how exactly resistor works - Page 1
Forum section: electronics / beginners

author: tester43
author_rank: Regular Contributor
timestamp: September 28, 2018, 10:41:09 am
content: sooo.... i am happy to come back with "simple" question.ResistorWhat we know: "Ohm's law", "current limiting", "heat / power limit", "material difference in conductivity / amount of energy required to move the electron".But:is the resistor a way to radiate overflow of energy as heat (wasted energy) - and that's how books are explaining it.is the resistor like a narrow tube between two large tubes limiting the water flow (no wasted energy).Which is is true?

author: glarsson
author_rank: Frequent Contributor
timestamp: September 28, 2018, 10:59:19 am
content: If you push water through a narrow tube then the resistance the water "feels" is converted into heat. Also true for any other liquid or gas. One example is all the heat produced by a compressor or a simple bicycle pump.

author: taydin
author_rank: Frequent Contributor
timestamp: September 28, 2018, 11:11:10 am
content: You can think of resistance like large diameter pipe, small diameter pipe. Or you can also think about a pipe with a polished internal wall, and a pipe with a rough internal wall. All work to imagine how a resistor would behave.But a better analogy would be this: there are a heap of melons, and a group of workers want to load it onto a truck. So the workers form a line. The first one picks up a melon from the heap and gives it to the next worker in line. The last worker puts the melon on the truck. So there is a flow of melons, changing hand from one worker to the other.Here, the flow of melons is "current", the manager shouting "come on guys, get moving!" is the "voltage", and the amount of laziness of the workers is the "resistance"

author: rs20
author_rank: Super Contributor
timestamp: September 28, 2018, 11:15:34 am
content: Just to perfectly clarify what glarsson is correctly pointing out;Quote from: tester43 on September 28, 2018, 10:41:09 amThe "no wasted energy" part is wrong. A narrow tube does dissipate energy.Quote from: tester43 on September 28, 2018, 10:41:09 amNow that I've correct your question, the answer is "both".

author: hamster_nz
author_rank: Super Contributor
timestamp: September 28, 2018, 11:51:42 am
content: Quote from: rs20 on September 28, 2018, 11:15:34 amCan you explain this to me a little more? I can't resolve this in my head.Unless the pipe is tapered (e.g. a nozzle) the same mass of fluid that enters the pipe exits the pipe, traveling at the same speed, so it is carrying the same energy, regardless of the diameter of the pipe.The change of temperature of air as it is compressed is related to adiabatic processes.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 28, 2018, 12:00:35 pm
content: Energy is the combination of two forces - related to water, the energy is the amount of water (current) times the pressure (voltage) moving it forward. the resistance is. Just water standing in pipe behind a closed tap is not delivering any energy - and as you maybe know, electricity without a flow of current isn't delivering any energy as well - whichever energy that might be

author: tester43
author_rank: Regular Contributor
timestamp: September 28, 2018, 01:13:50 pm
content: We know one for sure. Resistor can get warm.Limiting the current goes by wasting the current not wanted or is it limiting as in "putting an obstacle in the way of air/water..." and only a part of energy is wasted by the .... oh I dont know what...

author: a59d1
author_rank: Regular Contributor
timestamp: September 28, 2018, 01:15:19 pm
content: J=σΕ.

author: macboy
author_rank: Super Contributor
timestamp: September 28, 2018, 01:54:42 pm
content: Quote from: hamster_nz on September 28, 2018, 11:51:42 amIs the pressure the same on both sides of the pipe? If not, what does this imply?Does the hydraulic pump need to work harder to push a specific flow rate through a narrow pipe or a large one? So if the pump needs to output more energy into the narrow pipe system, where does it go?Your hydraulic pressure is analogous to electrical voltage. The flow rate is analogous to electrical current.

author: wolframore
author_rank: Newbie
timestamp: September 28, 2018, 01:57:09 pm
content: resistance is futile

author: IDEngineer
author_rank: Super Contributor
timestamp: September 28, 2018, 02:11:36 pm
content: Quote from: hamster_nz on September 28, 2018, 11:51:42 amIn a hydraulic system (which is what your pipe and fluid represents) there are two main variables: Pressure and flow. The amount of energy transferred is related to BOTH of them, and you can trade one for the other if you wish. That is, you can drop the flow volume by raising the pressure of the system and still deliver the same amount of energy.Likewise electronics. Energy transferred ("watts") is the mathematical product (multiplication) of pressure ("voltage") and flow ("current"). This is one solution to Ohm's Law, P = E * I. Like hydraulics, you can halve one variable by doubling the other and the energy/watts remains unchanged.Now let's "taper" (or constrict) the pipe, as you say. This introduces resistance against the flow. Surprise - we use the same term, "resistance", in electronics. As you introduce resistance, while keeping all else the same, the current (flow) is indeed reduced. But remember, that resistance is "working" to hold back the flow that would otherwise occur. Put yourself in its place... if there were a flow of water coming through a window, and your job was to hold a piece of plywood across the window to resist the flow, would you be "working"? Would you expend effort? Of course. And the more of the window you tried to cover - the more flow you held back - the harder you'd be working.Your body would start heating up while you worked. Your perspiration system would kick in and you'd start to sweat to cause evaporative cooling to maintain your body temperature. A resistor has no such cooling system to maintain a constant temperature, so that work performed by the resistor raises its temperature as it dissipates the heat. Stop the current flow and the resistor cools down because it takes no effort to hold a piece of plywood against a window with no water flowing through it (nor does it take effort to hold back electrons that aren't trying to move).Those are the extremes of the situation. Now think about the middle ground. The flow starts up again, a very slight trickle of water through the window (or a very small current through the resistor). How much work is it to resist that? Not much. But as the flow increases, the amount of work required to hold it back goes up. This is, in fact, a linear relationship.Kick those concepts around in your head for a while and see if it starts to make sense.

author: Brumby
author_rank: Unknown
timestamp: September 28, 2018, 03:31:02 pm
content: Quote from: wolframore on September 28, 2018, 01:57:09 pmThe collective wants you back.

author: Brumby
author_rank: Unknown
timestamp: September 28, 2018, 03:48:10 pm
content: As with all analogies, the water analogy might struggle a bit here.The first thing to understand is that energy is lost from the electrical system when current passes through a resistance - and the resistor will heat up. There is no way around this.The mental image I have is of a ball rolling down a plane. With nothing in the way, the potential energy it has at the top is turned into kinetic energy at the bottom - and there is no energy lost (except for a tiny, tiny bit in the rolling action, which we can ignore for this exercise). This would be the equivalent of a wire.Now place some posts on the plane and as the ball rolls down, it will hit some of the posts and lose some energy to those posts - sort of like how a cricket ball hitting a bat creates a hotspot:Add more posts and you are increasing the number of obstacles and the number of impacts which has the result of wanting to reduce the flow of balls down the plane. This is increasing the resistance.

author: rstofer
author_rank: Super Contributor
timestamp: September 28, 2018, 03:53:07 pm
content: I'm not a huge fan of the water analogy beyond about 10 minutes the first day of class.We have Ohm's Law and that tells us all we need to know about the relationship between voltage, current and resistance. It's a Law, not a suggestion!You know that if you have a current through a resistor, it drops voltage (E = I * R). You also know that Power = Current times Voltage (dropped in this case), (P = I * E). There are other expressions that are algebraic manipulations but, in the end, that current flow creates a voltage drop and the product is heat (power).https://www.electronics-tutorials.ws/dccircuits/dcp_2.html

author: ArthurDent
author_rank: Super Contributor
timestamp: September 28, 2018, 08:16:11 pm
content: So far we’ve had resistor analogies involving melons, water, balls, and cricket. I’d humbly like to use frosting, partly because I like food analogies.If you’ve ever seen someone decorate (embellish) a cake where they fill up a zip-lock bag with frosting and then cut a corner off the bag for the frosting to come out of, they have to supply quite a bit of pressure (intensity) to squeeze the frosting out of the small hole in the bag. The smaller the hole in the corner, the more pressure they have to apply because of the back-pressure (resistance) to force the same volume of the viscous frosting through the hole. If they want to embellish the same number of cakes per hour the decorator either have to increase the pressure (intensity) or decrease the resistance to get the same volume of frosting.The decorator has to use a formula to determine how to apply these variables and the hole size in the bag will determine the amount of frosting that will go through the hole. They use the equation of Embellishment=Intensity x Resistance or E=IR or as some like to state it, Volume=Intensity x Resistance or V=IR

author: iMo
author_rank: Super Contributor
timestamp: September 28, 2018, 08:53:52 pm
content: There is not such thing like electrons flowing through the pipes/wires generating the heat.The electrons are tiny and pretty week, and their speed is a few centimeters per second in the metal.The heat is created by fields, spanning over the whole Universe.. Weird..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2018, 09:04:14 pm
content: Just to emphasize:Energy is not power.Energy over time is power.Energy is more of a one-time thing. A resistor is more of a continuous thing. When you apply voltage and current to a resistor, you dissipate power. The dissipated energy counts up over time.Most precisely, energy is the integral of power with respect to time, or likewise power is the time derivative of energy. (If you've not had calculus, this probably doesn't help. On the upside, if you've had Newtonian mechanics, you will know this distinction. Energy is what it takes to get a car moving; power is what it takes to keep a car moving against wind resistance and such.)Tim

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 03:40:16 am
content: Quote from: rstofer on September 28, 2018, 03:53:07 pmBut it's not a law at all. It describes a common - but not universal - material property: that voltage is proportional to voltage by a fixed constant. But even where it is essentially true, such as in a carbon resistor or in a metal, I believe it is not 100% completely true. There are nonlinear factors.But we mortals really like to linearize things because it makes life much easier. And so we end up doing E=IR everywhere, even in transistor circuits for the "small" signal, because it's convenient - but it ain't right.

author: hamster_nz
author_rank: Super Contributor
timestamp: September 29, 2018, 04:28:26 am
content: Quote from: macboy on September 28, 2018, 01:54:42 pmHumm.... but what is the mechanism in the fluid that converts the power supplied by the pump into heat?It is altered by the viscosity of the fluid, and I guess you get the same heating effect (to different extents) in lubricating oils, maple syrup or thick tar. It only happens when they sheer or flow, so I guess it must be something like friction within the fluid itself.Time to get lost in Wikipedia on Fluid Mechanics (and most likely ending up reading about the design of Roman Sandals, via aqueducts and flume bridges) , I guess...

author: Brumby
author_rank: Unknown
timestamp: September 29, 2018, 05:15:53 am
content: Quote from: djacobow on September 29, 2018, 03:40:16 amIncorrect. Quite incorrect, in fact.Ohm's Law is a law - even in the strictest sense.Nobody has said that resistance is constant, which would appear to be the premise of your statement. We know about temperature coefficients and other factors that can affect the resistance of a component under specific conditions - but,under those conditionsOhm's Law is entirely consistent.

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2018, 06:07:14 am
content: Quote from: tester43 on September 28, 2018, 10:41:09 amYes, there is wasted energy. If there was no wasted energy then we could make all water pipes as small as we like. But we don't. For big water flows we have big pipes, and for small water flows we have smaller pipes. The size of the pipe is designed to minimize wasted energy while keeping the size and cost of the pipe within reasonable constraints.This is just like choosing the right size of wire for the desired current. We use bigger wires for higher currents, but we don't use unreasonably thick wires because that would be too expensive and the wires would be too difficult to install.Quote from: Brumby on September 28, 2018, 03:48:10 pmActually, it's pretty good.Quote from: hamster_nz on September 29, 2018, 04:28:26 amLet's look at this more closely.There are two quantities that can affect the capability of flowing water to do work. One is the flow rate, and the other is the pressure. These are directly analogous to current and voltage.When water flows through a constriction in a pipe and out the other side, the flow rate is indeed unchanged. However, the water loses pressure on the other side of the restriction, and the mechanical work done by the pump is turned into heat. The amount of work turned into heat (the power dissipated) is precisely proportional to the pressure loss times the flow rate (like voltage drop times current in a resistor).Here's how this happens. When the water enters the narrow section of pipe, it has to speed up to get through the smaller area for flow. In speeding up, it gains kinetic energy at the cost of pressure energy. So in the narrow section the pressure goes down (this is Bernoulli's principle). Now, when the water comes out of the narrow section and back into the original larger pipe, then indeed the flowing velocity returns to what it was. However, it may not get back to the original pressure. The fast flowing water when it leaves the narrow section and enters the larger pipe will suffer a lot of turbulence and this causes the kinetic energy of the water to be dissipated and not fully recovered.If the pipe is arranged to have a gradual narrowing and a gradual expansion without any sharp changes then the energy lost to turbulence can be reduced. However, the fast flowing water in the narrow section will still have a lot of friction with the pipe walls, and this will still turn pressure energy into heat. So whatever you do, friction in pipes will always cause energy to be dissipated as heat, just like resistance in wires.

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 06:18:37 am
content: Quote from: Brumby on September 29, 2018, 05:15:53 amSemiconductors and superconductors disagree.I was not taking about exogenous factors affecting "R", or even current and voltage indirectly affecting R, as they might through heating. I'm talking about E and I *directly* affecting R, which in many cases, they do. This makes the law not generally applicable, which sort of kicks it out of lawdom.

author: Nusa
author_rank: Super Contributor
timestamp: September 29, 2018, 07:20:43 am
content: I think you've sidetracked yourself by stepping out of the realm of resistors. If you've got a resistor, it follows Ohm's Law. If you're dealing with something that's non-ohmic (e.g. the voltage drop from a diode), it's not a resistor.

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 02:55:23 pm
content: Quote from: Nusa on September 29, 2018, 07:20:43 amBut this was my original point. Ohm's "Law" is a material property. It applies to the things it applies to, and not other things. This makes it far from being a basic law of anything.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2018, 03:07:48 pm
content: It might be better to say aresistance(the idealized concept) follows Ohm'slaw.It is regrettable that it was named as such in the first place; it should be called Ohm's Rule. Conversely, Kirchoff's "rules" should've been namedlaws(they are ultimately a consequence of fundamental physical laws, namely the conservation of charge and Gauss's law).For flavor: nothing is truly ohmic. Common metals are very good, in the ppm range, but environmental effects are always present, for example heating affecting resistance tempco (also a time-dependent effect, because of thermal mass). At very high current densities, there are electromigration and arc-over concerns. At ever-higher current densities, the resulting plasma is very nonlinear, more voltage drop causing exponentially more ionized matter (and therefore conductivity). Eventually the plasma becomes saturated (fully ionized), particles become relativistic (electrons first), and pair production occurs (creation of electron-positron pairs -- in a sense, space itself becoming torn apart and made conductive). At impossibly high current densities (energy density, really), space itself collapses into a black hole, which, well, isn't really conductive anymore, but doesn't much care what kind of particles you're putting into it...Tim

author: iMo
author_rank: Super Contributor
timestamp: September 29, 2018, 06:02:28 pm
content: Ohm's and Kirchhoff's and alike laws or rules are just simplifications people in the electronics use in order to make their life much easier.If you want to understand how the stuff works you must study this:https://en.wikipedia.org/wiki/Maxwell%27s_equationsBtw, a typical standard Q during exams from "Theory of Electromagnetic fields" subject the EE students usually get is to derive the Ohm's law from the Maxwell's equations.

author: palpurul
author_rank: Regular Contributor
timestamp: September 29, 2018, 06:26:04 pm
content: It's kind of irrelevant, but here is a quote for you from a very respectable physics book called "introduction to electrodynamics" by David J. Griffiths“I don't suppose there is any formula in physics more widely known than Ohm's law, and yet it's not really a true law.”

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2018, 06:48:22 pm
content: Every law or rule has an exception, if you're picky, but for real world practical use I'll go with what a sage once told me: "Ohm's law is definitive."

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 06:52:45 pm
content: Quote from: imo on September 29, 2018, 06:02:28 pmIf you examine such derivations, you'll see that they rely on the assumption that the number of free charge carriers and the relaxation time (the average lifetime of a charge carrier) are constants and do not change endogenously with the electric field or current density. But again, that is true only for certain materials.This is a beginners forum, so I understand that it is not a good idea to answer more than what was asked. But in giving a limited answer, I think it is important not to say something that is fundamentally wrong, least it become a fundamentally wrong building block for their entire understanding of electricity for their whole lives. Ohm's Law is presented incorrectly almost all the time when it could be presented correctly just as easily: ohm's equation explains the simple, linear relationship between current and voltage in many - but not all - common materials.

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 06:56:46 pm
content: Quote from: ArthurDent on September 29, 2018, 06:48:22 pmThe exceptions merely form the basis for the entire modern world as we know it.Life would be a LOT more boring if we only had ohmic materials.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2018, 07:58:08 pm
content: “The exceptions merely form the basis for the entire modern world as we know it.”Here we have the difference between the academic and real world. Ask any practicing engineer how many times they use complex equations to solve simple problems. Application is far different than research and development or theoretical research.If I’m planning a trip from Boston to NYC and ask someone the distance, I don’t think I really need to know the diameter of the earth to calculate great circle distance and how it isn’t a perfect sphere, or that the distance could be different depending on whether I go from Boston to NYC instead of NYC to Boston because of the effects of relativity and rotational speed of the earth, and whether my reference point is earth or the center of the universe.I’m into precise time and frequency and realize that talking to the average person about leap seconds or 5ns drift rates isn’t going to be a conversation starter. If someone asks me the time I look at my watch, which is synced to WWVB, and give them the approximate time.I believe the answer the OP wants is also a practical explanation, even if it isn’t to 12 decimal places. Yes, running a lot of current through a resistor generates heat which is what makes electric space heaters possible. They also have kind of an aura or field emanating from them.

author: rstofer
author_rank: Super Contributor
timestamp: September 29, 2018, 08:16:15 pm
content: Quote from: djacobow on September 29, 2018, 06:52:45 pmIn EE school, we worked with Ohm's Law, Kirchhoff's Laws and the other simple Theorems beginning in the first semester. I'm not sure but I think Maxwell's Equations didn't come up until 4 years later. Notably because it took 4 years of progressive math courses to get to a point where curl and divergence made any sense at all, mathematically. Yes, Maxwell's Equations can be hand-waved but sooner or later somebody wants to use numbers.Ohm, Kirchhoff, Norton and other Laws/Theorems are good enough for all practical purposes and certainly sufficient for a beginner.

author: djacobow
author_rank: Super Contributor
timestamp: September 29, 2018, 08:44:18 pm
content: Quote from: ArthurDent on September 29, 2018, 07:58:08 pmYou guy are killing me, so I think I'm just going to give up after this post. But you're still wrong as hell. Ohm's "Law" is misleading. It's as simple as that. It doesn't help to teach it as a "Law" even to a preschooler. There are cases where a simplification is helpful, there are cases where a simplification is necessary, because the truth is too complex. This is not one of those cases, because you do not have to explain the complex reality, only hint at its existence. This is actually very useful information for a student because it puts the seed into his or her head that E=IR is telling your something about the material under consideration, NOT about the universe.It's funny, you make a distinction between an academic and an engineer. As it happens, I'm an engineer and I'm happy to use simplest equations I can get away with. But I'm also often called on to teach, and I have a responsibility not to fill my students' heads with bullshit. Some of those students will continue on to more advanced learning, some will not, but regardless, it's nice to not tell them false stories. Hence, I always qualify Ohm's Law because it is perfectly trivial to explain to a day-one beginners of average intelligence that this equation, while generally useful, does not describe reality in many important circumstances and exactly describes reality in absolutely no circumstances. Engineering students definitely "get" this and it trips up nobody. The same goes for all the much worse crutch/analogies about water, drips, pipes, valves, pressure, flow, ball bearings, screens, hills, obstacles, etc, every single one of which ultimately becomes a hindrance to understanding electricity -- usually long before an undergraduate has finished his or her EE program.Finally, let me just state for the record that the OP's original question "how, exactly a resistor works" was absolutely an invitation for a nitpicky explanation. To borrow from your analogy above, the OP literally asked "how, exactly, do airplanes work?" and the answer he got was "you buy a ticket and it takes you where you want to go."

author: taydin
author_rank: Frequent Contributor
timestamp: September 29, 2018, 08:47:18 pm
content: Quote from: rstofer on September 29, 2018, 08:16:15 pmYep, that's what it comes down to. The word "law" is always used more or less loosely.If you take "law" in its most strict, absolute sense, for example, "every human will die" would be a law (fits observation, and there are no recorded exceptions of it). But there are very, very few things in life that are this certain, so it wouldn't be useful as a scientific categorization.

author: rstofer
author_rank: Super Contributor
timestamp: September 29, 2018, 09:14:36 pm
content: If somebody wants to call some kind of referendum to downgrade Ohm's Law to Ohm's Suggestions, go for it!I suspect that static inertia will preclude any change in the name or categorization.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2018, 10:38:09 pm
content: djacobow: “Finally, let me just state for the record that the OP's original question "how, exactly a resistor works" was absolutely an invitation for a nitpicky explanation. To borrow from your analogy above, the OP literally asked "how, exactly, do airplanes work?" and the answer he got was "you buy a ticket and it takes you where you want to go."”If we want to get nitpicky, the OP DID NOT ask "how, exactly a resistor works". That is not a quote, although you presented it as a quote. A quote is exactly what the person said, like the following.“• is the resistor a way to radiate overflow of energy as heat (wasted energy) - and that's how books are explaining it.• is the resistor like a narrow tube between two large tubes limiting the water flow (no wasted energy).Which is is true?”That is a quote, preserving any spelling or grammar the OP used .To wrongly equate your question of "how, exactly, do airplanes work?" (which can easily be explained by difference in pressure on the bottom/top of the wing and only takes a couple of short sentences) with “how do I hire someone to use their airplane to get me where I want to go” is entirely different and is a non sequitur.To answer your question of “"how, exactly, do airplanes work?" with an answer like you need to go to college and get an advanced degree in aeronautical engineering then work for years to learn how to apply the theoretical knowledge to practical applications would not be responsive.So as to how resistors work the answer is: They will follow Ohm’s law until someone proves that for all practical applications it’s been repealed.

author: tester43
author_rank: Regular Contributor
timestamp: September 29, 2018, 10:39:56 pm
content: sooooMaybe it would be better to rephrase.If I would take a battery... LiPo 3,7V 500mAh battery. Fully charged is 4,2V - nominal working value 3,7V.Then I am adding load: resistor 470Ohm to battery terminals.From Ohm's Suggestion () we can estimate the current going through R to be around 8mA.500/8 gives around 60Hours of battery power (without other factors typical for battery).using the same battery recharged with resistor 47Ohm gives current around 80mA.500/80 gives around 6 hours of battery life.<here comes the problematic part - do not hate please>Call me stupid but the way of my thinking is: if battery would be pushing constant current through resistor (by increasing voltage or other method if exists) then battery life would be always the same, ignoring the value of resistance. It's like battery would say: "i'm giving 1A no matter what - I want it to be over in 30 minutes" - this would lead to putting the resistor on fire. But it does not happen. It means that resistor is in fact stopping battery from pumping the whole current it could deliver. It limits the current flow. But then why Heat? So the question is:HOWresistor is limiting the current?

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2018, 11:12:00 pm
content: This is two questions.Quote from: tester43 on September 29, 2018, 10:39:56 pmWhy heat? Because when the battery is charged it contains energy. If you discharge the battery through a resistor you are letting the energy out of the battery and the energy has to go somewhere. Heat is a form of energy. The chemical energy in the battery gets turned into heat energy in the resistor.QuoteBecause resistors offer resistance to current flow. Ohm's Law says that if the resistance is higher for the same voltage, then the current is lower in inverse proportion. How does the resistor limit the current? Because it is a material property of the substance that resistors are made of. If you put a voltage across a resistor, then more resistance gives less current and less resistance gives more current. This is just what resistors do.

author: rstofer
author_rank: Super Contributor
timestamp: September 29, 2018, 11:30:26 pm
content: Quote from: tester43 on September 29, 2018, 10:39:56 pmA battery can be considered a voltage source (as opposed to a current source, more in a moment). It produces a certain voltage E and it produces a voltage drop across the one and only resistor which, obviously, drops all of the battery voltage. Now, we know E across the resistor, and we know R, the value of the resistor. Now all we need to do is drop the values in E=I*R and crunch.Say we had a 1V battery (assume perfect voltage source) and we had a 1 Ohm resistor. From the E=I*R equation, we can figure that 1A is flowing. Furthermore, the resistor is dissipating I2*R Watts - in this case, 1 Watt.Now use a 1K Ohm resistor. From E = I * R we get 1 mA of current and I2*R = 1 mWThere is a pretty dramatic change in power dissipation.Low value resistors -> higher current for a given voltage -> higher power dissipation for a given voltage.Current sources are a different animal - they want to deliver a specific amount of current and they will change their voltage to make it happen. Think about what happens when a 1A current source is left with no load. The voltage rises to near infinity and everything burns to the ground. Something like that...Voltage sources will deliver any required current and attempt to maintain a specific voltage. At some point, the load is more than the source can deliver and voltage drops off. Study Norton Equivalent Circuit for a battery and note that maximum power transfer occurs when the load resistance is the same as the source internal resistance.That perfect 1V battery above, if it had a 1 Ohm internal resistance would only deliver 1/2V to the 1 Ohm load because you now have 2 Ohms total resistance. 1V = 1/2A * 2 Ohms Try Excel and put in different load resistors, calculate the terminal voltage given the load resistance and internal resistance and then convert the load voltage and load current to Watts. Extra credit: Make a graph.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2018, 11:31:14 pm
content: Just looking at your problem mathematically, you have ohm’s law (or more of a very strong suggestion) which says E=IR. if you have a battery of about 4VDC and assume the voltage is going to remain fairly constant, if one term on the right hand side of the equation goes up, the other term must go down for the equation to balance. If you use a 4 ohm resistor the current has to be 1A and if you use a 400 ohm resistor the current will be 0.01A. the power that the resistor has to dissipate as heat is given by the formula of P=EI so in the first case you have 4VDC x 1A or 4 watts. In the 2nd case you have 4VDC x 0.01A or 0.04 watts. In the 1st case, dissipating 4W as heat means the resistor will get far hotter than the 2nd case where you will be dissipating 0.04 watts. In the 2nd case you won’t be able to tell by feel that the resistor is ever so slightly warmer than the ambient air temperature.A little generalization here but if the 4VDC battery has a 4AH capacity it means that with a 1A load it will discharge in 1 hour. In the second case the 4VDC battery at 0.01A will last 100 hours. Everything is mathematically related so using just a few simple formulas will allow you to calculate what you need to know.As to heat, the energy released into a resistor has to go somewhere, in some form. In a resistor it is heat, just like the resistor element in an electric space heater, in an electric car it is the power to propel the car. Basically (at our level) energy is not created or destroyed, it is only converted in form. The same holds true with charging your battery, the electrical power going into the battery is chemically converted and stored as potential energy (which is why its voltage is referred to as potential), ready to be used if you complete the circuit by putting a resistor across the battery.Ohm's law is like religion, you just have to believe it is true.

author: tester43
author_rank: Regular Contributor
timestamp: September 29, 2018, 11:42:39 pm
content: example with battery proves by "experiment" that resistors are limiting current flow and don't just radiate.Now I need to google how Ohm's Law was deducted.

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2018, 12:12:03 am
content: Quote from: tester43 on September 29, 2018, 11:42:39 pmWhat do you mean by "don't just radiate"?Resisting current flow and generating heat are one and the same thing. If a resistor did not limit current flow it would not get hot for any current. (Also, it would not be a resistor.) But all real wires have resistance, and so all real wires generate heat when passing current.So:1. If it is generating heat it is resisting current flow (because it will have a voltage difference across it when a current flows through it)2. If it is resisting current flow it is generating heat (because it has a voltage difference across it due to the current flowing through it)Note that "limiting" current flow is not a good word. "Resisting" current flow is a better word. That describes what resistors do.

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2018, 12:16:11 am
content: It case it helps, think of resistance as like friction, and friction and heat go together. If you apply the brakes on your car they resist the forward motion of the car. At the same time the brakes get hot. You cannot have friction brakes that slow down a car without getting hot. It is impossible.In the same way, you cannot have a resistor that is resisting the flow of electricity without getting hot. It is impossible.

author: rstofer
author_rank: Super Contributor
timestamp: September 30, 2018, 12:24:50 am
content: A good example of energy being converted to heat is a simple laptop computer. Rest is on your lap and see what you think after an hour or so. Even my Surface Pro gets uncomfortable.There are MANY places inside the laptop responsible for generating heat. If a device doesn't do useful work (turn a wheel, lift a bucket, etc) then all of its energy is converted to heat. A CPU doesn't do useful work regardless of what the existence of Facebook may imply. All of the power entering the chip is turned to heat.How is it turned to heat when there likely are very few resistors? Well, there is capacitance all over the place and there are high frequency changes in voltage levels. It takes infinite energy to change a voltage level in zero time in the presence of capacitance. Faster processors get closer and closer to zero time switching. That's why CPUs have radiators and all kinds of mechanical cooling systems. They want to go fast and speed takes energy. The old 8080 8-bit processors didn't go fast enough to need cooling!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2018, 05:09:38 am
content: Quote from: tester43 on September 29, 2018, 10:39:56 pmClearly, you describe something which is manifestlynota battery -- i.e., something with ~constant voltage output, and obeying a charge-current relationship.Instead, the constant-current analogy to a battery (which as it happens, doesn't seem to exist, outside of contrivances, whereas the battery is a simple chemical process!), would be a ~constant current output, obeying a flux-voltage relationship.An inductor would be the linear example, just as the capacitor is the linear version of a battery. By "linear", I mean to note that the charge-voltage or flux-current relationship is proportional, i.e., voltage increases proportionally with charge, current increases proportionally with flux.A battery is NOT a linear component (so, if we model one as a Thevenin voltage source (an ideal voltage source plus a resistance), we must observe that its equivalent resistance isnon-ohmic-- aha, bringing the off-topic discussion back into it, see?), so our analogous component must also be nonlinear. In particular, it needs to have ~constant current over most of its charge, until it becomes depleted and charge goes to zero.Real (ferromagnetic cored) inductors exhibit saturation, but this is the opposite effect: as flux goes up, the rate of flux per amp (the inductance) drops, so as you continue to charge it, the current rises exponentially, rather than leveling off.You could make a locally battery-like inductor, by pre-saturating the core with a permanent magnet. As flux builds up, the magnet is opposed, and inductance increases. Downside: this doesn't continue forever; once you go over the center hump, inductance goes back down again, mirroring its initial rise, merely offset.If moving parts are allowed*, a solenoid may be a better example. As magnetic field rises, magnetic force tugs on the armature, opposing the force of a spring. As the armature pulls in, the magnetic path is shortened, increasing inductance -- ah ha! As the path shrinks, more and more flux must be added to increment the current, i.e., inductance rises more and more. Eventually, the solenoid is fully charged (armature fully seated) and further charging only saturates the core again. Downside, most solenoids actually pull stronger and stronger, as they close, due to geometry; you may find a real solenoid actually has so much inductance in the active region, it's actually beyond infinite and becomes negative -- which is another way to say, it exhibits hysteresis (the solenoid tends to stay pulled in, until the (relatively low) holding current is removed). A solenoid could be shaped to have a flat flux-current curve, though.*A battery has moving parts: charged atoms (ions) moving between electrodes, through an electrolyte. They're just invisibly small...The downside to a magnetic component is, the time constant over which it stores useful energy is very limited. Current flow through the coil causes resistive losses, effectively giving a "shelf life" of milliseconds. Conversely, eddy current losses in the core (when the magnetic field is changing, or when there is relative motion) make rapid dis/charging extremely inefficient, and the mass of the armature itself limits how fast mechanical motion can be transformed into electrical energy (again on the order of ~ms).A solenoid made with superconductors would be pretty good, though.Incidentally, there are actually superconducting energy storage devices; a coil akin to an MRI magnet (just smaller; more like a chemical NMR machine, microwave-oven-sized) is charged with a few thousand amperes, and some very beefy transistors keep the coil short-circuited most of the time, but allow that current to flow into a load as needed. Because the inductor is air-cored, it doesn't suffer from saturation, and because superconducting wire is quite fine, it can be made with a great many turns, making it possible to carry kiloamperes in a fractional-henry value inductor, with the only steady-state loss being the switching circuit (which itself can be made superconducting, if the delay from operating a mechanical switch is acceptable).Incidentally, I think I once calculated that ITER uses a total around a kilohenry of superconducting coils, at a typical current of a few kiloamperes, for its various field coils around the meters-tall reactor; in total storing some gigajoules of energy! (They don't give you all the numbers, at least not without digging through design documents; but you can calculate these from some educated guesses and the numbers they do give.)Oh look, I'm rambling about things; cool things, admittedly (get it, because we don't have room-temperature superconductors?..), but ah... guess I should go to bed.Tim

author: westfw
author_rank: Super Contributor
timestamp: September 30, 2018, 06:15:09 am
content: Going back to the original question...In an ideal conductor, you could sort-of think of an electron going in one end with a particular energy, and coming out the other end with exactly the same energy. In a real conductor, things in the conductor interfere with the passage - in particular it interacts with the electrons that are already in the atoms that make up conductor (and probably the nuclei as well) - and it ends up spending some of its energy making those move around on a molecular and atomic level (ie, generates heat.) (it's one of the things that makes superconductors so amazing - how can you possibly move electronics through a medium without having them interact with anything?)The same thing happens with water in a pipe - some of the kinetic energy of the individual water molecules gets wasted interacting with the walls of the tube, bumping into each other, and so on.

author: Brumby
author_rank: Unknown
timestamp: September 30, 2018, 06:22:26 am
content: Getting back to the original question is exactly what should happen more often in the Beginners section.To the OP, the departures from what may be helpful to you in this thread are not new or something special thrown down to scare you off. It happens all too frequently.So much so, this little graphic needs to be flown every now and then....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2018, 02:34:03 pm
content: Another classic symptom: once the original question is answered satisfactorily, thread drift and quibble is inevitable.It is up to the reader to decide when the thread has lost its value. Yet another skill, but this time at least it's generally applicable to all parts of life, including IRL conversations, Facebook comments and so on.If the answer is not satisfactory to the OP, but OP does not respond in a timely manner to notify of that fact -- and give additional information with which to formulate a more appropriate answer -- replies will default to their own assumptions, and the above happens regardless.You have a duty, as the question-asker, to steer conversation in a useful direction. If you just leave it alone, what do you expect is going to happen?Most of all, this is a free forum, it's not like you've paid anyone to furnish an answer on a silver plate.(Which, if you have a few bucks to spare, I'm sure many would work with you for a very good answer. Tutoring isn't free!)Tim

author: Bassman59
author_rank: Super Contributor
timestamp: September 30, 2018, 05:38:08 pm
content: Quote from: imo on September 29, 2018, 06:02:28 pmI remember doing that, not as an undergrad, but in a graduate course in Microwave Electronics, which was really about Maxwell's Equations. And when the professor mentioned that youcouldderive Ohm's Law from Maxwell's Equations, it was a revelation.In all of this discussion about whether Ohm's Law is really a law or just an observation, there's a parallel in Newton's Laws of Motion. They were derived from observation and the model works extraordinarily well. So well, in fact, that they're called a Law. But they are simplifications of a more-general and more difficult physics.

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2018, 05:46:32 pm
content: I believe that Ohm's Law is a property of materials. And so to derive Ohm's Law from Maxwell's equations you would have to extend those equations to include parameters that describe properties of bulk materials. Wouldn't that make the derivation a circular argument?

author: james_s
author_rank: Super Contributor
timestamp: September 30, 2018, 06:35:50 pm
content: Don't get too caught up in analogies, they are useful for helping one to visualize a concept on a very simple level. A water analogy is useful in grasping basic electrical concepts because water flowing through a pipe behaves in a roughly similar way to electricity flowing through a wire except it's not really the same. It's helpful because you can see/feel/touch water and you can't really do that with electricity but beyond that it's just a simple analogy, it's not perfect.

author: tester43
author_rank: Regular Contributor
timestamp: September 30, 2018, 10:12:40 pm
content: Hi,Thank you everybody for this very active conversation.I stopped at the conclusion that:1. current islimitedby resistor to the value described by the Ohm's Law2.but, current above the limit of resistor will be changed into heat3. I do not understand why if current over limit is changed into heat then why "battery" life is different for each resistor from my example ( and I know "because different current" - I am talking about heat generated).4. To understand p3 above, I need to read on material conductivity and how Ohm's Law was formulated.

author: rstofer
author_rank: Super Contributor
timestamp: September 30, 2018, 10:39:33 pm
content: Quote from: tester43 on September 30, 2018, 10:12:40 pmI'm not sure I care for the idea that current is limited unless you add 'for a given voltage'. The resistor drops a voltage based on the current flowing through the resistor, nothing more, nothing less. The resistor itself may not be the only component in a circuit and, therefore, other components impact the current flow.I don't think I would use the term limit. The resistor simply doesn't do that all by itself. Think a little harder about E=I*R and the other permutations: R=E/I or I=E/RQuoteNo! Every single bit of voltage dropped across the resistor is turned to heat. If you want to think in current, every ampere through the resistor causes heat. P=I*E, P=E2/R, P=I2R. P is in Watts, E is in Volts, I is in Amps and R is in Ohms.There's no such thing as a limit. You are adding a discontinuity or asymptote (a limit, some kind of flattening in the graph of current versus voltage) to a linear relationship. There simply isn't some magic 'limit'. Ohm's Law is a linear, continuous function for all practical purposes.E=I*R for all reasonable values of the variables. Obviously there are other design considerations like flash-over voltage and maximum allowable dissipation. That's why there are resistors with different dissipation ratings. 1/8W, 1/4W, 1/2W...100W and so on.QuoteThe battery is NOT a perfect source. If it is rated, say, 1 Ah, that doesn't mean that it can provide 3600 Amps for 1 second. 1 hour = 3600 seconds. There is a very non-linear graph of output current versus depletion time. If you take the current out at a high value, the charge life will be a lot shorter and the Ah number doesn't apply. Usually the rate is given for some current and/or some time. It might actually be capable of providing 0.1A for 10 hours.Read the second paragraph here:https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2018, 10:39:46 pm
content: Unfortunately, you do not have this correct yet.Quote from: tester43 on September 30, 2018, 10:12:40 pmThe current is not limited, it is resisted. The current cannot be less than the value described by Ohm's Law, nor can it be greater. The current is alwaysexactly equalto the value given by Ohm's Law.QuoteThere is no current "above the limit" of the resistor (see above).Every time current flows through a resistor heat is generated. This is a fundamental property of resistors. There are no exceptions.QuoteThere is no "current over limit". The current is exactly what Ohm's Law says it shall be. Also (for an ideal battery) the battery life isnotdifferent for different resistors. If the battery is "perfect" than the capacity of the battery in milliampere-hours and heat generated will be the same for every resistor. (Note that real batteries are not perfect, but for suitably high value resistors this statement becomes approximately true.)QuoteSome more reading will help. Not only Ohm's Law, but also concepts like conservation of energy, electrical power, and conversion of energy from one form to another.

author: rstofer
author_rank: Super Contributor
timestamp: September 30, 2018, 11:52:51 pm
content: This electronics game is all math! You simply can't do one darn thing without using, at least, Ohm's Law and its permutations. The math goes all up hill from there.To see the equation E=I*R or P=I2*R doesn't leave an intuitive feeling about what is going on. It's only when you plug in sample values and solve for the unknowns that any real familiarity occurs. Hence homework...Take a piece of paper and draw a horizontal line about half way down the page - the is the X axis and for this project it will represent current through a resistor in amps. Draw a tick at the left end of the line and 5 more ticks to the right, equally spaced. Then label the ticks 0..5, left to right.Now draw a vertical line on the left end of the line going up the page and draw a tick at the intersection and up through 10, equally spaced. This is the Y axis and represents voltage dropped by our resistor.Assume a resistor value of 1 Ohm (EE textbooks like simple numbers, so do I). With 0 Amps flowing through a 1 Ohm resistor, it will drop 0 Volts according to E=I*R. This is at the intersection of the two axes at the lower left corner.Then assume 1 Amp on the X axis and crunching through the equation you come up with 1V so draw a dot at X=1 and Y=1 (current on X axis is 1 and voltage on Y axis is also 1). Continue up through 5 Amps on X axis.Connect the 6 dots with a straight line and that graph shows how a 1 Ohm resistor reacts to various currents in the range of 0..5V. Look at 1.5 Amps (half way between X=1 and X=2) and you should get a voltage of 1.5V - exactly what the equation says you should get for a 1 Ohm resistor. Just draw a line from 1.5 on the X axis up until it intersects the graph and then horizontal until it intersects the Y axis where you should be midway between 1 and 2 or 1.5V.Now, repeat the process with a 2 Ohm resistor. The maximum 5 Amps will now produce 10 Volts and the line will be twice as steep as the first line. If you look at 1.5 Amps, you should get 3 Volts.Repeat the entire process for different resistors until Ohm's Law is part of your DNA. It should be as automatic as breathing because it will be used just about as often when playing with electronics.Graphs! Engineers can't discuss much of anything without drawing a picture. Some of the great inventions started as a doodle on a napkin at some after-work party. Most of my career in electrical and project management revolved around napkin plans. I made a sketch, the contractor built the project. Pretty simple.Consider graphing power dissipation. Excel should become your new best friend. Charts and graphs, that's what this game is about. BTW, plotting power dissipation is hard because you have E on one axis and I on another and you want a point in space equal to the product. What you wind up with for, say, a 1 watt value is a line of the various combinations of E and I that result in a given value of P - the line itself. There will be similar lines for other dissipations. You will see these kinds of graphs in datasheets. A family of curves, if you will.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 30, 2018, 11:54:33 pm
content: tester43 “1. current is limited by resistor to the value described by the Ohm's Law”No, that isn’t what everyone has been telling you over and over again. Ohm’s law is an equation that explains proportional relationshipsbetween terms, there is no limit involved and it is baffling why you won’t accept this fact.tester43 “2. but, current above the limit of resistor will be changed into heat”Very wrong! This has also has been explained to you multiple times. Again many others and I have explained this to you. Way back in post #39 I said:“If you use a 4 ohm resistor the current has to be 1A and if you use a 400 ohm resistor the current will be 0.01A. the power that the resistor has to dissipate as heat is given by the formula of P=EI so in the first case you have 4VDC x 1A or 4 watts. In the 2nd case you have 4VDC x 0.01A or 0.04 watts. In the 1st case, dissipating 4W as heat means the resistor will get far hotter than the 2nd case where you will be dissipating 0.04 watts.In the 2nd case you won’t be able to tell by feel that the resistor is ever so slightly warmer than the ambient air temperature.”There is NO LIMIT above which heat is produced in a resistor but below which there is no heat. You are the only one who is misrepresenting what is being said. ALL energy in a resistor is given off as heat. Just because you don’t have the equipment or knowledge to measure the amount of heat produced at lower levels doesn’t mean it isn’t there.In your first post you misrepresented what you claim to have read in books: “is the resistor a way to radiate overflow of energy as heat (wasted energy) - and that's how books are explaining it.” No book would explain heat radiated by a resistor as “overflow” and heat from a resistor is sometimes exactly the product you are looking for as with a toaster, stove or electric heater and not wasted at all.My conclusion is that everyone here has been very patient explaining to you multiple times in multiple ways the correct interpretation of Ohm’s law and you admit reading books which I’m sure said the exact same thing. Because your RESISTANCE to take any sound advice has LIMITED your ability to understand these concepts, I’ll leave you on your own. Good luck.

author: a59d1
author_rank: Regular Contributor
timestamp: September 30, 2018, 11:55:05 pm
content: Quote from: ArthurDent on September 29, 2018, 07:58:08 pm

author: ArthurDent
author_rank: Super Contributor
timestamp: October 01, 2018, 12:04:23 am
content: Quote from: a59d1 on September 30, 2018, 11:55:05 pmI assumed you knew that was sarcastic although the glow of a space heater or light from a incandescent light bulb does appear to be an aura.

author: Brumby
author_rank: Unknown
timestamp: October 01, 2018, 01:04:20 am
content: To the OP:You seem to have a fixation about the limit of a current - and I would like to address three "limits" that are associated with resistors...Voltage limit:This is where excessive voltage (more than it was designed to handle) across the resistor is so high, that materials in the resistor break down and sparks can jump. Keep the voltages below this limit for the resistor to perform its job without damage.Power limit:This is where the power being dissipated by the resistor is higher than it is capable of physically withstanding and the materials will overheat. Keep the power dissipation below this limit for the resistor to perform its job without damage.When a resistor is usedwithin its specifications,neitherof these limits play any part in how the resistor operates, so please feel free to ignore them.Current limit:This is a simple mathematical calculation where, for a given resistance, the current flowing through the resistor is a function of the voltage available - Ohm's Law. For any circuit with this resistor and voltage, there will be a maximum current that can flow through the resistor. When that circuit contains ONLY the resistor, then the current will be what comes out of Ohm's Law.Where this can change is where you have anadditionalcomponent in series with the resistor.Take an LED, for example. For an LED with a Vf(forward voltage drop) of 2V, in series with a resistor in a circuit which has 5V available, the resistor has 3V across it. Using that 3V, apply Ohm's Law and you will get the current that will flow. Since you will not get any more current than this flowing, it could be classified as a maximum current, which implies it is a limit - but it is only a limit forthisparticularcircuit. Without a resistor, the above LED connected directly to the 5V supply would draw excessive amounts of current and would soon bake itself into oblivion. IN THIS ROLE, the resistor performs the function of limiting the current to a value defined by Ohm's Law - and is often called a "current limiting resistor".Such "current limiting" is a function of the resistance of the resistor AND the voltage applied.I will also place emphasis on the fact that a resistor is, for all intents and purposes, LINEAR. Put the smallest of currents through it and it WILL dissipate some heat. There is NO magic point where things suddenly change ... that's the sort of thing LEDs will do - but not resistors.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 01, 2018, 02:58:21 am
content: Physical examples seem to speak to you more than even simple mathematics.One example of using resistors to dissipate small amounts of heat is the arrays of resistors used to generate artificial infrared images for testing infrared imaging systems. In order to generate real scenes they have to be able to generate continuously varying temperatures (no large steps in temperature between adjacent resistors) and to test good imagers they need to adjust temperatures by small fractions of a degree Centigrade. The voltages across each resistor are adjusted to give the appropriate current to dissipate the power required to raised that resistors temperature by the appropriate amount.Even this simple (in concept) device requires a fair amount of math to operate properly. The power dissipated by the resistor is not linear with voltage (P=E^2/R) and the temperature rise is not linear with power having terms proportional to temperature difference with nearby objects and other terms proportional to the differences of the fourth power of the temperatures involved.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 01, 2018, 04:01:00 am
content: I'm not sure it's helpful at this point, but here's another model for how a resistor works, the Drude model:https://en.wikipedia.org/wiki/Drude_modelQuoteThe electrons are accelerated by the electric field, but keep bumping into the fixed atoms in the material, so that limits their net velocity. They transfer energy in the process, which warms up the material.This is not 100% accurate, but basically works, especially with some adjustments. Ohm's law falls right out of this model.

author: Brumby
author_rank: Unknown
timestamp: October 01, 2018, 04:58:14 am
content: That's the same concept as my cricket ball and bat - except there are lots of balls and bats.