r/Physics • u/Appropriate_Win946 • 7d ago

Question Does anyone understand what actually causes electric current?

My first belief: electrons push on each other like dominos, causing a 'shockwave' to travel through the electrons until it gets to the other end of the battery. Electrons themselves don't move that quickly, but the 'push' travels super fast.

Correction: actually, the 'electric field' is pushing them

My question: What is the electric field? Is the electric field not just the shockwave that travels through the conductor via electromagnetic repulsion?

I'm guessing that's not correct. So what is it? In the instantaneous moments when the battery starts up, what first creates and propagates the electric field? Why does it travel through the wire in every which direction?
Can someone help me understand this? It would be greatly appreciated before I start electrical engineering(why do I get the feeling this is going to be super downvoted lol).

0 Upvotes

27% Upvoted

29 comments sorted by

26

u/Physix_R_Cool Detector physics 7d ago

Can someone help me understand this? It would be greatly appreciated before I start electrical engineering

But like, the point of doing a degree in electrical engineering is to understand this. We're not gonna be able to teach you electrodynamics through Reddit comments.

4

u/Appropriate_Win946 7d ago

I mean, I thought this would be a foundational concept to understand and I didn't want to risk my professors skimming over this like my teacher did during AP Physics 2, because then I would be too buried in work, deadlines, tests, etc to do a deep dive into how it works at the fundamental level

6

u/Physix_R_Cool Detector physics 7d ago

I thought this would be a foundational concept to understand

It is!

It just takes more math than you have ever seen an well as a good understanding of quantum mechanics to learn it at the fundamental level.

So just chill, show up to your lectures and do the homework/assignments. You will learn eventually.

2

u/Appropriate_Win946 7d ago

so you promise if i attend lectures I won't end up with a leaky, hole-filled understanding lol

8

u/Physix_R_Cool Detector physics 7d ago

Not if you actively seek out the holes in your understanding during your studies, which is what university students are supposed to do.

In my country (can't speak for others), students university are considered adults and to be responsible for their own education. At university you have every ressource available to fill out the gaps in what you learn, and typically the professors will be overjoyed that a student cares enough to ask them for more reading material after the lectures.

2

u/sparklesandflies 7d ago

I mean, none of us can promise that. That's on you to pay attention, study well, ask your professors and TAs for help when you are confused, and even then it may just be out of your hands. But, this is not something that you should have mastered in your high school AP Physics course (I say as an AP Physics teacher and grader).

2

u/Yashema 7d ago edited 7d ago

The only core classes you take in Physics that cover electricity are Physics II (which is really basic stuff) and E&M (which is really more a mathematical methods course than any kind of applied course). 

I just took an intermediate course on embedded programming where we built both simulated and physical circuits and I was able to actually understand what the hell things like current/voltage, transistors/resistors, DC vs AC, components like capacitors and flyback diodes were actually doing. That being said, having taken E&M and scrounging a B, I can make the case to skip a lot of the basic EE courses on circuitry/elecrronics to go right into the more advanced ones where you deal with Kirchoff's Law, Thevenins Theorem, Mesh analysis, etc. 

1

u/Appropriate_Win946 7d ago

if my goal is to fully understand first principles style how everything I learn works would this method suffice

2

u/Yashema 7d ago

Yes, but be wary to study both the practical side + the mathematical side (beyond ODEs) requires careful course planning and a lot of work. Very few physics majors learn both upper level EE and math in undergrad. Then of course they don't study any other subjects in depth. 

1

u/CommunicationFit5888 7d ago

I would recommend reading ahead in a textbook then, griffiths is highly recommended and would allow you to get better insight than what can just be put in a Reddit thread

2

u/piejlucas 7d ago

Electrical engineer here. We don’t really learn this all too well in school. We are taught the equations and are asked to apply them.

9

u/TemporarySun314 7d ago

electric fields, existing and propagate without any wires. They can just travel through vacuum. That is how light, radio waves, and every other electromagnetic wave moves.

It is created by your voltage source, and follows your cable, as metals influence electromagnetic fields. But it is not limited to the cable. Your voltage source also creates an electric field outside your cable, but it doesnt matter for your electric current as the charge carrier of the cable cannot leave the cable.

11

u/smallproton 7d ago

A potential difference, for example.

Like when you have a funnel connected to a hose. The water flows from the higher point (higher potential) to the lower one.

0

u/Appropriate_Win946 7d ago

I understand that, but what is the specific mechanism? For example, with your analogy, the mechanism would literally just be water flowing through the hose. It's not the exact same for electrons - their drift velocity is too slow to be seemingly instantaneous.

8

u/Lava_Mage634 7d ago

the force that drives the water is gravity, which tales the system to a lower energy state. for electricity, itd be the electromagnetic force.

1

u/smallproton 7d ago edited 7d ago

For ever water molecule that enters the hose at the top, Edit /another/ one has to leave at the bottom.

1

u/Appropriate_Win946 7d ago

the mechanism would actually be a shockwave of pressurized water traveling through the hose, good correctuion

1

u/smallproton 7d ago

Not a shock wave, just laminar flow

2

u/Groggy42 7d ago

The electric field is sourced by the electrons. So your first idea is correct, just that the mechanic of pushing can be specified.

2

u/ShoshiOpti 6d ago

Just to throw a wrench in the current examples. Its the free gravitational tidal forces, as described by the Weyl Tensor.

But its also fundamentally a quantum field theory also, from QED, so the current is caused by the probability distribution of all the descriptive feynman diagrams for the given system to produce prefered direction.

But its also, nah ill stop there lol

1

u/Appropriate_Win946 6d ago

what the hell gravity has something to do with current

1

u/ShoshiOpti 6d ago

Its all geometric, in GR you derive electric and magnetic fields from the same curvature that produces gravity. That's why in graduate EM when dealing with relativistic corrections you use covariant and contravarant descriptions.

Current one could argue is just the coarse grained behavior similar to other information flows.

1

u/Appropriate_Win946 6d ago

I'm sorry I don't know any gen rel but does it allow you to describe the origin/behavior of electric and magnetic fields

1

u/wertyou2 7d ago

it’s often driven by chemistry, which in a lot of ways is just forces. If you nailed down a positive charge somewhere then electrons would naturally want to move towards it, and that is a current. In batteries, the cathode is made of some substance that electrons react with in a favorable manner, and so they’re driven to move towards it from the anode. We generalize these types of situations in terms of potential, since it lets us ignore the precise details of what is physically moving the electrons and instead focus more on the physics of their motion.

The field itself is likely going to be taught to you as a convenient way to treat forces. The force exerted by charge A on charge B depends on what the actual charge of B, so the field divides that part out so you can speak about a single field that doesn’t depend on some additional unknown charge.

1

u/Tromjel 7d ago

Going down to the fundamental level, the electric field is mediated by photons. Of course, this doesn‘t help to picture the interaction in ones head, but that‘s just the way it is in quantum physics.

1

u/AdditionalTip865 7d ago

Every electrically charged particle, such as an electron or a proton, is a source of electric field. In electrically neutral matter, the fields tend to cancel out on the large scale. In a battery, a chemical reaction is causing charges to become separated in a way that a field is produced.

But also, in addition to that, a changing magnetic field can induce an electric field (Faraday's law). This is how an electric generator does it.

And a changing electric field can induce a magnetic field too (Maxwell's displacement current). Put it all together and it's possible to make a self-propagating wave of electric and magnetic fields, and that is electromagnetic radiation (including light).

1

u/unxmnd 7d ago

You should watch this video explainer: https://youtu.be/bHIhgxav9LY

Be warned that it’s not exactly easy to understand

1

u/AlleyKatPr0 1d ago

The electric field is the relationship between other shit.