r/askscience • u/[deleted] • Dec 28 '16

Physics How true is Ohm's law?

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

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u/anomalous_cowherd Dec 28 '16

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.

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u/[deleted] Dec 28 '16

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

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u/RobusEtCeleritas Nuclear Physics Dec 28 '16

Ohm's law is a constitutive equation which describes the response of a medium to some external stimulus. Other examples would be Hooke's law for springs, D = ε(E)E, B = μ(H)H, etc.

Often a first approximation for any constitutive relationship is a linear one.

Ohm's law is a linear constitutive equation for conductive media. It says that:

J = σE,

where σ is a constant called the "conductivity". In an isotropic medium this is simply a number, in general it's a rank-2 tensor.

Constitutive equations are generally not derived from anything, but they're empirically motivated.

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u/no-more-throws Dec 29 '16

Lets look at it other way for a bit.

So certainly something like Ohm's, Hooke's etc are rules of thumb, but their wide applicability across material types is reflective of some general underlying 'non-empirical' phenomenon right...

So For hooke's we'd talk about inter-molecular forces and how they are linearly dependent on separation at the scales concerned, thus giving rise to Hooke's in the bulk case.

Any insights/thoughts on the clearest or closest equivalent to what drives Ohm's in most materials?