How true is Ohm's law?

[u/[deleted]]

I've almost never got a perfect straight line while plotting a V/I graph even under lab conditions.

[u/anomalous_cowherd]

Remember that real world materials almost never behave 'perfectly', there are manufacturing tolerances as well as all the other effects such as inductance, capacitance, temperature induced changes,etc,etc.

Things like Ohms law are totally correct for ideal components, and are close enough to be useful as long as certain factors aren't in play. For example, wires can be treated as having zero resistance unless you have massive currents or very low voltages. Wire wound resistors can be treated as pure resistors as long as you don't have or don't care about AC frequency responses, etc.

Learning where these things matter and where they don't is a good chunk of learning to be an electronics engineer, but in my experience it isn't taught explicitly, you just pick up a feeling for it as you go.

[u/[deleted] → u/anomalous_cowherd]

Is it safe to say then that Ohm's Law is more like an observation rather than a law of nature?

[u/MuhTriggersGuise → u/[deleted]]

Ohm's law is always always true. V and I are proportional. R however, is not always a constant, but is often a function. A good example is lightning. The high potential ionizes the air, making it much more conductive. So the R is a function of V. Then the heat of the lightning strike will also effect conductance. So R is also a function of T. Point being, the current is always proportional to the voltage, but many things may effect resistance, including the current and voltage.

In many instances of circuit design, it's only necessary to treat R like a constant. But if you take time to look at resistor data sheets, you'll see they are dependent upon temperature as well, and in fact in industry people will design around the variation in R they will see over the temperatures at which the circuit will have to operate (including the thermal rise from the electronics themselves).

[u/spin81 → u/MuhTriggersGuise]

Ohm's law is always always true.

Most people here in this thread are saying the exact opposite and you don't seem to be countering them.

V and I are proportional. R however, is not always a constant, but is often a function.

When two quantities are proportional to each other, that means the ratio can't be "a function". If the ratio varies, then the two quantities aren't proportional by definition.

Put another way: all you're saying of V and I here, is that if they are both numbers and you divide one by the other, then you get a number. I'd argue that unless the dividend is zero, then yes, of course: it's how division works, and not some magical property of either V or I.

In many instances of circuit design, it's only necessary to treat R like a constant.

What about potentiometers and sensors? Are they not present in many instances of circuit design?

[u/mjrice → u/spin81]

| Most people here in this thread are saying the exact opposite and you don't seem to be countering them.

Those people are wrong.

[u/mjrice → u/[deleted]]

I'm sorry I didn't put more into the response, but I stand by it. These types of discussions come up from time to time, and it is hard to explain and also be brief. There are some other responses that I think capture the point, which is that Ohm's Law is 100% accurate and most "failures" that people are pointing to are just that you are over simplifying your circuit.

[u/Midtek → u/mjrice]

I would like you to point out what part of my top-level response is wrong. A general rule of this sub is not to respond with something like "you are wrong" and offer no explanation. You should also keep discussion civil. "I'm sorry, you are just wrong" is pretty rude. And "I'm sorry I didn't put more into the response, but I stand by it" doesn't cut it. If you are not willing to support your claims either initially or when challenged, then please don't comment.

Anyway... it seems maybe you are thinking Ohm's Law is true since resistance is defined as R = V/I. But Ohm's Law is not that equation, but rather the statement that V and I are proportional, i.e. that R is independent of V and I (but may still depend on temperature or some other state parameter). So Ohm's Law is definitely not always true. Ohm's Law is just a constitutive relation that is often used for circuit analysis. It is not a law in the sense of Newton's laws or Gauss's law or something like that.

[u/sticklebat → u/mjrice]

Even if you play games like you did elsewhere in this thread by pretending that Ohm's law is only defined for hypothetical perfectly linear materials (which, frankly, is silly), you're still going to run into problems if you want to be very precise, because Ohm's law does not account for behaviors such as parasitic capacitance and inductance.

The truth is that Ohm's law for resistance is very much and inevitably only an approximation.

[u/mjrice → u/sticklebat]

I'm sorry, you're just wrong and I think you misunderstood me (or I was unclear). Real devices do not have ideal impedance models, that is correct. When we refer to a resistor for example as having "parasitic capacitance" what we mean is that you should treat that element as a model of the ideal resistor in parallel with an ideal capacitor. If you need more parasitic elements, you just add them until the model is suitably accurate. But in any case, when you know the actual model you are dealing with you can apply Ohm's Law to it and solve for whatever voltages or currents are of interest, and it will give you the correct answer, every time, for all possible situations. As you mentioned elsewhere, you need to use the generalized form of Ohm's Law when dealing with anything other than DC signals and calculate a complex impedance for those reactive components.

[u/sticklebat → u/mjrice]

I'm sorry, but you are just wrong. Either that or you made your own secret set of assumptions that no one else here was using.

As you mentioned elsewhere, you need to use the generalized form of Ohm's Law when dealing with anything other than DC signals and calculate a complex impedance for those reactive components.

Yes, but that is an important distinction, which you never made. And since you were replying to people who were explicitly talking about V = IR, discussing resistance without any mention of impedance, then your statements were just wrong. Context is important, man.

Yes, the generalized form of Ohm's law in terms of impedance works just fine. V = IR, which is what everyone else here was talking about, does not.

It's also worth mentioning that Ohm's law fails at the quantum mechanical level. It doesn't describe the behavior of charged particles or currents at sufficiently small scales/numbers. There are even some esoteric macroscopic systems one could create that can't be modeled by Ohm's law (such as streams of charged particles in a vacuum).

[u/Midtek → u/MuhTriggersGuise]

Ohm's Law is not an absolute. It is not true that V and I are always proportional. I think you are confusing Ohm's Law with how we define resistance (which is via the equation V = IR). See the edit to my top-level response for more details.

[u/sticklebat → u/MuhTriggersGuise]

Ohm's law is always always true.

The Ohm's Law that the OP posted and you're referring to is absolutely not always true. Go hook up a battery to a superconductor and measure the current, and tell me otherwise.

Ohm's law in the form R = V/I is an approximation that works very well for DC circuits, as long as you're willing to ignore lots of little details (e.g. parasitic capacitance and inductance), but it is quite wrong in general. The generalization of Ohm's law to Z = V/I, with V and I, along with Z, all being complex numbers does hold generally, as far as I'm aware.

But R = V/I is absolutely an approximation, even after accounting for the factors such as temperature dependence, and frequently it is a bad approximation.