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how exactly resistor works

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IanB:

--- Quote from: tester43 on September 28, 2018, 10:41:09 am ---is the resistor like a narrow tube between two large tubes limiting the water flow (no wasted energy).
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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.


--- Quote from: Brumby on September 28, 2018, 03:48:10 pm ---As with all analogies, the water analogy might struggle a bit here.
--- End quote ---

Actually, it's pretty good.


--- Quote from: hamster_nz on September 29, 2018, 04:28:26 am ---Humm.... but what is the mechanism in the fluid that converts the power supplied by the pump into heat?
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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.

djacobow:

--- Quote from: Brumby on September 29, 2018, 05:15:53 am ---
--- Quote from: djacobow on September 29, 2018, 03:40:16 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.

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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.

--- End quote ---

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.

Nusa:
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.

djacobow:

--- Quote from: Nusa 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.

--- End quote ---

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.

T3sl4co1l:
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

Tim

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