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4:24 AM
Good morning sir :-) @JohnRennie
 
@user8718165 morning :-)
 
JR SIR GOOD MORNING :) @JohnRennie
 
@yuvrajsingh hello :-)
 
@yuvrajsingh morning :-)
 
5:07 AM
Morning @user8718165
 
6:04 AM
@JohnRennie hi
 
@yuvrajsingh hi
 
I am confuse in one chemistry question
 
@JohnRennie sir I have a little qn
@yuvrajsingh oh.sorry...you go first :-)
 
I have a compound 1 cholrotriptycene @JohnRennie
Will it go $Sn_2$ if I react with ethanolic agno3
 
I have no idea, sorry. You probably know more organic chemistry than I do.
 
6:11 AM
I have a idea but I am not too sure, halides here is 3 degree, if I remove cl ion an Carbocation will produce, which cannot become planner because of steric requirement. Is it right. @JohnRennie
 
It's hard to see how a chlorotriptycene cation could possibly be planar.
 
I have a structure if you want I can upload @JohnRennie
 
@yuvrajsingh I know the structure.
 
OK sorry
 
@yuvrajsingh Not from memory - I Googled it :-)
 
6:22 AM
@JohnRennie sir this one :-)
 
Is my reason look correct @JohnRennie
 
I guess so. Chlorotriptycene can't adopt a planar configuration whether it's ionised or not.
 
If I increase the temperature let say 150 centigrade
@JohnRennie I have one more small question.
 
@yuvrajsingh yes?
 
I heard in childhood that mass is a measure of inertia,
How would I define a mass in space, where body is under no force, let say I have a space craft moving in a space. @JohnRennie
 
6:35 AM
The term inertia isn't well defined in physics. The way it's common used suggests it basically means momentum.
 
Can you elaborate sir
 
Well what do you mean by the term inertia?
 
Inertia, which allows the body to remain at it current state, until and unless an external force applied to it. @JohnRennie
 
@JohnRennie sir...are you busy now?
 
6:50 AM
@user8718165 no, I'm not busy. If you want to ask something go ahead.
@yuvrajsingh that sounds like conservation of momentum to me.
 
Again how.
 
@yuvrajsingh $dp/dt = F$ i.e. momentum changes only if an external force is applied.
 
Yup, can you derive how a mass relate momentum, for all conditions. @JohnRennie
If I have a body which is at rest, so momentum is zero, and velocity is zero $p=mv$, will give absurd result. @JohnRennie
 
I don't see why that is an absurd result. What's wrong with momentum being zero?
 
I derive mass by it gives $0/0$ which is senseless.
 
6:55 AM
Momentum is frame dependent and there is always a frame in which the momentum is zero.
The best definition is from special relativity, but that's a bit advanced. In SR the rest mass is the relativistic magnitude (called the norm) of a 4D vector called the four-momentum.
The norm of a four vector is not frame dependent, so the rest mass is the same in every frame.
 
So mass never dependent on frame right sir. @JohnRennie
 
Correct.
 
But momentum is a frame dependent quantity as you said, so either momentum is inertia definition is wrong. @JohnRennie
 
No, because both momentum and velocity are frame dependent.
When you write $p = mv$ you have a frame dependent quantity on both sides of the equation i.e. $p$ and $v$.
If you write $m = p/v$ the frame dependencies of $p$ and $v$ cancel out making $m$ frame invariant.
 
This lead to me more confusion, if I take you above equation correct and assume mass to be frame independent which says momentum is directly proportional velocity
It mean If I have given momentum of light, by de broglie wavelength, and I now the speed of light I can get mass of a photon. @JohnRennie
 
7:04 AM
The equation $m = p/v$ is non-relativistic so it can only be used at speeds much less than the speed of light. Photons always travel at the speed of light so any attempt to use $m = p/v$ is pointless. The relativistic equation is:
$$ E^2 = p^2c^2 + m^2c^4 $$
where $p$ is the relativistic momentum. For all particles $p = h/\lambda$ where $\lambda$ is the de Broglie wavelength.
For photons the de Broglie wavelength is just the wavelength.
For massive particles we use:
$$ p = \frac{mv}{\sqrt{1 - v^2/c^2}} $$
This is the relativistic equation for the momentum of a massive object.
 
Aah, indeed, can you conclude our whole conversation, so that I can mark it and write in copy for future reference. About momentum and a mass, inertia
 
I think all of this is a bit of a diversion and not something you need to worry about for JEE level questions. You won't study relativity until you start a physics degree.
 
@JohnRennie sir I have a question
 
Thanks, sir as curious teenager I can, t control, it is habit to learn something new, I hope you will understand, sir do you know what is bubble chamber data. @JohnRennie
 
@user8718165 yes?
 
7:14 AM
Is it a place, @JohnRennie
 
@yuvrajsingh physics is good fun :-) I was just as curious as you when I was a teenager, and it was even worse because the Internet didn't exist!!
4
 
@JohnRennie sir, suppose current is flowing steadily through a diode in forward biased condition...
 
@yuvrajsingh a bubble chamber used to be used to detect particles in colliders, but they aren't used any more
@user8718165 yes?
 
Do you know about seven particles @JohnRennie last sorry @user8718165 for interrupting.
 
@yuvrajsingh ;) go on
 
7:17 AM
@yuvrajsingh seven particles? Or was that a typo?
 
I mean CERN experiments @JohnRennie.
 
@JohnRennie suddenly switch is opened...current stopped so there will be a lot of electrons in the p side...I mean the holes will be filled up right sir?
 
@yuvrajsingh yes I have been following the work at CERN because it's really interesting.
 
Yup. Sorry I think time up for me. @user8718165 go on. @JohnRennie
 
@user8718165 remember that for every electron that flows across the junction into the P type side an electron flows out of the P side into the wire connected to the diode. So the total number of conduction electrons in the P side doesn't change during the flow.
What does happen is that when current is flowing the depletion zone is removed. When the current stops the depletion zone is recreated by electrons diffusing across the junction.
 
7:23 AM
@JohnRennie the electron which flows out of the p region into the wire...where does it come from sir?
@JohnRennie I mean, sir...isn't there a stream of e's flowing thro' the diode?
 
In effect when an electron flows from the N side across the barrier into the P side all the electrons in the P side move along one atom and the electron nearest the wire moves from the P side into the wire.
It's like water flowing in a pipe. When water flows into one end of the pipe an equal amount of water must flow out of the other end of the pipe.
 
@JohnRennie those are the bound electrons sir...do they move :-/
 
@user8718165 Are you asking how current flows in a P type semiconductor?
 
@JohnRennie sir first of all, the electrons should diffuse from the n type across the junction and come all the way upto the p-type/wire contact to go into the wire...is it sir? because there aren't any free electrons at the p-type/wire edge to go into the wire
@JohnRennie I'm saying that sir...please help me if that's incorrect sir :-(
 
The electrons in the valence band of a P type semiconductor move by hopping between holes.
I've just looked for diagrams to show this, but surprisingly I can't find any. I can draw
a diagram if you want.
 
7:36 AM
@JohnRennie the electrons in the p type don't more...isn't it sir?
@JohnRennie okay sir
 
Yes the electrons in a p type semiconductor flow.
Obviously they do because current is the flow of electrons.
 
@JohnRennie hi, I got some part of my dad, s old computer I have an image can you help me what it is
 
@yuvrajsingh yes, post a picture of it and I'll have a look.
@user8718165 that's my attempt to show how conduction works ina P type semiconductor.
The white circle is a hole and the green circles are electrons.
The electrons move right into a hole leaving a hole behind them, so the net effect is that the electrons move right and the hole moves left.
 
7:52 AM
@JohnRennie I crop , little bit i.stack.imgur.com/ss2aa.jpg
Do you know what it is.
 
That looks like the circuit board out of a Quantum hard drive
 
I can't upload the full image, it look like a hard disk to me. @JohnRennie
It shape is like this, which is not clear in this.
 
From back side image is similar to this. @JohnRennie
 
It's an old hard disk. It isn't of much use.
 
8:02 AM
Should I throw it @JohnRennie
 
@JohnRennie sorry for being late sir
 
@yuvrajsingh If it is an old hard disk then I suspect it is worthless.
 
One thing more do you remember old big TV desktop. @JohnRennie
 
@yuvrajsingh you mean the old type monitors that look like TVs?
 
I have two desktop like, I do not know whether they are working, yes yes @JohnRennie
 
8:05 AM
 
Yup
I have a cpu with Windows 10 should I use them with it, or they are also use less now.
 
You might be able to find someone to buy them in India because computer stuff is harder to get in India than in the UK. In the UK they would be worthless because no-one would want them.
@yuvrajsingh have you got a flatscreen monitor for your Windows 10 PC?
 
I'd be surprised if old monitors work well with a modern PC, but you can always try them.
 
8:22 AM
@JohnRennie sir I've one last qn
@JohnRennie hello sir
 
@user8718165 hi
 
@JohnRennie sir, if a battery is connected to a simple p-type semiconductor, after a while all the holes will accumulate where the (-) lead of the battery is connected to the p-type...right?
 
@user8718165 no
 
@JohnRennie why sir :-(
 
Because when you separate charges it creates a potential difference.
As the holes start to move towards the -ve wire connected to the diode this creates a potential difference that pulls the holes back in the opposite direction.
The end result is that we get a depletion zone around the junction but in the rest of the diode the concentration of holes is at the normal level. And the depletion zone is very thin.
 
8:30 AM
@JohnRennie oh sir...but the holes are actually neutral sir..they don't have net charge
 
Holes carry a positive charge ...
 
@JohnRennie isn't it just assumed sir? how do they indeed have a positive charge ? I mean there aren't any ions in the semiconductor
 
Holes form when an electron in the valence band jumps into a gap state, where it is fixed and can't flow. This creates a negative gap state and a positive hole in the valene band.
 
@JohnRennie sir...I studied it just days ago and now forgot it...so sorry for troubling you...got it now...
 
:-)
 
9:24 AM
@JohnRennie hello sir, what languages do you know other than English? :-)
 
I only speak English
 
@JohnRennie sir only English?
 
I used to know a bit of French, Italian and German, but I haven't used those languages for so long that I've mostly forgotten them.
Because English is such a dominant language in the world it's usually the case that native English speakers never need to learn a second language.
So the British, Americans, Australians, are rarely bilingual.
 
@JohnRennie ever heard of Bengali sir :-)
@JohnRennie yeah sir...got it :-)
 
Yes. My father lived in India for several years and he spoke reasonably good Urdu. But the only thing he taught me were some Urdu insults :-)
I know of most of the Indian languages though i don't speak them.
 
9:30 AM
@JohnRennie LOL sir...that's verrry funny :-)😂
 
@JohnRennie hi
 
@yuvrajsingh hi
 
Does your father told you about gandhi, and India during British rule. @JohnRennie
 
@yuvrajsingh I think this is a really complicated subject, and one that I'm reluctant to get involved with as it is so important to many Indians.
 
what were your views during British rule. @JohnRennie
About india
 
9:36 AM
@JohnRennie nothing can be more complicated than physics XDXD
 
@yuvrajsingh I feel like it's none of my business. It's like foreigners commenting on the Queen.
 
@JohnRennie, Hi. Are you free right now sir?
 
@Intellex hi, yes I'm free :-)
 
Thanks
 
Sorry sir. I ask it in journey nothing more. @JohnRennie
 
9:39 AM
What will happen to a mixture of oil and water in the absence of gravity @JohnRennie sir?
 
@yuvrajsingh it's no problem. It's just that I don't think I should comment.
@Intellex Nothing very exciting. You will just get droplets of oil floating in larger drops of water.
 
Sources on the internet give contradictory answers sir
 
The formation of spherical drops of oil is a result of surface tension not gravity, so you'd still get drops.
 
@JohnRennie How can they float sir? There is no true notion of up or down
 
Float in the sense of not moving anywhere. I didn't mean float on top of the water.
 
9:42 AM
@JohnRennie Will we get drops of oil in the centre of water bubble or vice versa sir?
@JohnRennie Understood sir.
 
If no surfactant is present then there is nothing to stablise the oil water interface, so if two oil drops meet they will merge to form a single larger drop.
But without gravity the oil drops won't move around so they won't meet.
I guess there could be an Ostwald ripening process, but that will be very slow.
 
@JohnRennie Sorry sir, I have to learn about it.
 
Ostwald ripening is really simple.
The pressure inside a drop is inversely proportional to the radius i.e. larger drops have a lower pressure than smaller drops.
 
@JohnRennie Yes sir. P=2S/R or 4S/R
Depending on whether its a hollow or complete sphere
 
So suppose you take two drops of slightly different sizes and connect them with a tube so the oil can flow along them. Then the oil will flow from the high pressure small drop to the low pressure large drop i.e. the small drop will shrink to nothing and the large drop will grow.
@Intellex OK so far?
 
9:46 AM
Yes sir
 
But oil is very slightly soluble in water. So oil can dissolve out of the small drop into the water, diffuse to the large drop and come out of solution enter it.
The end result is that oil molecules can travel between the drops through the water, just as if there was a tube between but much, much slower.
It's slow because the solubility of oil in water is very small, and diffusion through water is very slow.
 
And this is that Ostwald's process. Am I right sir?
 
@Intellex yes. There's probably a Wikipedia article on it.
 
@JohnRennie Understood sir. Thank you.
@JohnRennie I'll read it sir.
 
@JohnRennie sir can I ask a little question ?
 
9:50 AM
@user8718165 yes, go ahead.
 
@JohnRennie, Have you seen a British Black Arrow (rocket) launch?
Oops sorry for interrupting
 
@Intellex no, I'm not that old! :-)
 
@JohnRennie Were you too young then?
 
Actually I suppose I could have seen a launch. Wikipedia tells me the last launch was in 1971 and I would have been ten years old then.
But I didn't.
 
Thanks a lot for the info sir.
Bye :)
 
9:52 AM
Bye
 
@JohnRennie sir we can ionize the valence e's to the acceptor band just by providing sufficient energy...right sir? (in p-doped)
 
In P doped semiconductors the doping creates gaps states just above the top of the valence band. The energy difference between the gap states and top of the valence band is comparable to $kT$ so electrons get thermally excited into the gaps states leaving holes behind.
 
@JohnRennie yeah sir...got it...but if at 0K, say we apply a very high voltage across a slab of p-type semiconductor..will the ionization happen sir
@JohnRennie we discussed earlier that conduction isn't possible at 0K...but please sir...once more
 
I suspect you'd get an avalanche effect. As you increased the voltage there would come a point where one electron moved, and it would collide with other electrons and excite them. Then they would in turn collide with other electrons and the number of mobile electrons would grow exponentially.
But the point is that the motion of the electrons would heat the semiconductor so it wouldn't be at 0K any more.
 
@JohnRennie yeah sir...I remember...thank you soooo much for telling once again sir
 
 
5 hours later…
2:37 PM
@JohnRennie hello sir...good evening :-)
 
@user8718165 hi :-)
 
@JohnRennie sir in an ideal wire connected to an ideal battery...the charges will still flow...so can we say that there lies an E-field in wire
 
It's not a well defined system because the current is given by a division by zero.
 
@JohnRennie aha...sorry sir...say we include a resistor...then there will be an E-field in the resistor...right
 
Yes, when the resistance is non-zero there is a potential gradient along the resistor.
 
2:42 PM
@JohnRennie and in the ideal wires...there isn't E-field right sir...because there isn't any potential gradient...(in our new setup)
 
Yes. Ideal wires have no resistance so there is no potential gradient when a current flows through tem.
 
@JohnRennie yeah sir...my qn is based on this topic sir
@JohnRennie if there is no E-field in the wire...why does the current flow sir through the wire...is it just due to the e-field in across the resistor? I'm confused :-/
 
If there is no resistance to flow then it takes no energy to make the current move.
i.e. you don't need to exert any force on the electrons to make them move.
 
@JohnRennie thank you very much sir...
@JohnRennie you always help everyone :-)
 
:-)
 
2:47 PM
@JohnRennie is lunch finished sir?
 
I'm eating lunch now :-)
 
@JohnRennie Good Evening sir, How do you convert Laplace Operator to Spherical Coordinates? (derivations are excessively lengthy)...?
 
@JohnRennie yeah :-)
 
@AbhasKumarSinha Google it. I don't remember it and as you say it's messy to derive.
 
3:20 PM
@JohnRennie hi
 
3:41 PM
Sir
 
3:52 PM
@yuvrajsingh hi Yuvraj. Sorry, I was working and didn't see your post.
 
No issue, sir are you available now. @JohnRennie
 
@yuvrajsingh yes I'm free now for about an hour.
 
OK sir I have an answer can you verify it, if there is some issue please let me know. @JohnRennie
0
Q: Potential drop in a resistor

aditya siroutiyaWhat causes a potential drop in a resistor or load? Why a wire having neglible resistance have same potential across it?

 
@yuvrajsingh reading now ...
@yuvrajsingh there's nothing wrong with your answer but I suspect it doesn't address what the Aditya Siroutiya is actually asking. I suspect he's asking about the physical mechanism that causes resistance. Bob D's answer makes the analogy with friction, which is actually pretty good. In effect the electrons flowing through a resistor experience a force analogous to friction and it's the work done against this force that causes the potential drop across the resistor.
 
4:10 PM
Should I update it, or leave it @JohnRennie
 
Unless it gets downvotes I'd leave it.
What actually happens is the electrons scatter off phonons in the resistor and the electron kinetic energy gets converted to energy of the phonons.
The energy that goes into the phonons gets distributed around the resistor and ends up as heat.
 
Aah thanks for clarification @JohnRennie
One more @JohnRennie
 
4:26 PM
@yuvrajsingh hi
 
Do you know Peter skies @JohnRennie
 
@yuvrajsingh no. Give me a moment to Google him.
Do you mean Peter Sykes?
 
Yes
Sorry that was typo.
 
I've met him. He gave lectures in organic chemistry at Cambridge. But I didn't know him well.
 
Were on a same batch @JohnRennie
 
4:32 PM
I've still got one of his books :-)
 
Yup, his book are so famous here, @JohnRennie
Specially this one which you have.
 

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