Electricity doesn't work the way I thought it did
Discussion
speedy_thrills said:
speedy_thrills said:
The Poynting vector describes the flow of energy through a static EM field but the description in the video is not correct because the question is about a dynamic system (you are flipping the switch.) This is an electrodynamics problem and that means you need an equation that includes a "t" for time and, as you may have noticed, the pointing vector does not have a time variable.
Someone sent me this video today and it's a far better visual demonstration of what actually happens when you flip the switch on a circuit:https://youtu.be/2AXv49dDQJw?si=nxD0aQEJimEvgl7Q
I'd rather not be but I have such basic questions when anyone (or a video) seeks to enlighten me that either: must infuriate the teacher and they give up or the video doesn't answer.
Yet, I managed to follow that video.
Brilliant explanantion and demonstration.
Nice find, S_T!
dukeboy749r said:
speedy_thrills said:
speedy_thrills said:
The Poynting vector describes the flow of energy through a static EM field but the description in the video is not correct because the question is about a dynamic system (you are flipping the switch.) This is an electrodynamics problem and that means you need an equation that includes a "t" for time and, as you may have noticed, the pointing vector does not have a time variable.
Someone sent me this video today and it's a far better visual demonstration of what actually happens when you flip the switch on a circuit:https://youtu.be/2AXv49dDQJw?si=nxD0aQEJimEvgl7Q
I'd rather not be but I have such basic questions when anyone (or a video) seeks to enlighten me that either: must infuriate the teacher and they give up or the video doesn't answer.
Yet, I managed to follow that video.
Brilliant explanantion and demonstration.
Nice find, S_T!
Yes, sometimes finding someone explaining something only slightly differently, can be the difference between continued ignorance or a 'light bulb' moment. Well, for me at least.
Plus, you get variations on a theme and more fullsome explanations/debates, which is helpful.
All great stuff.
Plus, you get variations on a theme and more fullsome explanations/debates, which is helpful.
All great stuff.
The battery has a relatively high impedance, so faced with a (near) short circuit, the voltage will decrease practically to zero almost immediately.
It would be interesting to see the same demonstration with a high current power supply (only needs to be a few mV to avoid melting the wires), to see if the same theory applies - i.e. without the subsequent drop in voltage from the battery - would we still see the same effect given that the voltage / current will be sustained?
In the case of the demonstration in the video, as the voltage is applied a small magnetic field is generated which then promptly collapses as the battery is unable to maintain sufficient current to keep the circuit energised.
It would be interesting to see the same demonstration with a high current power supply (only needs to be a few mV to avoid melting the wires), to see if the same theory applies - i.e. without the subsequent drop in voltage from the battery - would we still see the same effect given that the voltage / current will be sustained?
In the case of the demonstration in the video, as the voltage is applied a small magnetic field is generated which then promptly collapses as the battery is unable to maintain sufficient current to keep the circuit energised.
The thing I like about this explanation is that it gets you a lot further in a very intuitive way. You now understand that electromagnetic fields are set up, they aren't just instantaneously willed into existence and that you get propagation even in dead legs.
It also gets you down the road to start thinking about concepts like the capacitive nature of grids and inductive or resistive nature of circuit loading and why it might matter.
It also gets you down the road to start thinking about concepts like the capacitive nature of grids and inductive or resistive nature of circuit loading and why it might matter.
Edited by speedy_thrills on Wednesday 22 May 10:18
speedy_thrills said:
Someone sent me this video today and it's a far better visual demonstration of what actually happens when you flip the switch on a circuit:
https://youtu.be/2AXv49dDQJw?si=nxD0aQEJimEvgl7Q
Thanks for posting that, really enjoyed it. Hope that bloke is teaching outside you tube as well, given his obvious talent for it.https://youtu.be/2AXv49dDQJw?si=nxD0aQEJimEvgl7Q
The simple fact is the copper wire is made up of atoms of copper, funnily enough atoms are made up of protons, neutrons and electrons, this means the electrons are already in the cables so when a circuit is completed the battery 'pops out' and electron from the negative terminal it is effectively pass the parcel down the down the cable and an electron gets received back at the positive terminal.
On the branch which is not joined, there isn't another atom to play pass the parcel with because the cables aren't joined and a current does not flow.
The general convention is current flows from the positive terminal to the negative terminal, when looking at electrons, they flow from negative to positive but for that to happen there is a flow of holes going the other way.
So a coventional flow of current is really a constant stream of holes going from positive to negative as electrons flow from negative to positive at the same time.
The newtons cradle is a good analogy but there should be 2 cradles side by side going in different directions because current only flows in loops.
Confused?
On the branch which is not joined, there isn't another atom to play pass the parcel with because the cables aren't joined and a current does not flow.
The general convention is current flows from the positive terminal to the negative terminal, when looking at electrons, they flow from negative to positive but for that to happen there is a flow of holes going the other way.
So a coventional flow of current is really a constant stream of holes going from positive to negative as electrons flow from negative to positive at the same time.
The newtons cradle is a good analogy but there should be 2 cradles side by side going in different directions because current only flows in loops.
Confused?
"holes" are a useful model in some materials, but the idea that the "absence of a thing" is itself a thing is not a generally useful idea. Obvious example is an electric current that consists of a physical stream of charged particles travelling through a vacuum. The idea of holes flowing the other way doesn't help. Holes are a particularly useful idea when thinking about doped semiconductors.
Anyway ... if we want to get super anal, replacing the block circuit model with a field model using Maxwell's equations ... well we all "know" that's also "b
ks" as we don't like non-local classical fields giving rise to spooky action at a distance so what you really need to do is use quantum electrodynamics to describe how long it takes the light to switch off in the fridge when you close the door based on particles interacting with each other. Or ... you use a model that is appropriate for the scale of the system you are trying to predict and you don't pretend that any of them models is correct in a general sense.
Anyway ... if we want to get super anal, replacing the block circuit model with a field model using Maxwell's equations ... well we all "know" that's also "b
ks" as we don't like non-local classical fields giving rise to spooky action at a distance so what you really need to do is use quantum electrodynamics to describe how long it takes the light to switch off in the fridge when you close the door based on particles interacting with each other. Or ... you use a model that is appropriate for the scale of the system you are trying to predict and you don't pretend that any of them models is correct in a general sense.Gassing Station | Science! | Top of Page | What's New | My Stuff