Infinite Resistance
- Thread starter physis
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crossman
Senior Member
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- Southeast Texas
Re: Infinite Resistance
okay dadgummit, someone give me some insight!
A 1.5 volt battery is connected to a .1 ohm resistance and forms a complete circuit. Less current flows than would be indicated by ohm's law. Does the internal resistance of the battery mathamatically account for ALL of the missing current?
I'm thinking it doesn't.
okay dadgummit, someone give me some insight!
A 1.5 volt battery is connected to a .1 ohm resistance and forms a complete circuit. Less current flows than would be indicated by ohm's law. Does the internal resistance of the battery mathamatically account for ALL of the missing current?
I'm thinking it doesn't.
crossman
Senior Member
- Location
- Southeast Texas
Re: Infinite Resistance
And another one!
At what level of resistance is a circuit considered incomplete?
And another one!
At what level of resistance is a circuit considered incomplete?
Ed MacLaren
Senior Member
Re: Infinite Resistance
Let's say the current was only 12 amps when you connected the .1 ohm load to the 1.5 volt battery.
All you know for sure is that the total circuit resistance is .125 ohms. If we disregard the wire resistance the extra .025 ohms is the internal resistance of the battery.
Ed
Yes, that and the connecting wires, connection resistance, etc.
Let's say the current was only 12 amps when you connected the .1 ohm load to the 1.5 volt battery.
All you know for sure is that the total circuit resistance is .125 ohms. If we disregard the wire resistance the extra .025 ohms is the internal resistance of the battery.
Ed
physis
Senior Member
Re: Infinite Resistance
I can't, at least at the moment, find a value of resistance for empty space. I think there is some value of conductance but I can't be sure.
My assumption is that you cannot acheive zero conductance.
I'm at a very disappointing disadvantage because I have almost no text resourses.
I can't, at least at the moment, find a value of resistance for empty space. I think there is some value of conductance but I can't be sure.
My assumption is that you cannot acheive zero conductance.
physis
Senior Member
Re: Infinite Resistance
You've obviously never seen shorted Ni-Cads melt down. The internal resistance can be way less than an ohm. The only thing holding them back from, I suppose, vaporizing instantaniously, is the internal resistance, and whatever other resistance in the melting metal and such.
crossman
Senior Member
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- Southeast Texas
Re: Infinite Resistance
I have never seen a 1.5 volt AA flashlight battery melt down under short circuit conditions and I am assuming it is because there is not enough chemical reactant inside to support a large current flow.
I have never seen a 1.5 volt AA flashlight battery melt down under short circuit conditions and I am assuming it is because there is not enough chemical reactant inside to support a large current flow.
physis
Senior Member
Re: Infinite Resistance
I think those are carbon zinc, or used to be, I don't know what the alkaline are made of. I dont know what the internal resistance is and I don't have a meter that can measure that that's working right now. Maybe someone can measure it?
Chemical reactions are usually pretty, how you say, "dadgum" fast. I can't be 100% but I think the limiting factor will be resistance.
I think those are carbon zinc, or used to be, I don't know what the alkaline are made of. I dont know what the internal resistance is and I don't have a meter that can measure that that's working right now. Maybe someone can measure it?
Chemical reactions are usually pretty, how you say, "dadgum" fast. I can't be 100% but I think the limiting factor will be resistance.
Re: Infinite Resistance
All so called voltage sources have an internal resistance. Flashlights typically draw a half Amp or so. A dead short should produces a few amperes for a short while. Of course, the internal resistance changes wildly under these conditions. So, Ohm's law applies if we take into account this changing resistance.
I think an alkaline AA cell is rated at about 1.5 Ampere-hours and produces a higher current than would a carbon zinc cell.
All so called voltage sources have an internal resistance. Flashlights typically draw a half Amp or so. A dead short should produces a few amperes for a short while. Of course, the internal resistance changes wildly under these conditions. So, Ohm's law applies if we take into account this changing resistance.
I think an alkaline AA cell is rated at about 1.5 Ampere-hours and produces a higher current than would a carbon zinc cell.
crossman
Senior Member
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- Southeast Texas
Re: Infinite Resistance
I agree with the points about batteries. Also, I am certain that you cannot measure the internal resistance of a battery with an ohm-meter, it would have to be done with a bunch of equations involving the sub-atomic properties of the chemicals themselves, and taking into account the length and area involved, etc. This is entirely speculation on my part.
Another simpler way to figure the internal resistance would be to connect a known load, measure the current and voltage, and do ohm's law to determine the actual internal resistance of the battery.
I also agree that this internal resistance will change for different circumstances of load and battery life.
I stand by my statement that the limited current flow of a battery is due to the limited amount of chemical reactant and hence the number of available electrons that are contained within. As an example, if you have only 2 moles of a reactant that could provide one electron per atom, at most you could have 2 amps of current flow.
*note: I am basing all this on my elementary knowledge of chemistry and electricity and could be totally wrong.
I agree with the points about batteries. Also, I am certain that you cannot measure the internal resistance of a battery with an ohm-meter, it would have to be done with a bunch of equations involving the sub-atomic properties of the chemicals themselves, and taking into account the length and area involved, etc. This is entirely speculation on my part.
Another simpler way to figure the internal resistance would be to connect a known load, measure the current and voltage, and do ohm's law to determine the actual internal resistance of the battery.
I also agree that this internal resistance will change for different circumstances of load and battery life.
I stand by my statement that the limited current flow of a battery is due to the limited amount of chemical reactant and hence the number of available electrons that are contained within. As an example, if you have only 2 moles of a reactant that could provide one electron per atom, at most you could have 2 amps of current flow.
*note: I am basing all this on my elementary knowledge of chemistry and electricity and could be totally wrong.
Re: Infinite Resistance
Crossman, you are half wrong. The amount of reactant, that is the size of the battery determines the energy delivered which is related to the Ampere-hour rating of the battery. I can see no reason to equate two moles of reactant to two amperes of current. Current is the time rate of charge flow in Coulombs/Sec. Depending on the battery geometry, you might have 100 Amps for a millisecond or a microsecond. I don't want to bother my brain calculating today.
If I recall correctly,
The charge on one electron is -1.6 x 10^-19 Cb.
Crossman, you are half wrong. The amount of reactant, that is the size of the battery determines the energy delivered which is related to the Ampere-hour rating of the battery. I can see no reason to equate two moles of reactant to two amperes of current. Current is the time rate of charge flow in Coulombs/Sec. Depending on the battery geometry, you might have 100 Amps for a millisecond or a microsecond. I don't want to bother my brain calculating today.
If I recall correctly,
The charge on one electron is -1.6 x 10^-19 Cb.
crossman
Senior Member
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- Southeast Texas
Re: Infinite Resistance
My take:
1 amp = 6.25 x 10^18 electrons flowing past a given point in one second.
1 mole of any element or any compound is equal to 6.25 x 10^18 atoms or molecules.
Also, if one molecule of reactant is capable of giving up one electron (it could be two, maybe three, depending on the actual chemical reaction), then one mole of the reactant will contain enough electrons for one amp of current.
Now, that is assuming that the reactant molecules are all in parallel, and the voltage produced would be very very low. So actually a lot of the molecules in the mole of reactant would be in series to increase voltage and this would reduce the amount of current way below one amp for each mole of reactant.
My take:
1 amp = 6.25 x 10^18 electrons flowing past a given point in one second.
1 mole of any element or any compound is equal to 6.25 x 10^18 atoms or molecules.
Also, if one molecule of reactant is capable of giving up one electron (it could be two, maybe three, depending on the actual chemical reaction), then one mole of the reactant will contain enough electrons for one amp of current.
Now, that is assuming that the reactant molecules are all in parallel, and the voltage produced would be very very low. So actually a lot of the molecules in the mole of reactant would be in series to increase voltage and this would reduce the amount of current way below one amp for each mole of reactant.
crossman
Senior Member
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- Southeast Texas
Re: Infinite Resistance
oops, hold on I am seeing the flaw in my argument! Never mind! Thanks! :roll:
Still I agree that the amount of reactant present will have an effect on the maximum possible current flow and this is fairly obvious.
oops, hold on I am seeing the flaw in my argument! Never mind! Thanks! :roll:
Still I agree that the amount of reactant present will have an effect on the maximum possible current flow and this is fairly obvious.
physis
Senior Member
Re: Infinite Resistance
Hmm.
On the one hand, I'm thinking, hot diggity, I'm learning a mole is directly related to a Coulomb.
Chemistry is probobly my weakest subject. I used to know what a mole is. Anyway, I'm concerned about accuracy.
Crossman, what is going to regulate the chemical process in such a way as to prevent the entire gobb of stuff from completing all of it's reaction immediately?
Maybe you have a point. Ive never heard of a battery exploding. That's what uncontrolled chemical reactions do.
Edit: ok, lead acid, but that's hydrogen gas.
[ January 13, 2005, 05:05 PM: Message edited by: physis ]
Hmm.
On the one hand, I'm thinking, hot diggity, I'm learning a mole is directly related to a Coulomb.
Chemistry is probobly my weakest subject. I used to know what a mole is. Anyway, I'm concerned about accuracy.
Crossman, what is going to regulate the chemical process in such a way as to prevent the entire gobb of stuff from completing all of it's reaction immediately?
Maybe you have a point. Ive never heard of a battery exploding. That's what uncontrolled chemical reactions do.
Edit: ok, lead acid, but that's hydrogen gas.
[ January 13, 2005, 05:05 PM: Message edited by: physis ]
peter d
Senior Member
- Location
- New England
Re: Infinite Resistance
The scary thing is, this thread is becoming infinite!!
The scary thing is, this thread is becoming infinite!!
Re: Infinite Resistance
I have heard that cell phone batteries have been known to explode or perhaps just start a fire.
Indeed the size of the battery would be expected to help determine the short circuit current. Remember though that a high voltage battery would have a higher internal resistance than a low voltage battery of the same mass.
I have heard that cell phone batteries have been known to explode or perhaps just start a fire.
Indeed the size of the battery would be expected to help determine the short circuit current. Remember though that a high voltage battery would have a higher internal resistance than a low voltage battery of the same mass.
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