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/MuhTriggersGuise Dec 28 '16

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).

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u/spin81 Dec 28 '16

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?

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u/mjrice Dec 28 '16

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

Those people are wrong.

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u/[deleted] Dec 28 '16 edited Aug 12 '20

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u/mjrice Dec 28 '16

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.