B Which makes a bulb glows brighter?

Khashishi

Ohm's law is certainly a law, and not simply a definition. We usually just quote the equation $I = V/R$, but of course, some additional understanding of the variables is necessary to appreciate the law. Really, this is the case of all laws, including Newton's laws. But, Ohm found that $R$ is mostly independent of $V$ and $I$, or that current is proportional to the voltage, and the proportionality constant can be called $R$. (It's not true for all materials, but a law doesn't cease to become a law just because we found its limits.)

$F=ma$ could be regarded as a mere definition of force, or of mass, depending on what you take for granted. It's the same story here: we understand that force is proportional to acceleration, so $m$ is a constant of proportionality. Of course, the law needs to be emended for relativity.

Pretty much all laws can be explained this way. A lot of them look like simple definitions, but it wasn't necessarily obvious that they should be defined in this way before their discovery. It's simple to define $m'=m/a$ such that $F=m'a^2$, but this would be a WRONG law.

sophiecentaur

Gold Member
How does "only ohmic conductors obey Ohm's Ratio" sound to you ?.
It's one way of putting it and you could extend it to saying Ohm's Law applies to metals (which is what it is all about). If you see the light light when you tell it to some keen young thing then you will have confirmed that it's ok.
But the point is that Constant Temperature is a very relevant factor in yer real Ohm's Law. Under those conditions, V/I is constant for a metal. That's the other way round from your way of looking at it, which is totally practical and serves a purpose.
We usually just quote the equation I=V/RI=V/RI = V/R,
That way round is a derivative from the actual law and on it's own is simply a definition.
My argument is that R=V/I is not Ohm's law. V and I apply to all elements of a circuit and V/I will always give an answer - for a diode, a capacitor or a light bulb. Ohm's law doesn't apply to them so how can it be correct to say that the ratio of V and I "is Ohm's Law" for all those non-ionic elements? Ohm's Law describes how ideal metals behave at constant temperature and, when used appropriately, it is a 'proper' law. But just because it is commonly misused, doesn't mean that it's misuse is correct.
In an Electronic Constructors' Forum, using words like Ohm's Law in a loose way is fair enough but PF is about Physics, essentially so surely the Physics should count.

Angela Liang

It's one way of putting it and you could extend it to saying Ohm's Law applies to metals (which is what it is all about). If you see the light light when you tell it to some keen young thing then you will have confirmed that it's ok.
But the point is that Constant Temperature is a very relevant factor in yer real Ohm's Law. Under those conditions, V/I is constant for a metal. That's the other way round from your way of looking at it, which is totally practical and serves a purpose.

That way round is a derivative from the actual law and on it's own is simply a definition.
My argument is that R=V/I is not Ohm's law. V and I apply to all elements of a circuit and V/I will always give an answer - for a diode, a capacitor or a light bulb. Ohm's law doesn't apply to them so how can it be correct to say that the ratio of V and I "is Ohm's Law" for all those non-ionic elements? Ohm's Law describes how ideal metals behave at constant temperature and, when used appropriately, it is a 'proper' law. But just because it is commonly misused, doesn't mean that it's misuse is correct.
In an Electronic Constructors' Forum, using words like Ohm's Law in a loose way is fair enough but PF is about Physics, essentially so surely the Physics should count.
Thx. I think my question was incorrect. Since V is fixed and we hv put a bulb in the circuit, the current should be fixed. There is no way to increase the current without changing the voltage, right?

jim hardy

Gold Member
2018 Award
Ohm's law is certainly a law, and not simply a definition. We usually just quote the equation I=V/RI = V/R, but of course, some additional understanding of the variables is necessary to appreciate the law. Really, this is the case of all laws, including Newton's laws. But, Ohm found that RR is mostly independent of VV and II, or that current is proportional to the voltage, and the proportionality constant can be called RR. (It's not true for all materials, but a law doesn't cease to become a law just because we found its limits.)
Seems a fine semantic point
but might it have didactic value ?

Ohm derived his law by parallel thought to contemporary experiments with conduction of heat through metal.
He was using galvanic cells and wire.

http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/Jeremie Smith/page2.htm
A little bit about the life and times of Georg Simon Ohm:

Georg Simon Ohm was a German physicist born in Erlangen, Bavaria, on March 16, 1787. As a high school teacher, Ohm started his research with the then recently invented electric cell, invented by Italian Conte Alessandro Volta. Using equipment of his own creation, Ohm determined that the current that flows through a wire is proportional to its cross sectional area and inversely proportional to its length. Using the results of his experiments, Georg Simon Ohm was able to define the fundamental relationship between voltage, current, and resistance. These fundamental relationships are of such great importance, that they represent the true beginning of electrical circuit analysis. Unfortunately, when Ohm published his finding in 1827, his ideas were dismissed by his colleagues. Ohm was forced to resign from his high-school teaching position and he lived in poverty and shame.
Like so many pioneers he wound up face down in the mud and full of arrows.

Anyhow he indeed derived his law for metal wire.

One can take the ratio of voltage across to current through anything and get a number for it
but if one plots that ratio at varying currents it will be a straight line only for reasonably well behaved materials like metal, and then only until the current starts heating the wire.

In a diode
i = eqv/kt q = charge of electron, v = volts, k = Boltzmann,
so ln(i) = qv/kt
and
v = kt/q X ln(i)

R = v/i = ( kt/q) X ln(i)/ i

and at every i you will get a different Ohm's Ratio.

Seems a plausible way to teach it,
Ohm's Ratio is linear only for ideal circuit elements
and we call that linear relationship Ohm's Law
and Ohm's Law is the very basis of circuit analysis.

"In improving our language we reason better " ??

old jim

jim hardy

Gold Member
2018 Award
But the point is that Constant Temperature is a very relevant factor in yer real Ohm's Law.
this is the best stuff i know of, 266 times better than copper

Drakkith

Staff Emeritus
2018 Award
Thx. I think my question was incorrect. Since V is fixed and we hv put a bulb in the circuit, the current should be fixed. There is no way to increase the current without changing the voltage, right?
You can change the resistance of the circuit. Increasing or decreasing the temperature of part of the circuit is one way to do this.

sophiecentaur

Gold Member
and Ohm's Law is the very basis of circuit analysis.
Hmmm. It isn't much use assuming Ohm's law applies to a diode is it - despite the fact that it will have a Resistance because it will have a V and an I? That will really mess up your analysis.

Let's go elsewhere then. Hooke's Law applies well to a metal spring but we know it doesn't work for a rubber band or a plastic bag. Would you say that the ratio for a rubber band (= force/extension), under a particular load "is Hooke's Law"? No; stiffness is the word that's used for it for it. I think that years of using a term inappropriately has actually done a disservice to a many students of EE. I'd hate to suggest that Mechanical Engineers are better disciplined than Electrical Engineers . . . . . . .Especially as I am / was a chartered Electrical Engineer.

jim hardy

Gold Member
2018 Award
Hmmm. It isn't much use assuming Ohm's law applies to a diode is it - despite the fact that it will have a Resistance because it will have a V and an I? That will really mess up your analysis.
Nope. You'll have to remember it's only that many ohms at that many amps. That's Ohm's Ratio (to use my term) .

But you have to remember that anyway.

sophiecentaur

Gold Member
You are incorrigible dear boy.
We're just going to have to disagree on this one.

jim hardy

Gold Member
2018 Award
We're just going to have to disagree on this one.

I thought we were agreeing. But i'm mildly dyslexic.

Some difference in our respective concept of "law" ?

Angela Liang

You can change the resistance of the circuit. Increasing or decreasing the temperature of part of the circuit is one way to do this.
How to increase or decrease the temperature?

sophiecentaur

Gold Member
How to increase or decrease the temperature?
You could increase the temperature of a light bulb filament by shining a high power light source on it. You could decrease the temperature by placing the bulb in a blackened container that's held a a very low temperature - say in liquid nitrogen and that would reduce the surface temperature by about 200°C.

Angela Liang

You could increase the temperature of a light bulb filament by shining a high power light source on it. You could decrease the temperature by placing the bulb in a blackened container that's held a a very low temperature - say in liquid nitrogen and that would reduce the surface temperature by about 200°C.
Wow. Thanks :)

houlahound

So power in a light is proportional to diameter of filament assuming constant length tungsten filament?

Merlin3189

Homework Helper
Gold Member
So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
I'm not sure which element of the thread this has come from - Ohm's ratio, temperature dependence, power in a resistor? But whichever it is, perhaps we need to be clear about the details of the context, else statements like this are hard to evaluate.
Assuming for the moment, it is talking about the light FROM an incandescent bulb, plugged into mains - approximating a constant voltage source, then I would think that:
increasing the diameter would lower the resistance in inverse proportion to the cross sectional area, so inverse proportion to the square of the diameter;
reducing the resistance increases the power dissipation according to V2/R, so inversely proportional to the resistance or proportional to the are or the square of the diameter;
greater power dissipation would increase the temperature until the radiated power equalled the new electrical power;
since the radiated power is also proportional to the surface area, which is proportional to the square of the diameter, so there is no need for any temperature rise to increase the radiated power.

Net result, increased light power is proportional to square of diameter for a constant length filament, so long as the the resistance of the filament remains much larger than the resistance of the power supply. (If you carry on increasing the diameter of your filament, eventually nearly all the power is dissipated in the supply and very little in the filament - or massive metal bar, as it might then be called.) The temperature does not change (if the filament is in empty space) so the colour does not change.

sophiecentaur

Gold Member
So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
That's what the standard formula relating length CSA and Resistivity to Resistance says. Resistivity is temperature dependent. Edit: The Power will not have a simple relationship with Resistance because of this because the temperature changes with Power and the R will change accordingly etc...

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Merlin3189

Homework Helper
Gold Member
So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
That's what the standard formula relating length CSA and Resistivity to Resistance says. ...
I hate to be pernickety, but CSA is proportional to D2 not D, so I don't think it is what the standard formula says.

Possibly more interestingly your rider, that resistivity varies with temperature, provoked me to think that length and CSA would also vary with temperature. Then the thermal increase in length would be more than compensated by the thermal increase in CSA , leading to a lower resistance with increasing temperature. Unfortunately the effect of thermal expansion is about 1000x less than the increase in resistivity with temperature (for Tungsten), so I can see why no one ever mentions it.

sophiecentaur

Gold Member
I hate to be pernickety, but CSA is proportional to D2 not D, so I don't think it is what the standard formula says.

Possibly more interestingly your rider, that resistivity varies with temperature, provoked me to think that length and CSA would also vary with temperature. Then the thermal increase in length would be more than compensated by the thermal increase in CSA , leading to a lower resistance with increasing temperature. Unfortunately the effect of thermal expansion is about 1000x less than the increase in resistivity with temperature (for Tungsten), so I can see why no one ever mentions it.
The expansion affects the radius which affects CSA and the length. But it wouldn't affect the number of atoms / electrons involved. Gets harder and harder , dunnit?
And then there's the increase in surface area of the whole filament. That will reduce the equilibrium temperature for a given rate of dissipation of energy.

houlahound

may have missed it, CSA is ?

Drakkith

Staff Emeritus
2018 Award
may have missed it, CSA is ?
I believe it stands for Cross Sectional Area.

David Lewis

Ohm derived his law by parallel thought to contemporary experiments with conduction of heat through metal. He was using galvanic cells and wire.
Ohm used a thermocouple as his voltage source. (He realized source impedance of galvanic cells varied as a function of current and state of charge, which obscured the underlying principle he was trying to discover.)

sophiecentaur

Gold Member
Ohm used a thermocouple as his voltage source. (He realized source impedance of galvanic cells varied as a function of current and state of charge, which obscured the underlying principle he was trying to discover.)
That's a fascinating thing.
Life was tough for experimenters in those days. No radio Shack just down the road. Imagine having to wind your own insulation round wire that the local blacksmith would have probably drawn out for you.

jim hardy

Gold Member
2018 Award
Ohm used a thermocouple as his voltage source.
I believe you are right and thanks for the correction .

sophiecentaur

Gold Member
I now realise that Ohm can have had no particular Model for this - no idea of what was actually going on in the substance except the parallel with thermal conduction. The range of conditions that he could use must also have been fairly limited - how many Amps can you get out of a thermocouple (his particular type), I wonder? A bit of luck, you could say, that it later turned out to cover a huge range of current values.

David Lewis

Ohm inserted a separate term in his formula to represent source impedance. All test resistances were made up from the same spool of resistance wire, so that the only independent variable (during each run) was the length of the wire:

Current = voltage / (source impedance + wire length)

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"Which makes a bulb glows brighter?"

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