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Thread: Why is anything an Ohmic Device?

  1. #1 Why is anything an Ohmic Device? 
    New Member James Burroughs's Avatar
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    Hello,
    I understand that a (filament) bulb is classed as a non ohmic device, as when it heats up, electrons are more likely to collide with the vibrating lattice, etc (correct me if wrong please, or I would love a more detailed explanation, I'm not sure if this explanation is even legit since I'm only in high school), but this got me thinking how can anything be classed as an ohmic device? Surely this concept of temperature increase from colliding electrons/resistance applies to anything in which a p.d can be induced across, and thus nothing is strictly ohmic?

    Or is this effect negligible? Since the temperature increase is so small for a wire for example, in comparison to a filament bulb.

    Thanks very much for your input

    James


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    "Ohmic" means that the device behaves according to Ohm's Law, that is, it is not an active but a passive device. Resistors, simple wires, and direct contacts from metal-to-metal (for most metals) are all ohmic. Thus, as the voltage increases, the current will increase proportionally. You will see this from Ohm's Law, which I'm sure they've shown you by now; get used to looking at equations like this and seeing how if you change one variable the others must change; you will be doing a great deal of this in the future, I assure you, as an Electronics Engineer.

    The reason the light bulb is non-ohmic is because its resistance changes as the temperature of the filament changes. And this is its normal operation. The filament material is deliberately chosen for this property, otherwise the temperature would run away and any possible filament would vaporize. The material therefore has a negative temperature coefficient; you probably also know what that means, but if not tell us and someone nice will help you understand what that means; it's another way of talking about how the variables vary, like I was speaking of above.

    Thus, if we were to make an "Ohm's Law for light bulbs," it wouldn't be the simple E = IR, but a more complex equation with a term that contained a temperature variable. And it is this that makes it "non-ohmic," in fact it is the very definition of non-ohmic.

    Does that help?


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  4. #3  
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    It is about if the device follows ohm's law or not.
    Because a lightbulb's resistance and current change in a different way than Ohm's law dictates it is non-ohmic.
    As the filament in a bulb heats up the resistance increases.

    Other examples of non-ohmic devises are diodes, transistors, thermistors,....

    Again the point is that they don't follow ohm's law without a modification of the law.
    Last edited by dan hunter; March 6th, 2014 at 05:30 PM. Reason: typo
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    Brassica oleracea Strange's Avatar
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    All resistors are non-linear for the same reason, but to a very small extent. This can normally be ignored (so you can treat them as "ohmic"). For high precision work you can buy resistors with a temperature coefficient close to zero so they work (almost) like perfect resistors.

    But, in the end, nothing in the real world is perfect and all our engineering and physics "laws" are just good approximations.

    BTW I have been en engineer for nearly 50 years and I don't think I have ever heard the word "ohmic" before!
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    Strange is correct but probably a bit advanced for high-school electronics unless it's gotten a lot more ambitious in the last thirty years or so.

    Have they started introducing you to component tolerances, yet, James Burroughs?

    Quote Originally Posted by Strange
    BTW I have been en engineer for nearly 50 years and I don't think I have ever heard the word "ohmic" before!
    It's chip-designer-speak for figuring out how to bond a wire to a silicon contact pad such that it is an ohmic contact between them and no extra semiconducting or other weird semimetal behavior results. It's why they use gold-ball and gold-wedge bonders and gold wires; you have to do it just right or it alters the characteristics of the chip, especially if it's an analog chip, like an A-D or amplifier or comparator or something. But that's like super-advanced beyond where James Burroughs is, unless they're now teaching chip manufacturing techniques and recipies in grade school!
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    Quote Originally Posted by James Burroughs View Post
    Hello,
    I understand that a (filament) bulb is classed as a non ohmic device, as when it heats up, electrons are more likely to collide with the vibrating lattice, etc (correct me if wrong please, or I would love a more detailed explanation, I'm not sure if this explanation is even legit since I'm only in high school), but this got me thinking how can anything be classed as an ohmic device? Surely this concept of temperature increase from colliding electrons/resistance applies to anything in which a p.d can be induced across, and thus nothing is strictly ohmic?

    Or is this effect negligible? Since the temperature increase is so small for a wire for example, in comparison to a filament bulb.

    Thanks very much for your input

    James
    Ohm's law posits a perfect proportionality between voltage and current: V = IR, and all that. Some elements conform very well to that law, others not so much. If, in a given circumstance, the deviations from perfect proportionality ("linearity") are small, we can go ahead and call it ohmic. It need not be passive; active elements can be ohmic (example: feedback around a transistor can cause it to behave ohmically).

    A light bulb can act ohmically as well, despite the nonzero (positive) temperature coefficient of resistance. How can this be? If a conventional incandescent bulb is driven with an ac voltage (as light bulbs typically are), you rarely notice a flicker. That's because the thermal inertia of the light bulb is high enough for the filament to stay at a roughly constant temperature throughout the ac cycle. The corresponding current will be proportional to the voltage, and thus the bulb acts "ohmically."
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Schneibster View Post
    It's chip-designer-speak for figuring out how to bond a wire to a silicon contact pad such that it is an ohmic contact between them and no extra semiconducting or other weird semimetal behavior results.
    I must have heard it in that context but I've never had much to do with the manufacturing side so I guess I didn't pay it any attention.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Schneibster View Post
    It's chip-designer-speak for figuring out how to bond a wire to a silicon contact pad such that it is an ohmic contact between them and no extra semiconducting or other weird semimetal behavior results.
    I must have heard it in that context but I've never had much to do with the manufacturing side so I guess I didn't pay it any attention.
    Yeah, I never heard it either until I started working for a company that made microinspection equipment, which of course goes in the chip foundries and is used during manufacturing to ensure materials aren't wasted and problems are detected early before they can do much damage.
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    For metals, Ohm's law applies over a surprisingly large range of applied voltages provided that the temperature is held constant. I think that part of the problem here is that statements of Ohm's law do not always specify that the constancy of temperature is a prerequisite.
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    Quote Originally Posted by JonG View Post
    For metals, Ohm's law applies over a surprisingly large range of applied voltages provided that the temperature is held constant.
    The conditions for Ohm's law to hold are even more restrictive than that. The general constraints are that linearity and time-invariance must hold.

    At low fields, the drift velocity of electrons (or, more generally, charge carriers) is well approximated as proportional to applied electric field (voltage), and Ohm's law emerges. At high enough fields, though, the drift velocity approaches thermal velocity. In that regime, any increase in applied field does not cause a proportionate increase in drift velocity; energy instead goes into scattering by high-energy ("optical") phonons. In that regime, the current changes little with increases in applied voltage.

    If temperature is not held constant, then temperature-dependent quantities, such as the mean-free path, will change, and cause the resistance to vary in a signal-dependent manner. So too with pressure and other second-order variables.

    I think that part of the problem here is that statements of Ohm's law do not always specify that the constancy of temperature is a prerequisite.
    More generally, the problem is that the word "law" misleads people. The lesson is that equations have a limited domain of validity. It is important to know what those limits are for a given equation, so that one does not use it outside of that limited domain.
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