What is R when you apply that formula to a battery? I does not apply to batteries because they are not resistors.

There is a different law for each component that relates voltage to current.

For a capacitor the law is:

I = C dV/dt

Which is a differential equation.

That equation applies to capacitors, not resistors or batteries.

The I V equation for l an ideal 12V battery is

V = 12

Note that there is no I or R in the formula meaning it dies not depend in the current. For a real battery, that changes

Applying Ohm l’s law to a battery is equivalent to this:

Gas milage = distance travelled / gas used

So what is the gas milage a tree?

 

I guess I was just looking for a more discrete reason for why it doesn't apply to batteries et al. My intuition told me it very clearly shouldn't apply to batteries. But if anyone asked me why it didn't apply, I wouldn't be able to give a better answer than, "because!" I feel like that could to lost confidence. It's very hard to teach if you don't have a student's confidence and trust.

My take away is this:

When current passes through a resistor I and R both have finite, non zero values, therefore, it makes sense for V to have a finite, non zero value.

But for an open circuit battery I = 0 and R = ∞. To me: "V = 0 / ∞" shows clearly enough the limit of Ohm's law and that we would need a different tool/formula to understand open circuit voltage of a battery.

In this case I'm not concerned with finding a definition for describing open circuit voltage of a battery. I just wanted to find an adequate reason for why Ohm's law didn't apply to batteries other than just, "because."

This may seem overly pedantic but it's been my experience, in my trade, that vague ideas lead to techs being confused and mislead which leads to things being misdiagnosed which leads to revenue and customers lost.

Ohm's Law is not a universal law that applies to everything. It is a mathematical model of the relationship between various parameters that is exhibited by a certain class of materials. That class of materials are referred to as "ohmic materials".

Most materials and devices are non-ohmic, which merely means that they do not obey Ohm's Law. Inductors, capacitors, diodes, and, yes, batteries, are non-ohmic devices.

Even materials, like the best resistors you can find, that are generally classified as ohmic are non-ohmic to some degree. So what we really mean when we say that a device obeys Ohm's Law is that it obeys it well enough for our purposes at the moment.

Similarly, even the most non-ohmic devices almost always have some small portion of their behavior that is ohmic -- meaning that a change in current results in a proportional change in voltage. This is usually modeled by including an ohmic resistance as part of a larger circuit model. For a battery, we call this the internal resistance of the battery.

 

Search around and you will be able to find online courses and tutorials specifically targeted to automotive electricity and electronics. This absolutely essential, as you are well aware, everything in automobiles are controlled by electronics today.

During my time in this field I've read many tutorials/courses, watched many videos and been to many seminars. It's because of this that I'm wanting to make my own. Most of them put a focus on making an intuitive model. One that you can visualize and understand clearly. My focus is on the effectiveness of the model. I care only about how easy it is to APPLY the model with accuracy and consistency. I care about ability to solve problems on cars in the work environment. Not about ability to solve problems in coursework at some desk.

I find trying to visualize electricity to be extremely counter productive. It's not water in a pipe. Water is a completely separate entity than the pipe. It's free to radiate anywhere it can. It has no need to, "go back to the source." I find all of that leads to "clear" models but models that require a lot of steps to translate to something applicable to the car in front of you. An environment that's incredibly busy and messy.

It's this reason why I want to make my own course. I find it better to just think of a circuit as having certain properties and capable of being in certain states. Most devices we can just treat as black boxes. It's helpful to be aware of some of their characteristics but for the most part we just need to know what they need in order to function and what they're supposed to do. If they're not doing what they're supposed to do and they have what they need then the device is faulty.

We can treat the voltage from our power sources (battery and alternator) as pretty stable. If we have a supply issue it's VERY apparent. So really all we concern ourselves with is if the device is working and the circuit has adequate current carrying capacity. As in, no extra, unwanted resistance (corrosion, bad connection, break in wire, etc).

All very simple stuff. Our complexity comes from having such a noisy environment. It can be difficult to filter out all the non relevant stuff. Plus it can be very difficult finding out what a device's proper inputs and outputs should be...

Wow, I'm rambling. Sorry for that. Thanks for letting me babble on if you got this far.

 

No problem, most of the users here like helping people who are honestly trying to learn. It is obvious you have a long way to go but you have started a very interesting trip.

 

V = 0 x ∞

The formula is still correct and any value of V satisfies the formula.
But V is the driving force of the battery and is 12V. You cannot change that. V = 12V is a constant value in this application.

Okay, I can dig that. My goal is to create a framework focused on its ease of application to my industry. Not necessarily on ease of understanding the model, but on ease of applying it to get consistently accurate results. It'll be a gross simplification of many things but I'd like to have it grounded in enough reality so as to not get, "kicked off a physics bus". Sure I'd get some scowls... be thrown to the back... possibly even shoved in a suitcase... Just not get kicked off altogether.

 

You can differ as much as you please but that is blatant misinformation which we are desperately trying to correct in this valid query.

Well if it's that desperate, and you consider re-arranging the terms in Ohm's law "blatant misinformation" then by all means, we'll go with you pedantic view.

 

Well if it's that desperate, and you consider re-arranging the terms in Ohm's law "blatant misinformation" then by all means, we'll go with you pedantic view.

Pedantic? Yes.

Have you ever taught Ohm's Law?

I teach physics and electronics at the university level.
I teach Ohm's Law as

I = V / R

not
V = I x R

 

I find trying to visualize electricity to be extremely counter productive. It's not water in a pipe. Water is a completely separate entity than the pipe. It's free to radiate anywhere it can. It has no need to, "go back to the source." I find all of that leads to "clear" models but models that require a lot of steps to translate to something applicable to the car in front of you. An environment that's incredibly busy and messy.

The water model to electricity sometimes work and sometimes it doesn't.
For this reason it is better to stay clear from using this model because it leads to confusion and misinformation.

 

Have you ever taught Ohm's Law?

No, but I've used it in all its algebraic forms for my 40 years of electronic design work as an EE.

I teach physics and electronics at the university level.

Bully for you.
That doesn't excuse your condescending replies.

 

Pedantic? Yes.

Have you ever taught Ohm's Law?

I teach physics and electronics at the university level.
I teach Ohm's Law as

I = V / R

not
V = I x R

Neither of them is Ohms law, if you want to be pedantic. From Wikipedia

He found that his data could be modeled through the equation

x = a / (b+l)

where x was the reading from the galvanometer, l was the length of the test conductor, a depended on the thermocouple junction temperature, and b was a constant of the entire setup.

Note that it is solved for current, not voltage.

 

Try Wikipedia.

https://en.wikipedia.org/wiki/Ohm's_law

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship:[2]



where I is the current through the conductor, V is the voltage measured across the conductor and R is the resistance of the conductor.

 

No, but I've used it in all its algebraic forms for my 40 years of electronic design work as an EE.
Bully for you.
That doesn't excuse your condescending replies.

My responses to you are not condescending.
You ought to know better.

I have explained this perfectly and clearly to the TS and anyone else who wants to argue.
I am done with this.

 

My responses to you are not condescending.

People who are condescending seldom think the are.

You ought to know better.

And ironically, in this very next sentence you give me an example of a condescending reply (or perhaps the word is patronizing).

I am done with this.

You won't be missed.

 

Guys just a suggestion, let's keep it on topic. This thread is about ohms law not personalities.

 

This thread is about ohms law not personalities.

Okay.
But I wasn't referring to personalities, just to the tenor of the one of the responses, which I found patronizing, and I've been around way too long, to put up with that.
Since that responder is one of your moderators, I suggest you have a conversation with him (unless of course, you find the way he responds acceptable).

 

Yes, I think it does and to clarify it, just consider the resistance increasing without limit and approach but not reach infinity, in that case the current will decrease without limit and approach but not reach zero. What is important is the product of both that is I x R that will approach a constant and that is the battery voltage. This happens in calculus all the time. In this case it is the 'open circuit voltage' that happens to be a limit when 'I' moves towards zero and 'R' moves towards infinity. All batteries have an 'internal resistance' thus the voltage will always be something less as the current increases. You are looking at a limit problem, but because the DMM has sufficiently high resistance the measurement will obviously be close to the final limit to the point that the measurement is close enough.

I got this graph off the internet which IMHO speaks volumes:

 

I think it is important to also realize that just because V=IR, this does not mean that if I halve the resistance, the current will increase by a factor of 2 because the voltage may also change due to the chemistry of the battery. Ohms law still holds but that does not mean that only one variable may change, most likely both variables V and I could change if R changes. The same holds for the other two as well. We cannot assume that one of the others remains constant just because we forcibly change one of the individual contributors.

 

Someone very old school might say that a battery has emf (Electromotive Force) and voltage is something that is developed according to Ohm's law.

EMF is the correct term but voltage is independent of Ohm's law.

 

My responses to you are not condescending.
You ought to know better.

I have explained this perfectly and clearly to the TS and anyone else who wants to argue.
I am done with this.

Condescending means you called him out for being wrong. It's a common occurrence.

 

EMF is the correct term but voltage is independent of Ohm's law.

Interesting, being that Voltage is Joules per C, Current is C per second and resistance is J*s/C^2 that is:

J/C=C/s * (J*s/C^2) =
J/C = J/C

unit analysis seems to indicate it does, unless I am misunderstanding what you are trying to state?

The unit for ohm came from the following link:

https://www.rapidtables.com/electric/ohm.html