Contradiction in electric circuits - Page 1
Forum section: electronics / projects

author: ElectronicsGeek
author_rank: Contributor
timestamp: July 13, 2017, 10:06:37 pm
content: If you consider a circuit with a voltage source V and 2 resistors R1 and R2, then we know that the total energy provided in 1 second by the source is V volts times the current I, and this energy is given out in the circuit as heat in the resistors.We also know that the potential energy V*I is converted into Kinetic energy as it goes back to zero potential, so here's the contradiction:If the energy is dissipated in the resistors, then there's no kinetic energy left so that the charges can go back to the battery. But clearly they do go back, so this means the kinetic energy is not converted to heat in the resistors because if it was, then charges would have no energy left to go back to the source.The wire considered here is ideal, and the resistance of non ideal wires are lumped into R1 and R2.So what's the solution?Clearly energy is given off as heat in the resistors, so how come the charges have kinetic energy left to go back to the source?

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 10:14:22 pm
content: First, you seem to be confusing power and energy. The total power is P = V * I (assuming a DC voltage source), the energy is P * t.Wires will have a non-zero resistance, which also means that there will be voltage drop. So not all voltage will be dropped and power dissipated by the resistors.

author: ElectronicsGeek
author_rank: Contributor
timestamp: July 13, 2017, 10:26:43 pm
content: Quote from: alm on July 13, 2017, 10:14:22 pmI'm not confusing it because I am talking about power in 1 second!Anyways I've solved it:Actually, the ideal wires have zero length, so after passing through the resistors, the charges are ALREADY back into the source.

author: IanB
author_rank: Super Contributor
timestamp: July 13, 2017, 10:31:13 pm
content: Quote from: ElectronicsGeek on July 13, 2017, 10:06:37 pmNo. We don't know that. Why do you think that is true?Kinetic energy is energy associated with masses in motion. It has nothing to do with electricity.(Oh, by the way, V*I is power, not potential energy.)

author: cdev
author_rank: Super Contributor
timestamp: July 13, 2017, 10:35:28 pm
content: No!

author: Benta
author_rank: Super Contributor
timestamp: July 13, 2017, 10:41:52 pm
content: alm, IanB, cdev et al: why are you responding to this kook?If you contradict him, he'll get rude, and concerning electronics he lives in a parallel universe. Just stop.https://www.eevblog.com/forum/projects/real-inductor-iv-relationship/

author: ElectronicsGeek
author_rank: Contributor
timestamp: July 13, 2017, 10:53:23 pm
content: For those interested:An ideal wire has no length and has zero impedance so for the circuit in discussion when charges leave a voltage source, they immediately are at the first resistor, and so that resistor plus the second resistor form a length of non uniform resistance, given by rho * l / A, and these two resistors have nothing in between them since ideal wires have zero length, and hence, in one second one coulomb of charge leave the voltage source with potential energy V*I, and they start gaining kinetic energy, but lose it as soon as they start hitting the resistive materials in R1 and R2, so they lose the kinetic energy, and then start regaining it and losing it and so giving off heat. They will keep gaining and losing KE until they have left the resistive materials.The electric field only exist inside the resistors and nowhere else in the ideal wires. Remember that E = J*resistivity, and so for an electric field to exist, we need a current density and a resistivity. In an ideal conductor, the resistivity is zero, so the electric field is zero!

author: rs20
author_rank: Super Contributor
timestamp: July 13, 2017, 11:03:40 pm
content: TLDR:https://physics.stackexchange.com/questions/247740/energy-of-electrons-in-wireYou don't really deserve any effort put into an answer with your rudeness, but here goes nevertheless.There are plenty of apparent "contradictions" that emerge when you mash together talk about macroscopic phenomena (temperature, voltage, current, the arrow of time) and the microscopic phenomena from which the macroscopic phenomena emerge (electrons, particles, elastic collisions that are completely reversible in time). Here's a long (but excellent) talk that goes into a huge range of things, including touching on this point:link. These apparent "contradictions"dohave good resolutions, but nevertheless you're going to tie yourself in knots if you describe a scenario with macroscopic terms (voltage, current) and then just think about a single microscopic question with tunnel vision.When designing and analyzing electronic circuits, we use macroscopic terms: voltage, current, power. This works great, and is explains everything an EE needs day-to-day. This is why it is tempting to say that "kinetic energy has nothing to do with electricty". This is a pragmatic, realistic model of reality that works great, as opposed to a pedantic, impractical observation that electrons in motion must have kinetic energy.Anyway, if onefully commitsto the microscopic model which one must do (no half-arsing it), then we have the following:-- As electrons bump into the lattice of the resistor, they lose kinetic energy (converted to thermal energy).-- As the top wire is at a higher voltage than the bottom wire (immediately breaking my own rule), there is an electric field along the wire that is tending to accelerate the electrons faster, which is giving kinetic energy back to the electrons.The system will rapidly reach an equilibrium where these losses and gains of kinetic energy cancel out; so the electrons in the top and bottom wire both have the same drift velocities/"average" kinetic energy. Kirchoff's current lawemergesfrom this observation.

author: mikerj
author_rank: Super Contributor
timestamp: July 13, 2017, 11:13:41 pm
content: Quote from: ElectronicsGeek on July 13, 2017, 10:59:44 pmSchool has broken up early I guess, we can look forward to plenty of child trolls like this for the next 6-7 weeks.

author: ElectronicsGeek
author_rank: Contributor
timestamp: July 13, 2017, 11:16:40 pm
content: Quote from: rs20 on July 13, 2017, 11:03:40 pmThank you!Really a google talk where sean carroll talks about this ? Hard to believe but I'll watch that

author: ElectronicsGeek
author_rank: Contributor
timestamp: July 13, 2017, 11:18:03 pm
content: ahhh you meant micro vs macro ic icthanks buddy

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2017, 08:30:44 am
content: Quote from: ElectronicsGeek on July 13, 2017, 10:53:23 pmThe amount of charge transferred is unknown because the current was not specified.

author: Zbig
author_rank: Frequent Contributor
timestamp: July 14, 2017, 09:51:26 am
content: Quote from: Hero999 on July 14, 2017, 08:30:44 amThis guy calls people idiots and nazis left, right and center, whenever someone points out his incompetence. Then deletes some of his most offensive posts when he regains some of his senses (or moderators do this). He doesn't deserve anyone's time here. Please don't make him think this kind of shit will be tolerated by investing your time and energy in actually helping him.

author: 3db
author_rank: Frequent Contributor
timestamp: July 14, 2017, 10:08:53 am
content: Quote from: ElectronicsGeek on July 13, 2017, 10:53:23 pmBollocks !!

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2017, 10:56:07 am
content: Quote from: Zbig on July 14, 2017, 09:51:26 amI posted not for the benefit of the original poster but for others who might be interested.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 11:03:57 am
content: Quote from: Zbig on July 14, 2017, 09:51:26 amMy understanding is that the OP has now been banned. The moderator(s) have removed the offensive post(s).