The EMF generated across the resistor due to I*R is the opposing force! This goes back to my comment about diodes...is the forward voltage across a diode an EMF, or something else

This is still rubbish.

The EMF applied to the resistor (object B in mechanics) by the battery (object A in mechanics) is the analog of the action in N3.

The resistor applies no corresponding reverse emf to the battery, corresponding to the reaction in mechanics.
The resistor is incapable of generating an EMF, by definition.

I do not know how to put it any plainer.

I have attached three circuits containing a diode.

Please feel free to show a KVL analysis describing the voltages seen at points A, B C, and D in each case.

Sometimes you can do a loop analysis with a modified form of KVL, but you can't guarantee that it will work.

Remember that Kirchoff, if he was still alive, would be nearly as old as I am, and had never heard of diodes before he died.

 

As I said earlier, we must agree to disagree. I've had my say.

But I am rather annoyed that, for someone with such supposedly great respect for Kirchhoff, you repeatedly misspell his name.

 

But I am rather annoyed that, for someone with such supposedly great respect for Kirchhoff, you repeatedly misspell his name.

You are right at that, perhaps that's why he never liked me at school!

I never noticed the double h before.

But then you will have noticed, if you have read many of my posts, that spelling is not my strong point.

 

I think "Kirchoff" is a very common misspelling of his name. Working in the electronics lab at the university, I have been able to observe some classes, and I think most of the professors here spell his name wrong as well.

Then again, it's not as bad as my physics professor back when I was in school. He insisted Tesla's name was spelled "Telsa". It bugged me to no end, and I actually spoke up (several times) in class to correct him.

 

I am going to regret this, studiot, but I feel like arguing just a bit more. And, I just finished releasing some code today, so I have a bit of time.

Back to your pebble on a table example. Let's remove the table...it's just complicating the example.

Let the pebble float static free in space. Now, this is just an ordinary pebble. It's got no source of power, no means of changing speed or direction of its own volition.

Now, let's throw in a massive body near the pebble, say a planet. Suddenly, the pebble finds itself in a gravitational field of magnitude g, producing a force vector on the pebble of magnitude mg (where m, of course, is the gravitational mass of the pebble).

N3, of course, states that that force must be countered by an opposite force of equal magnitude. That force is the inertia of the pebble -- equivalent to product of the inertial mass of the pebble and its acceleration -- and arises regardless of the fact that the pebble has no means of generating power. Empirical evidence suggests that inertial mass and gravitational mass have the same value for all matter (and Einstein himself has argued that there is no need to differentiate between the two).

But now, observe this same pebble relative to a reference frame accelerating at g in the same direction as the pebble (which is perfectly valid according to GR), and you will see that the forces, both with respect to gravity and inertia, disappear. The pebble is now just the same old pebble at rest that we started with.

From that vantage point, the pebble is not providing a reactionary force. It could also be said to *never* have provided such a force. Further, as the pebble has no self-contained source of power, one could say as long as we choose our reference frames properly, the pebble will continue to eternity as just a simple, static, free floating pebble.

Of course the resistor, just like the pebble, has no means of providing a countering force. For the pebble to provide such a force, it must be accelerated by the gravitational field. So you could say the pebble is performing a bit of jujitsu and reflecting gravity back on itself via the pebble's inertia.

Likewise, for the resistor to provide a countering EMF, it must be driven by the resulting current derived from the voltage source (times the resistance, of course), performing an identical jujitsu move against the voltage source.

This is my story, and I'm sticking to it.

 

Still rubbish I'm afraid.

Let's remove the table...it's just complicating the example.

The action and reaction is between the pebble and the table!

So if you remove the table you remove both.

You might like to know that

The Law of gravitational attraction is


\(F = {\lambda _g}\frac{{{m_2}{m_1}}}{{{r^2}}}\)

It depends upon and acts upon both masses equally.

This can be compared with Coulombs law for electric charge


\(F = {\lambda _e}\frac{{{q_2}{q_1}}}{{{r^2}}}\)

The difference is that the force of attrataction is just that - it is always positive. there are no negative and positive charges in gravity.

However the electric version may be attractive or repulsive.

 

I knew I'd regret it. We are speaking past each other.

I must admit, though, you are quick with the Tex.

 

I think you guys are arguing a moot point. There is no such thing as a perfect voltage source, OR a perfect short. It can't exist in the real world, hence math really can't explain it, or even solve it. If it was explainable by math then it would probably exist, but since it doesn't exist, real definitive mathematics cannot be applied.

If you ask me, there is no correct answer, nor does there need to be. There is absolutely no way to prove it one way or another.

 

Thank you for your thoughts, DerStom.

But did you not have any thoughts on the original post or issue rather than the side show?

 

Thank you for your thoughts, DerStom.

But did you not have any thoughts on the original post or issue rather than the side show?

I posted my thoughts on the original post already, but I think you two were too busy arguing to see it

 

"Apply an ideal 9v source to an ideal insulator and zero current flows, by definition."

Sorry, Studiot, I don't buy it. You're bypassing rigor by using the vague term "ideal".

What's the resistance of an "ideal" insulator?

If we accept Ohm's law, then given a voltage of 9 volts, if that insulator has any non-infinite value of resistance then a non-zero current will flow. You state that the current is zero, so what does that say about the resistance?

If you reject the term infinity, then what else is there to call it?

 

Just send the 9V battery into free space well away fro the solar system. Once the capacitive displacement current (if any) "flows" within the battery one should have zero current. The battery may die in free space because of it having approached absolute zero temperature. But we could assume it is "ideal" [same problem]. Alternatively we could connect the battery on earth across a pre-charged ideal capacitor [same problem] having the exact same terminal voltage [same problem] as the battery.

 

Hey, y'all left out "Dark Matter".

And, "Dark Energy".

 

Apply an ideal 9v source to an ideal insulator and zero current flows, by definition."

Sorry, Studiot, I don't buy it. You're bypassing rigor by using the vague term "ideal".

What's the resistance of an "ideal" insulator?

If we accept Ohm's law, then given a voltage of 9 volts, if that insulator has any non-infinite value of resistance then a non-zero current will flow. You state that the current is zero, so what does that say about the resistance?

If you reject the term infinity, then what else is there to call it?

Thank you davebee, you have come to the point I wanted to bring out in the first place.

How to avoid 'infinity'. Or more particularly dividing by zero.

Infinity is a convenient symbol we can write to avoid saying something much longer.
But it will never give a result (number) we can put into calculations.

If we can have an ideal voltage source and an ideal resistor, why not an ideal insulator, defined such that zero current flows?

And if we have a component such as a switch that does not follow Ohm's Law, and many components don't, why do we need to demand that our insulator follows Ohm's Law?

What is the resistance of a switch in the off position?

My answer is that this is an inappropriate question. You can call it infininte for brevity if you like, but you can't calculate with it.

I stated in post that physics (and therefore electrical engineering) treats infinity differently from mathematics. There is some common ground, but difficulties arise when someone tries to apply the part of maths that is not common to physics and finds nonsense because the physical world does not respond in the same way as maths treats that infinity.

 

...
No, both situations can be realised in practise without faults being required. Even if there were to be a fault it should still be capable of analysis.
...

Now I'll call you out on that, from my area of expertise (decades of making my living from logical analysis/deduction/simplification).

Your puzzle was stated as;
"A 9N force is applied to a body with a result of zero acceleration. Determine the mass. "

Your puzzle contains 3 elements only;
1. 9N force is applied
2. acceleration 0 occurs
3. there is a mass, what is it?

The only solution for the three elements involves the situation of infinite mass. Let's ignore that for now. Where there is NOT infinite mass, the puzzle is faulty, ie; it requires another condition or situation outside the 3 elements.

I covered the fault possibility in my first answer where I said; ""Zero acceleration" can only occur if the mechanism is faulty/blocked etc" and Newtons 3rd law would definitely be a blocking fault. Some external influence (outside your 3 puzzle elements) which when introduced, causes your 3 element test to be non-valid.

Just because the fault you chose is "Newton" does not in my mind give it credibility in the answer. You could equally chosen any other blocking fault (like an air jet, or glue) which would be just as effective in destroying the validity of the test.

If you still want to do the "infinity * 0" thing I thought of a good simple proof the other night that proves infinity*0=0, which is immediately visually accessible.

 

The battery may die in free space because of it having approached absolute zero temperature. But we could assume it is "ideal" [same problem].

Sorry t_n_k, just being picky here, but outer space is not at absolute zero temperature, it is actually about 3 Kelvin above absolute zero.

Infinity is a convenient symbol we can write to avoid saying something much longer.
But it will never give a result (number) we can put into calculations.

...

I stated in post that physics (and therefore electrical engineering) treats infinity differently from mathematics. There is some common ground, but difficulties arise when someone tries to apply the part of maths that is not common to physics and finds nonsense because the physical world does not respond in the same way as maths treats that infinity.

studiot, I'd have to argue that you can't have ideal sources/resistors and NOT deal with infinity. An "ideal" resistor would have to be one of infinite resistance, in order to prevent ALL current flow. Now you're right in the sense that infinity is just a symbol--a concept, really--rather than a number, but in this case it must be treated as a number. An ideal voltage source by definition would have to allow an infinite amount of current to flow if placed across a perfect short. And by definition, 1/0 is ∞. Not by real maths, but by concept. You cannot apply real math to ideal sources or resistors. If you insist on having one, you must be willing to accept the other as well.

Guess that makes $0.04 total from me

Matt

 

And by definition, 1/0 is ∞.

but in this case it must be treated as a number

Reference to an acceptable authority please. Particularly the second quote that allows ∞ to be treated as a 'number'.
What sort of number, by the way, did you mean?

An "ideal" resistor would have to be one of infinite resistance, in order to prevent ALL current flow

Yes indeed, but an insulator is not a resistor, ideal or otherwise. If it were there would be no point having two identical circuit elements with different names.

 

The only solution for the three elements involves the situation of infinite mass. Let's ignore that for now. Where there is NOT infinite mass, the puzzle is faulty, ie; it requires another condition or situation outside the 3 elements.

Actually after some logical thought about three elements (and my question what holds it in place?) it should become obvious that there must be other agents involved. The three 'elements' I listed are not compatible, by themselves, with Newtons three Laws.

I did not say that you may not introduce any other agent you wish, only that the end result must be as specified. It was not one of those exam questions where you are 'given' all the necessary information. Rather it was like a real life situation where you have to determine what information is required, what laws apply and then work it out.

Remember my objective is to examine ways to approach a problem that avoid attempting to manipulate 'infinity' or divide by zero.

Surely that is a profitable objective for all concerned?

 

Reference to an acceptable authority please. Particularly the second quote that allows ∞ to be treated as a 'number'.
What sort of number, by the way, did you mean?

In the second quote I mean that it should be treated as a number rather than, say, a distance or anything else for that matter. I should have brought up the limit, rather than basic division:

\(\stackrel{lim}{x->0}\) \(\stackrel{1}{x}\) = ∞, or "does not exist"

I think we can all agree that infinity is not an actual thing we can define (hence why it is in-finite). It is simply a concept that we use to describe the highest possible number. Now again, we know that there can't be a "highest number" because they are infinite--you can always count one number higher. So any time you hear someone (me, for example) say "infinity", I am referring to something that has no mathematical limit. The same concept works for a resistance. If a resistance is infinite, there is no limit meaning that current cannot flow. So "infinity as a number" was probably a bad way for me to word it, what I meant was a value without an upper limit.

Yes indeed, but an insulator is not a resistor, ideal or otherwise. If it were there would be no point having two identical circuit elements with different names.

I beg to differ. An insulator with an infinite resistance is the same as a resistor with infinite resistance. A perfect insulator is a resistor of sorts, it is just assumed to have an infinite resistance. Resistors, on the other hand, are assumed to have a finite resistance, which is why they are treated as two different things. In the real world, there is no such thing as a perfect insulator--everything has a finite resistance. But since we're talking about the theoretical "perfect world", where you can have ideal insulators, ideal voltage sources, and so on, then you will have to accept the theoretical value of infinity. It is what binds this "perfect world" together. Why are you so eager to accept the impossible idea of ideal voltage sources and insulators, but not the impossible idea of infinity? They go hand-in-hand!

 

I think we can all agree that infinity is not an actual thing we can define (hence why it is in-finite). It is simply a concept that we use to describe the highest possible number. Now again, we know that there can't be a "highest number" because they are infinite--you can always count one number higher. So any time you hear someone (me, for example) say "infinity", I am referring to something that has no mathematical limit. The same concept works for a resistance. If a resistance is infinite, there is no limit meaning that current cannot flow. So "infinity as a number" was probably a bad way for me to word it, what I meant was a value without an upper limit.

This is getting much better, (except for the can't define bit) you are working towards the appropriate terminology.

"A value without an upper limit"

The correct term is "unbounded above"

But this idea "you can always count one number higher." is the one.

Instead of your limit / tends to formulation how about this

A non empty set, S, unbounded above, such that

\({\rm{Whenever r }} \in {\rm{ S then (r + 1) }} \in {\rm{ S}}\)

There are other possible formulations.

An insulator with an infinite resistance is the same as a resistor with infinite resistance. A perfect insulator is a resistor of sorts, it is just assumed to have an infinite resistance.

Is an ideal capacitor a resistor then?