. It is rather like piping system of a heating circuit - metallic conductors, making a circuit, are literally full of free electrons ('electricity particles'), same as water pipes are full of water molecules. When you close a circuit (remove an isolating gap) a directed movement of electrons appears - and that is current. Similarly, when you remove an obstacle in a water circuit by opening a valve, water starts to run. Then, of course, there is some time needed for the electrons from one end of a cell to arrive at the other end, just like some time is needed for water molecules pushed from one end of a water pump to arrive at the other end of the pump. However the flow (electric or water current) appears almost immediately in the whole circuit. Additionally, electrons are not distinguishable, so you can not tell when 'the same electrons' come at some point which left some other point some time ago. They have some average velocity which can be determined, but some may move slower and other move faster than that, racing each other. So the question when 'they' (meaning 'precisely the same') reach the other terminal of a cell can not be answered precisely. However the 'mean' flow of electrons reaches the other terminal precisely at the same moment as the current, because that is current. --CiaPan (talk) 10:04, 20 September 2021 (UTC)

There is a flow of traffic from New York to San Francisco, formed by moving cars. Which reaches faster at the other end of the road: the moving cars or the traffic flow? The question is meaningless.  --Lambiam 23:18, 20 September 2021 (UTC)

Actually, it's better conceptualized as the amusement park ride Caterpillar (ride), where there are no "ends". After all, it is an electric circuit. Electrons do move, in a sense, but as you can't actually track the motion of a single electron anyways, thinking about what one does is a meaningless exercise. You can describe the bulk motion of the electrons, what you're looking for there is called the Drift velocity of electrons. In a direct current, there is an example worked out in the drift velocity article. Assume a current I = 1 ampere, and a wire of 2 mm diameter (radius = 0.001 m). This wire has a cross sectional area A of π × (0.001 m)2 = 3.14×10−6 m2 = 3.14 mm2. The charge of one electron is q = −1.6×10−19 C this comes out to 23 micrometers per second; at that speed a hypothetical "electron" (which again, we can't track, but let's pretend we can for demonstration purposes) would travel about 1.38 millimeters in a minute, or 8.3 centimeters in an hour. Electrons move pretty slow. But again, this belies the fact that we can't identify (even hypothetically; it's not a limit of our technology, it is baked into the physics) individual electrons to say where they are going. We can just measure the group of electrons, and describe how fast the bulk of them moves, on average, statistically. --Jayron32 16:03, 21 September 2021 (UTC)