Timeline for A few basic questions about simple electric circuits
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
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| Aug 22, 2018 at 12:06 | comment | added | Elliot Alderson | @wbeaty I didn't say that a voltage could appear without current flow, I said that a voltage could exist without current flow. | |
| Aug 22, 2018 at 11:12 | comment | added | wbeaty | @ElliotAlderson not correct: OP is asking about circuitry: therefore, in a network of conductors, for potential-diffs to appear at nodes, we certainly need a brief "charging" currents. Networks are sets of parasitic capacitances. Without brief currents, all voltages would equal zero.Your example of batteries is misleading: actually there is a brief current when the cell is first assembled, otherwise there will be no voltage across the battery terminals. (A battery is a charge-pump: the "pump" only runs long enough to produce the two halfcell potentials. It's the OP's "charging current.") | |
| Aug 7, 2018 at 23:32 | comment | added | Barleyman | @ElliotAlderson you said energy flows at almost at the speed of light. 50-ish is not insignificant difference when it comes to wavelength. In any case, note the quotation marks. Its something you might say to non-EE, its actually not uncommonly asked question. Wavefronts, propagation speeds, impedance discontinuities and the like are shop talk.. Just getting someone's head around the fact that the propagation time for a signal is not nothing and seemingly trivial trace length mismatches can ruin your data bus takes a bit of technical background. | |
| Aug 7, 2018 at 15:51 | comment | added | Elliot Alderson | @Barleyman I didn't say anything about the "speed of electricity"...that phrase doesn't have much meaning in engineering. I said that the electrons move slowly and energy moves quickly. Yes, the propagation of the wave depends on the dielectric constant but this is still many orders of magnitude faster than electron motion. | |
| Aug 7, 2018 at 13:05 | comment | added | Barleyman | @ElliotAlderson "Speed of electricity" depends heavily on the dielectric constant of whatever surrounds the conductor. If you have exposed copper hanging in air, it's practically c. If it's got rubber insulation, you're looking at about 55% c. On PCB it's more complicated situation but usually the end result is something between 0.5 and 0.7c. This does actually matter for high speed and/or long distance transmission lines. For most purposes this makes your life harder since the wavelengths get smaller. | |
| Aug 7, 2018 at 12:45 | history | edited | Steve T. | CC BY-SA 4.0 |
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| Aug 7, 2018 at 11:02 | comment | added | Elliot Alderson | @wbeaty The water analogy is also misleading because a conductor with a higher electric potential doesn't have more charge in it like the rising level of water in a tub. Analogies like this tend to confuse students because they lose sight of the important bit, which is the transfer of energy rather than matter. | |
| Aug 7, 2018 at 10:56 | comment | added | Elliot Alderson | @wbeaty No, you misread my statement. I said we don't need current to flow to have a voltage difference. Take any battery out of your parts box...there is a potential difference between the terminals whether current flows or not, whether you measure the potential or not. I'm talking about the definition of electric potential, not your common observations of it. | |
| Aug 7, 2018 at 1:16 | comment | added | wbeaty | @Steve T. "Charging" signals will see the line-impedance of conductors. That determines current: if we charge a 75-ohm coax, it briefly looks like a 75ohm resistor, and w/12V supply, would draw 160mA. But line-reflections are then immense, and they bounce through lossy components to make most circuit behavior deviously complicated. Here's a central theme: when you step into a bathtub (or a lake,) the water surface rises instantly, everywhere at once. But look close and you'll find a fast wave spreading at well over 3000MPH. Potentials in networks behave like that: big jumps from tiny flows. | |
| Aug 7, 2018 at 1:03 | comment | added | wbeaty | @ElliotAlderson not correct: insulators connected to power supply won't display potential difference like conductors do (zero current, zero surface charge.) Brief 'capacitive' currents are certainly necessary before potentials become established. (brief: nanosecond scale.) All circuits are composed of small parasitic capacitors ~pF and below, which are charged when the supply is connected. | |
| Aug 6, 2018 at 22:51 | comment | added | Criticizing Israel not allowed | AC is just constantly changing DC. | |
| Aug 6, 2018 at 22:50 | answer | added | Dave Tweed | timeline score: 3 | |
| Aug 6, 2018 at 22:06 | answer | added | Voltage Spike♦ | timeline score: 2 | |
| Aug 6, 2018 at 21:37 | comment | added | D.A.S. | Since I(t)=dQ/dt just understand that current is the rate of charge. Then knowing the exact impedance from R,L,C circuit or transmission line values you can expect a certain load impedance at DC or any frequency where these values are normally constant. From that V(t)/I(t) = Z(t). You can also replace t with frequency f. Z(t) for reactive parts will have an exponential curve and for R a linear fixed value. Z(f) will have a a rising slope for L with f with a negative slope for C load. | |
| Aug 6, 2018 at 21:32 | comment | added | Elliot Alderson | No, we don't need any current to flow to have a potential difference. The electrons actually flow very slowly through a circuit; you would be better to think about energy flowing through a circuit at almost the speed of light. And there aren't two kinds of current. Sorry. | |
| Aug 6, 2018 at 21:21 | history | asked | Steve T. | CC BY-SA 4.0 |