Naked Science Forum
General Science => Question of the Week => Topic started by: EvaH on 19/05/2020 09:50:25
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Rakesh wants to know:
So typically when electrons flow for the electric current, do they come out from the atoms and flow as electric current? Is it not true that when electrons come out from atoms light and energy is released? So why don't electric wires change their colours?
What do you think?
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In a metallic conductor, the conduction electrons are already "free" and delocalised in a conduction band, i.e. not associated with any particular nucleus, so they drift with the applied field like a classical gas, without undergoing any quantum transitions.
Matters are different with semiconductors. Typically a light-emitting diode demonstrates quantum transitions with photon emission (hence light-emitting!) but even these are a property of the condensed structure rather than the individual atoms.
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Is it not true that when electrons come out from atoms light and energy is released?
It actually takes energy to liberate an electron from an isolated atom or molecule (eg in a gas, or an electrical insulator).
- This energy can come in the form of a high-energy photon, like ultraviolet light, which will expel an electron from an atom.
See: https://en.wikipedia.org/wiki/Photoelectric_effect
When the electron rejoins the isolated atom, energy is released, often as a cascade of lower-energy photons; for molecules, some of this energy is turned into vibrations, which we sense as heat.
However, with metal atoms formed into a wire, the electrons are not isolated in different atoms or molecules, they are in physical and electrical contact with nearby atoms.
- These electrons are described as being in the "conduction band": when you apply a battery to the wire, these negative electrons will drift towards the positive terminal of the battery, to be replaced by new electrons from the negative terminal of the battery
See: https://en.wikipedia.org/wiki/Valence_and_conduction_bands#Electrical_conductivity
So typically when electrons flow for the electric current, do they come out from the atoms and flow as electric current?
Yes, that is a reasonable description, but to add some details....
- Electrons in a solid or liquid metal are not associated with a particular atom - they tend to float freely between and around the atoms - you could imagine the electrons floating around like a gas in a pipe (the wire).
- So you don't need to inject energy to make an electron break free from a particular atom - it is already free
- And the electron does not release energy when it attaches to another atom, because it remains free
Removing the electrons from an atom doesn't take energy, but the electron gains energy by being accelerated by the battery voltage.
- At room temperature, these moving electrons keep crashing into atoms in the wire.
- This transfers energy from the electrons into the atoms of the wire, and the wire gets hot
In contrast, near absolute zero (-273C), some metals become "superconductors"; the behavior of the electrons change, and they can flow through the wire without crashing into the atoms. The most familiar example of superconductors is the powerful magnets of an MRI machine.
So why don't electric wires change their colours?
They do: If the wire heats up, it will change colour from a dull red, to orange and yellow: you have an incandescent lamp
- This energy comes from the moving electrons bumping into the metal atoms.
Oops! Overlap with alancalverd..
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And we've got the answer!
https://www.thenakedscientists.com/podcasts/question-week/how-do-electrons-flow-electric-current