Ohmic conductor, when R tends to zero I=V/R

[PainterGuy]

hello fine people,

ohm law is V=IR. suppose V is a constant value, as say 5. then it is I=5/R. when R is zero current is infinite. me thinks this is case with superconductors. me have been telled that limit of I=5/R does not exist. what did it mean in this real world problem? will this mean R=0 is not possible, but it is as in superconductor. will this mean ohm law is only valid under certain condition? will it mean as R tends toward 0, the conductor stop behaving as a ohmic conductor?

this is sub-question:--- in case of superconductor if you have applied voltage to it once the current will b there as long as something do not take it out. which means as long as current is not usedd to do some work.. me have seen a picture magnet flying over a very cold superconductor.. some my friend was saying current is doing work to keep that magnet flying at fixed position. but me thinks this is not work from physics view. if me stands holding a bucket of water me not doing any physics work.

this is another sub-question:--- me once read to achieve zero kelvin is impossible because atoms can not never stop moving around completely. which means there is always some small movement of atoms and this is temperature. and if there is always some atomic motion, then itt is impossible to have zero resistance because some electrons will struggle against those moving atoms.

please show me the light. every help is appreciated. cheers

edit:--- please don't use quantum things and complex math formulas. take care

 

[sophiecentaur]

You would have to assume that the WHOLE circuit had zero resistance - including the power supply (only nominally a voltage source). This wouldn't happen, so the catastrophic scenario you propose wouldn't occur. There would always be some resistance to limit the current. You would still melt the generator or battery, though!

 

[PainterGuy]

thanks, sophiecentaur.

please someone help me with other questions also.

cheers

 

[sophiecentaur]

"Please don't use quantum things" is a bit like asking why things fall down without using the notion of Gravity.

Resistance is there because electrons interact with the atoms they are drifting through. Resistance represents lost energy and not 'something pushing against electrons'.

If the lattice of the metal has so little 'movement' / energy that it can't interact with electrons then there will be no energy loss, so no resistance. Why is this? Because of Q(hush hush) effects.

 

[PainterGuy]

Resistance is there because electrons interact with the atoms they are drifting through. Resistance represents lost energy and not 'something pushing against electrons'.

If the lattice of the metal has so little 'movement' / energy that it can't interact with electrons then there will be no energy loss, so no resistance. Why is this? Because of Q(hush hush) effects.

sophiecentaur many thanks again. in my opinion (which can be very muich wrong, so forgive me) electrons drifting thru metal or conductor will always encounter atoms on their passage from one side to another. its just like a ball passing thru a ground with standing posts without hitting anyone of them. its not possible. so there will always be resistance. me don get how resistance goes to zero. you say this impossible to have zero resistance but its possible in case of superconductors. please teach me now (and yes without those quantum formula!). many many thanks for this help.

cheers

 

[sophiecentaur]

Unless you accept that QM comes into it you cannot explain this. Your mechanical 'pinball' description is just not enough.
Regards

 

[kaymant]

Even in case of everything being superconductor, the inductance (and the back emf generated) will limit the short circuit current.

 

[ZapperZ]

sophiecentaur many thanks again. in my opinion (which can be very muich wrong, so forgive me) electrons drifting thru metal or conductor will always encounter atoms on their passage from one side to another. its just like a ball passing thru a ground with standing posts without hitting anyone of them. its not possible. so there will always be resistance. me don get how resistance goes to zero. you say this impossible to have zero resistance but its possible in case of superconductors. please teach me now (and yes without those quantum formula!). many many thanks for this help.

cheers

You should learn a little bit about solid state physics before you make such assertion. To say that it is impossible to have zero resistance implies that you clearly have not understood the phenomenon of superconductivity and HOW it occurs. Please do a quick research for yourself before making such statements. The PF Rules that you had agreed to prohibits making such speculative and WRONG description about something we know very well.

Zz.

 

[Dale]

hello fine people,

ohm law is V=IR. suppose V is a constant value, as say 5. then it is I=5/R. when R is zero current is infinite. me thinks this is case with superconductors. me have been telled that limit of I=5/R does not exist. what did it mean in this real world problem? will this mean R=0 is not possible, but it is as in superconductor. will this mean ohm law is only valid under certain condition? will it mean as R tends toward 0, the conductor stop behaving as a ohmic conductor?

this is sub-question:--- in case of superconductor if you have applied voltage to it once the current will b there as long as something do not take it out. which means as long as current is not usedd to do some work.. me have seen a picture magnet flying over a very cold superconductor.. some my friend was saying current is doing work to keep that magnet flying at fixed position. but me thinks this is not work from physics view. if me stands holding a bucket of water me not doing any physics work.

this is another sub-question:--- me once read to achieve zero kelvin is impossible because atoms can not never stop moving around completely. which means there is always some small movement of atoms and this is temperature. and if there is always some atomic motion, then itt is impossible to have zero resistance because some electrons will struggle against those moving atoms.

please show me the light. every help is appreciated. cheers

edit:--- please don't use quantum things and complex math formulas. take care

Please use proper English. I don't know if there is a language barrier, but this is very distracting.

Regarding your question, the point is that it is impossible to apply a voltage to a superconductor.

 

[sophiecentaur]

Even in case of everything being superconductor, the inductance (and the back emf generated) will limit the short circuit current.

At DC!?

 

[PainterGuy]

Please use proper English. I don't know if there is a language barrier, but this is very distracting.

Regarding your question, the point is that it is impossible to apply a voltage to a superconductor.

sorry. english is not my mother tongue. i started learning just very few years ago. i do every effort to use proper english. yes using "me" is wrong i was told by another member in another thread. next time such mistake will not repeat. thanks

cheers

 

[PainterGuy]

You should learn a little bit about solid state physics before you make such assertion. To say that it is impossible to have zero resistance implies that you clearly have not understood the phenomenon of superconductivity and HOW it occurs. Please do a quick research for yourself before making such statements. The PF Rules that you had agreed to prohibits making such speculative and WRONG description about something we know very well.

Zz.

i am sorry if you think this. my intention was not to break any rule. i was only trying let sophiecentaur know where problem was in my mind and what bother me. so sophiecentaur can help better way. i said it in my post that my opinion can be very much wrong. still sorry.

cheers

---------
sophiecentaur many thanks again. in my opinion (which can be very muich wrong, so forgive me) electrons drifting thru metal or conductor will always encounter atoms on their passage from one side to another. its just like a ball passing thru a ground with standing posts without hitting anyone of them. its not possible. so there will always be resistance. me don get how resistance goes to zero. you say this impossible to have zero resistance but its possible in case of superconductors. please teach me now (and yes without those quantum formula!). many many thanks for this help.

 

[sophiecentaur]

As I said before, electrons do not behave like little ball moving through a maze of obstructions. There is no explanation of resistance that does not involve Quantum mechanics. You may not like it but that's the way it is. You need to read about it.

 

[kaymant]

At DC!?

Well at dc the current won't limit. For idealized model the current should increase linearly. If at t=0, an emf \mathcal{E} is suddenly applied to a superconductor element having inductance L, then there being no resistance, the voltage won't drop across the element implying that the back emf L \dfrac{\mathrm dI}{\mathrm dt} will be at all times equal to \mathcal{E}. So we get
\mathcal{E} = L\dfrac{\mathrm dI}{\mathrm dt}
This gives
I = \dfrac{\mathcal{E}}{L} t
using the initial current of zero.

 

[sophiecentaur]

Well at dc the current won't limit. For idealized model the current should increase linearly. If at t=0, an emf \mathcal{E} is suddenly applied to a superconductor element having inductance L, then there being no resistance, the voltage won't drop across the element implying that the back emf L \dfrac{\mathrm dI}{\mathrm dt} will be at all times equal to \mathcal{E}. So we get
\mathcal{E} = L\dfrac{\mathrm dI}{\mathrm dt}
This gives
I = \dfrac{\mathcal{E}}{L} t
using the initial current of zero.

This is all fair enough and true and it is good to know that you know about these things. But introducing Inductance opens up a whole can of worms for someone who is trying to understand Resistance. Is it helpful? You might also demand that the actual routing of the wires (zero resistance) in the schematic diagram of any circuit should always be covered with an attached caveat.
The fact is that the Resistance of a superconducting circuit is Zero. Anything else is incidental - although it is relevant to a many practical situations.

 

[PainterGuy]

As I said before, electrons do not behave like little ball moving through a maze of obstructions. There is no explanation of resistance that does not involve Quantum mechanics. You may not like it but that's the way it is. You need to read about it.

hello sophiecentaur,

i thanks for this help. i agree with you there is no explanation without quantum mechanics. actually i hoped that you may make it non-quantum so i can understand it. now i think making it non-quantum will still involve some of quantum mechanics. no problem. at least now i know why i can not understand it. many thanks. will ask any question here about this topic.

cheers