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Offline tester43Topic starter

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how exactly resistor works
« on: September 28, 2018, 10:41:09 am »
sooo.... i am happy to come back with "simple" question.
Resistor :)
What 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?
 

Offline glarsson

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Re: how exactly resistor works
« Reply #1 on: September 28, 2018, 10:59:19 am »
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.
 

Offline taydin

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Re: how exactly resistor works
« Reply #2 on: September 28, 2018, 11:11:10 am »
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"  ;D
« Last Edit: September 28, 2018, 11:12:52 am by taydin »
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Offline rs20

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Re: how exactly resistor works
« Reply #3 on: September 28, 2018, 11:15:34 am »
Just to perfectly clarify what glarsson is correctly pointing out;

a narrow tube between two large tubes limiting the water flow (no wasted energy).

The "no wasted energy" part is wrong. A narrow tube does dissipate energy.

But:
  • is the resistor a way to radiate overflow of energy as heat (energy dissipated) -  and that's how books are explaining it.
  • is the resistor like a narrow tube between two large tubes limiting the water flow (energy dissipated).
Which is is true?

Now that I've correct your question, the answer is "both".
 

Offline hamster_nz

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Re: how exactly resistor works
« Reply #4 on: September 28, 2018, 11:51:42 am »
Just to perfectly clarify what glarsson is correctly pointing out;

a narrow tube between two large tubes limiting the water flow (no wasted energy).

The "no wasted energy" part is wrong. A narrow tube does dissipate energy.

Can 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.
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Offline HB9EVI

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Re: how exactly resistor works
« Reply #5 on: September 28, 2018, 12:00:35 pm »
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
 

Offline tester43Topic starter

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Re: how exactly resistor works
« Reply #6 on: September 28, 2018, 01:13:50 pm »
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...  :(
 

Offline a59d1

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Re: how exactly resistor works
« Reply #7 on: September 28, 2018, 01:15:19 pm »
J=σΕ.
 

Offline macboy

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Re: how exactly resistor works
« Reply #8 on: September 28, 2018, 01:54:42 pm »
Just to perfectly clarify what glarsson is correctly pointing out;

a narrow tube between two large tubes limiting the water flow (no wasted energy).

The "no wasted energy" part is wrong. A narrow tube does dissipate energy.

Can 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.
...
Is 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.
 

Offline wolframore

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Re: how exactly resistor works
« Reply #9 on: September 28, 2018, 01:57:09 pm »
resistance is futile  ;D ;D ;D
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Offline IDEngineer

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Re: how exactly resistor works
« Reply #10 on: September 28, 2018, 02:11:36 pm »
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.
In 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.
 

Offline Brumby

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Re: how exactly resistor works
« Reply #11 on: September 28, 2018, 03:31:02 pm »
resistance is futile  ;D ;D ;D

The collective wants you back.
 

Offline Brumby

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Re: how exactly resistor works
« Reply #12 on: September 28, 2018, 03:48:10 pm »
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.
 

Offline rstofer

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Re: how exactly resistor works
« Reply #13 on: September 28, 2018, 03:53:07 pm »
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
« Last Edit: September 28, 2018, 03:56:43 pm by rstofer »
 

Offline ArthurDent

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Re: how exactly resistor works
« Reply #14 on: September 28, 2018, 08:16:11 pm »
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
 

Offline iMo

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Re: how exactly resistor works
« Reply #15 on: September 28, 2018, 08:53:52 pm »
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..
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Offline T3sl4co1l

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Re: how exactly resistor works
« Reply #16 on: September 28, 2018, 09:04:14 pm »
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.)

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Online djacobow

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Re: how exactly resistor works
« Reply #17 on: September 29, 2018, 03:40:16 am »
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!

But 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.
 

Offline hamster_nz

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Re: how exactly resistor works
« Reply #18 on: September 29, 2018, 04:28:26 am »
Just to perfectly clarify what glarsson is correctly pointing out;

a narrow tube between two large tubes limiting the water flow (no wasted energy).

The "no wasted energy" part is wrong. A narrow tube does dissipate energy.

Can 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.
...
Is 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.
Humm.... 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...
Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing.
 

Offline Brumby

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Re: how exactly resistor works
« Reply #19 on: September 29, 2018, 05:15:53 am »
But 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.

Incorrect.  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 conditions Ohm's Law is entirely consistent.
 

Offline IanB

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Re: how exactly resistor works
« Reply #20 on: September 29, 2018, 06:07:14 am »
is the resistor like a narrow tube between two large tubes limiting the water flow (no wasted energy).

Yes, 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.

As with all analogies, the water analogy might struggle a bit here.

Actually, it's pretty good.

Humm.... but what is the mechanism in the fluid that converts the power supplied by the pump into heat?

Let'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.
 

Online djacobow

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Re: how exactly resistor works
« Reply #21 on: September 29, 2018, 06:18:37 am »
But 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.

Incorrect.  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 conditions Ohm's Law is entirely consistent.

Semiconductors 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.
« Last Edit: September 29, 2018, 06:21:43 am by djacobow »
 

Offline Nusa

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Re: how exactly resistor works
« Reply #22 on: September 29, 2018, 07:20:43 am »
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.
 

Online djacobow

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Re: how exactly resistor works
« Reply #23 on: September 29, 2018, 02:55:23 pm »
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.

But 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.
 

Offline T3sl4co1l

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Re: how exactly resistor works
« Reply #24 on: September 29, 2018, 03:07:48 pm »
It might be better to say a resistance (the idealized concept) follows Ohm's law.

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 named laws (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... :-DD

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Electronic design, from concept to prototype.
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