How does electricity transfer energy?

Question

I am new to electronics and was just looking for some intuition behind how electrons give energy to components.

I have been attempting to understand this under the condition of voltage being the measure of the pressure of electrons, while current is the flow of electrons through a given wire cross-section. Essentially, the best explanation I have come up with is that elements (resistors, lights, motors, etc.) decreases the momentum of the electrons. I will describe the following only in a simple circuit of elements in series with a source.

Given momentum P as mass*velocity, for a given cross-section since the flow of electrons is steady (velocity or current is constant) and density or voltage drops.

This also allowed me to understand AC sources as the momentum drops on let's say the right side of an element, and as the AC voltage becomes the right side builds back pressure or voltage and forces the flow to lose momentum getting to the left side of the element.

I now ask whether this is correct or if there are any insights to add. Also, I'd love some understanding of how components take the momentum/energy of the electron by simply having it pass through. Additionally, on top of that could we create circuits exactly in the same way but using other fluids or particles instead of electrons or are electrons special?

Answer 1

I'm afraid you're headed off into the weeds. Electrons do not transfer useful energy via their mechanical momentum — although that is how electrical energy gets transformed into heat.¹

Electrons are charged particles; they gain energy when they are forced to move "uphill" with respect to the potential gradient of an electric field. This can be done in a number of ways — most commonly by the use of magnets (in a generator) or by chemical reactions (in a battery).

That energy is released with the electron is allowed to move "downhill" along the potential gradient. This can be used to create another magnetic field (in a motor, solenoid or transformer), to create light (in the P-N junction of an LED), or to control transistors by manipulating the fields and currents inside them.

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¹ When electrons hop from atom to atom in a conductive or resistive material, some of their mechanical momentum gets coupled to the nucleus of the atom, causing it to vibrate a bit. The cumulative effect of all this random vibration is what we call heat.

Answer 2

The nearest thing to what you are describing is the Drude Model, proposed around 1900.

It sort of explains electrical conduction in some metals, but can't be extended to cover them all properly, and fails horribly for semiconductors.

'Ballistic' electrons are quite a good fit where energies are high and their density is low, in vacuum tubes and plasma for instance, but in metals, not so good a fit.

The hydraulic analogy is greatly under-rated, as it has potential baked right into the intuitive model, something that people concentrating on electrons often have a hard time conceptualising. You can make and understand a DC-DC boost converter with hydraulics for instance. What is usually considered to be a big flaw, that it's manifestly non-physical, I see as an advantage. When it's time to let the model go, either 'down' to the pure charge and voltage of circuit theory so you can get on with some electrical engineering, or 'up' to the quantum mechanics needed to engineer semiconductors, it's easy to let it go. Electrons seem just so enticing, it can be more difficult to let them go when you need.

Answer 3

Since momentum is always conserved, if a resistor could take momentum from an charged particle it would have to recoil when exposed to current to carry that momentum. However, resistors do not experience a force and are not pushed around on circuit boards by current, so your understanding is not accurate. Instead of momentum, electrical loads extract potential energy from current, and since energy is also conserved, they become hot (resistors), emit light (LEDs), or can perform work (motors). You can see this from the units of voltage, which are energy per charge. The voltage between two points is therefore how much potential energy each unit of charge has as it moves from one point to the other.

Answer 4

I would have to say that I think that the idea of "momentum of the electrons" is misleading and useless. The amount of momentum that the electrons have is, for all practical purposes, zero. Energy is transferred through the electric and magnetic fields, not by means of momentum.

The question "how do components take the momentum/energy of the electron by simply having it pass through?" simply is not meaningful. That's not how it works.

Here's how electric current transfers energy, from an electrical engineering perspective. When positive charge moves in the same direction as the electric field (or, to be precise, a "more same than opposite" direction), it releases energy, much as a heavy object releases energy when it is allowed to move downwards. When positive charge moves in the opposite direction to the electric field (or, to be precise, a "more opposite than same" direction), it takes in energy, much as a heavy object takes in energy when it is allowed to move upwards.

(That is not the only way energy is transferred electromagnetically. Energy can also be transferred in the absence of a current, if there's a magnetic field.)

You might be wondering, "How, exactly, does an electric charge take in or release energy when it moves in the presence of an electric field?" I don't know how to answer that question. What I will tell you is that that's a physics question, not an electrical engineering question. There's nothing wrong with that, of course! If you want to study physics, study physics. And if you want to study electrical engineering, study electrical engineering.

Answer 5

My understanding is that at the end energy is transferred by electromagnetic waves, so variations in the electrical and magnetic fields.

A charge has an electric field, and when you move it, it produces a magnetic field. The electromagnetic field influences other particles around, close or far away, and this influence travels empty space at ligth speed. This is what transfers the energy.

The energy 'travels' from one particle to another. For example, from one side of a capacitor to the other side, if you change the voltage up and down fast. Or if you push a magneto near another magneto, the energy applied to the first one will be 'perceived' by the second one. Electric and magnetic forces are different sides of the same coin.

Answer 6

It's always nice having people that try to understand the physics of electricity and possibly give a contribution.

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If you want to describe the loss of Energy of electrons during their travel in electrical components in terms of inelastic collisions and scattering you have to deal with Maxwell's equations.

You have to divide the circuit in subsections, apply Maxwell's equations in each subsection, make assumptions and derive your cross-section equations.

You also have to take into account the physics of the material where electrons flow: copper, Silicon et cetera.

I think it's not a trivial task. It's something that researchers might already have investigated.

It's really important the assumptions you make because they simplify the problem.

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Back in 1996 I simulated an HEMT transistor using IBM's DAMOCLES semiconductor devices simulator. That simulator used the principle of inelastic collisions and scattering between electrons and the underlying lattice.

You might want to start from studying the Poisson's equations used by DAMOCLES:

https://chairs.utdallas.edu/biographies/dr-massimo-max-v-fischetti/

https://link.springer.com/chapter/10.1007/978-1-4757-2124-9_16

Answer 7

A short answer: an explanation from physics is that electrical energy is transferred by electromagnetism via photons.

This interaction can be expressed with a Feynman diagram. Read more about these here: https://physics.stackexchange.com/questions/576435/simple-explanation-for-feynman-diagrams

The study of this interaction is within the field of Quantum Electrodynamics (QED). More here: https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics_III_(Chong)/05%3A_Quantum_Electrodynamics

The Drude Model of collisions and momentum is useful for classical electricity work as described by Ohm’s Law and Maxwell-Ampere. But the Drude Model fails to explain, for example, how radio waves propagate in a vacuum where there are no electron-electron collisions happening.

Further, electrons in a conductor move much more slowly than the speed of light, at Fermi velocity, yet electromagnetic waves in transmission lines travel at a significant fraction of C.

How? Again, back to photons. We can observe photonic energy transfer with an LED, which emits photons when electrons move from a higher energy state to a lower one. We can feed that light to a solar cell, which captures photons which push electrons to a higher energy state.

Conductors work the same way, we just can’t observe the photons visually.

Answer 8

The electrons do not lose momentum on their way from one place to another. What they lose is electrical potential energy, and due to conservation of energy, that energy is transferred to whatever the electron encounters along its journey.

Say you have a 9V battery powering a resistor and an LED in series. Each electron starts its journey at the negative terminal of the battery, having 9eV (nine electron-volts) of electrical potential energy. It's rather like an object being released from 9m height, and that will begin to fall under gravity, accelerating towards the ground. In this case, electrons will "fall" towards battery positive.

If permitted to "fall" the entire way from the negative terminal, through the resistor and LED, all the way to the battery positive terminal, an electron will arrive with 0eV of potential energy, having donated all 9eV to the various elements it encountered on the way.

The battery imposes 9V of potential difference between the two ends of the resistor/LED chain. That's another way of saying that charges at one end of the battery have 9 electron-volts (9eV) more energy than charges at the other terminal. This establishes an electric field along that chain. It is this field that imparts a force on every electron present within that field, causing them to accelerate, gaining kinetic energy, and propelling them towards the battery positive.

As it travels, an electron encounters different obstacles that hinder its acceleration. Sometimes it's atomic nuclei in the way, which will take kinetic energy from the electron (due to electrostatic forces between them), and cause heating. That's what happens in the resistor.

Sometimes the electron will pass through a magnetic field from some source outside the circuit. Its own magnetic field (due to charge motion) will interact with that field, causing the external source to experience a force, and accelerate. This is what happens for electric motors, and it will take away the electron's kinetic energy.

When the electron encounters the LED's PN junction, it must make a rather sudden leap to a place where it would have several electron-volts lower potential energy. This is like jumping off a cliff, but the extra kinetic energy possessed by the electron when it arrives at the bottom is released as light as it re-combines with "holes". Again, that's kinetic energy that the electron had momentarily gained, but has now lost to the environment.

All this means that the electron is never permitted to accelerate freely, and the kinetic energy it arrives with at the battery positive terminal is negligible. Therefore, the falling object analogy is flawed. Perhaps it's better to imagine a ball rolling down a hill, never able to gain much speed, instead donating any kinetic energy it acquires to bending grass, and thumping trees.

It starts its journey with negligible kinetic energy, and 9eV of potential energy, and it ends its journey with negligible kinetic energy and 0eV of potential energy. All 9eV was delivered to the environment along that journey, the electron never really got very fast, and we can ignore the kinetic energy (mass and momentum) of the electron.

That story is not necessarily true, because it's possible for any individual electron to do something absolutely out of character at any time, do its own thing, and completely fail to follow the rules I just described. However, we are talking about trillions of electrons, and on average they will appear to have behaved exactly as described.

Also, the electron isn't something you can pin-point and say what it's doing. It's a quantum object, with only probabilities describing where its likely to be found, and where its likely to be going (and how fast). As such, everything I just said about individual electrons is rubbish. I stress that this behaviour is only average behaviour when many, many electrons are involved.

In reality what's going on is far more complex, but relatively easy to describe. When there are electrons bunched together more densely in one place, and less densely in another, all electrons will experience a force accelerating them in a direction which corrects this imbalance. The imbalance is what we call "potential difference", and what you described as "pressure". It is this imbalance which endows an electron with potential energy, and which gives rise to different "voltages" (potentials) around the circuit. This doesn't mean the imbalance is huge, it generally isn't, unless we are talking about thunder-clouds or other extreme conditions.

When you measure a potential difference of 10V, this means that electrons are slightly more densely packed at near the lower potential end, and they all experience a force pushing them towards the higher potential end. However, as I said before, this is an "average" state of affairs. In reality what happens is that charge migration occurs in waves, rather like sound propagates through the air in waves of compression and rarefaction.

These waves travel through the medium (the "sea" of electrons) very fast, something like half the speed of light, and energy is carried on these waves. This is the actual mechanism of electrical energy delivery in the circuit, and even though the individual electrons have very little average velocity (millimetres per second, perhaps), when any single electron moves, it has a knock-on effect on the next electron, and that electron pushes on another, and so on, carrying energy at near-light-speed.

Strictly speaking, a physicist would argue that all charges in the system interact by exchanging photons, giving rise to "coulomb forces" repelling and attracting them. They would say that all these waves are electro-magnetic (light waves), since the electric field and all the forces on charges within it are mediated by photons of light. At an even deeper level, it's not even appropriate to refer to individual charges. Speaking in quantum terms, everything going on in there is probabilistic, there's no such thing as an electron, only a "cloud of something" that behaves as if it were lots of classical electrons. How you understand all this depends on how far down the rabbit hole you want to go.

The waves reflect, diffract and refract in the same way sound waves, or water waves do. The conditions at any point in any conductor in the circuit are the superposition of waves which are travelling through that point at any instant, waves that have reflected off boundaries where electrical impedance suddenly changes (like sound waves reflecting off a wall), for example.

It sounds like a mess, but it's not. On average, the result is very ordered. We may predict with great precision the average velocity of electrons passing any point, any we can predict exactly the average electrical potential energy of electrons (the voltage) at any point in the circuit. Even though the individual waves contributing to conditions at some point seem chaotic and even random, they are well choreographed, and their superposition is highly predictable.

As for your point about fluid circuits, that's what hydraulic systems are. They function on the same principle of waves of compression and rarefaction that travel through the fluid at the speed of sound, reflecting, refracting and diffracting, and superimposing on other waves to produce very predictable pressure gradients (analogous to electric fields) and fluid flow (analogous to electron flow). When dealing with trillions of molecules, hydraulic systems obey very similar mathematical relationships, like power = flow × pressure difference, and suffer very similar problems such as reflections at the ends of long fluid lines, requiring proper damping (transmission line termination).

Pumps are like batteries, valves are like variable resistors and switches, fluid lines can "balloon", like electrical capacitance, pistons have momentum, like electrical inductance, and so on. And the mathematics is very similar between the two fields.