Problem Solving Strategies

3:56 AM

@JohnRennie hi, morning,Alesha here actually we know the heisenberg uncertainty principle, is it apply to the train which is moving at the speed of 180km/hr, I can judge the exact position of the train, so at what speed , we see the heisenberg uncertainty principle working.

I have one more stupid question can human be treated as a quantum particle in space.

user8718165

4:58 AM

@YuvrajSingh... pretty much accurately. Because compare the size and mass of an electron to that of the train. The train is so huge we can exactly find out it's position. The uncertainty is extremely less and negligible hence has no real use.

That's a common 1 Mark question asked in boards. You'll also find one regarding matter waves.

Yuvraj Singh...

5:26 AM

@JohnRennie hi.

John Rennie

@YuvrajSingh... morning :-)

Yuvraj Singh...

Have you seen the question?

John Rennie

@YuvrajSingh... If you consider a single particle like an electron, then it only has a position and a velocity. It has no internal structure. OK so far?

Yuvraj Singh...

Yes.

John Rennie

Now consider an atom. It has internal structure because it has electrons and a nucleus. The whole atom has a position and velocity, but now we can have different states inside te atom i.e. the electrons can be excited into different energy levels.

If we now move to a molecule there is even more internal structure because now the molecule can vibrate so the positions of the atoms can vibrate around.

And if you move up one step to, e.g. a grain of dust, then now the molecules can vibrate relative to each other so it has even more internal structure.

The point of all this is that as we increase in size there is more and more internal structure to the object.

user8718165

5:34 AM

@JohnRennie good morning sir :-)

John Rennie

And at some point this increasing internal structure means the object stops behaving like a single quantum particle and it no longer makes sense to apply the uncertainty principle to it.

@user8718165 morning :-)

@YuvrajSingh... does this make sense so far?

Yuvraj Singh...

Yes, sir, but actually I was reading about how Einstein read up, about uncertainty principle.

John Rennie

OK, what I was working towards was to say that it makes no sense to apply the UP to a train becuse it's too large and doesn't behave like a quantum particle.

Yuvraj Singh...

But now it is clear to me, but sir one last after this argument uncertainty principle depends upon system.

John Rennie

What's the connection to Einstein?

Yuvraj Singh...

5:37 AM

The Uncertainty Principle | Genius

►

Have a look sir.

@JohnRennie I do not know whether this video is correct or not?

@JohnRennie hi. Yes?

John Rennie

@YuvrajSingh... in the video Einstein makes the point that quantum mechanics should apply to large objects as well. Yes?

Yuvraj Singh...

OK.

John Rennie

As I mentioned above, the difference between large and small objects is that large objects have lots of internal structure, and it is this that means they don't behave like electrons or other simple particles.

Yuvraj Singh...

OK.

John Rennie

There is a theory that explains why this happens, but I'm afraid it is rather complicated. It is called decoherence.

Quantum decoherence

Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics. If a quantum...

Cars don't behave like quantum particles because they decohere.

Yuvraj Singh...

5:53 AM

OK sir can you show decoherence, using slits experiment?

@JohnRennie

John Rennie

What you can do is try the double slits experiment with larger and larger particles and see how big you can get before you stop getting an interference pattern.

The pattern disappears when decoherence means the objects stop behaving as quantum particles.

I'd have to Google to find out what the largest particles that show interference are, but we're talking about particles small than a micron so far too small to see.

Yuvraj Singh...

So sir let start with it. $$\psi(x) = \alpha\, \psi_1(x) + \beta \,\psi_2(x)$$ where $\psi_{1,2}(x)$ are the wave-functions

John Rennie

You mean for two different particles 1 and 2?

Yuvraj Singh...

No, a particle passes through an ideal pair of slits, it' is the wave function. @JohnRennie

John Rennie

So $\psi_1$ and $\psi_2$ are the waves emerging from the slits?

Yuvraj Singh...

6:01 AM

Yes.

@JohnRennie

John Rennie

That isn't really the particle wavefunction, but this is still a useful way to understand the situation.

We get the fringes because $\psi_1$ and $\psi_2$ interfere. Yes?

Yuvraj Singh...

Yes,

John Rennie

To see an interference pattern with light we need to use a coherent source. Yes?

Yuvraj Singh...

Yes.

John Rennie

With light either we use a laser, which is coherent, or we use an incoherent source like the sun but we have to pass the light through a single slit first to make it coherent.

With a particle diffraction experiment the particle acts as the source and it passes through both slits to generate the $\psi_1$ and $\psi_2$ that interfere. OK so far?

Yuvraj Singh...

6:09 AM

Yes.

John Rennie

When the particle gets too big it behaves like an incoherent light source. The wavefunctuions $\psi_1$ and $\psi_2$ at the slits are no longer coherent so we no longer get an interference pattern.

Yuvraj Singh...

Why?

Coherence means phase is constant.

Increasing the size effects the phase.

John Rennie

This is where the theory of decoherence comes in.

The loss of coherence happens because an object is always interacting with its surroundings.

And as a general rule the bigger the object the more quickly it interacts with its surroundings. It is this interaction that causes the loss of coherence.

Yuvraj Singh...

Sorry, sir, but still I Don, t get how the interaction with surrounding result into loss of coherence.

@JohnRennie

John Rennie

That isn't easy to explain I'm afraid.

To be honest I don't understand the details of the theory of decoherence myself.

Yuvraj Singh...

6:17 AM

No issue. I have want do discuss something general, can I talk to you in chit chat.

@JohnRennie

So there were two cylinder, with same cross section,and length, and are connected linearly. @JohnRennie

@JohnRennie hi.

M. Guru Vishnu

@JohnRennie, Good morning sir :-)

Yuvraj Singh...

With resistivity k1 and k2.

John Rennie

← 1 message moved from [Chit-chat and stuff for physics enthusiasts ](chat.stackexchange.com/rooms/100303/…)

@M.GuruVishnu hi :-)

Yuvraj Singh...

I need to find the current at exit and entrance from the cylinders.

John Rennie

6:33 AM

Isn't that just two resistances in series?

Yuvraj Singh...

Yes.

John Rennie

So the resistance of each cylinder is $R = \rho\ell/A$

Yuvraj Singh...

:yes.

John Rennie

So the total resistance will be $R_t = R_1 + R_2 = \rho_1\ell/A + \rho_2\ell/A$

This seems too simple. I feel I must have misunderstood the question ...

Yuvraj Singh...

Yes that, s what I feel when done the same.

John Rennie

6:42 AM

With JEE questions it's best if you can upload a picture of the question so I can see exactly what it says.

McSuperbX1

6:53 AM

@JohnRennie hello, are you free to help out with a question?

help me*

John Rennie

@McSuperbX1 hi :-) Yes, what's the question?

McSuperbX1

Hello! Just a sec, uploading.

Struggling with part (C). How is it possible?

John Rennie

@McSuperbX1 Suppose the piston moves in a distance $dL$ in a time $t$, then it's velocity is $u = dL/t$. Yes?

McSuperbX1

Yes

John Rennie

During that time the particle will collide with the piston $n$ times for some number $n$, and each time the particle velocity $v$ increases by $\Delta v = 2u$ because it's bouncing off a surface that is moving towards it.

McSuperbX1

7:02 AM

Ah yes

But... wouldnt $t$ be very small? So shouldn't $n$ be a tiny tiny fraction?

But $n$ must be a natural number..?

John Rennie

It depends how large $dL$ is.

McSuperbX1

Oh I assumed $dL$ is very small. okay okay.

John Rennie

We have to assume $dL$ is large enough that the particle has time to bounce between the eds of the tube many times.

McSuperbX1

Right. Now that makes sense.

Else if dL was a small number, the position of the small particle would become critical. If the particle was right beside the pistin (heading towards the piston) and dL was a small number - we would get atleast one collision. However if the particle happened to far away from the piston, we wouldnt get any collision in that time span.

I see $dL$ must be large or else the issues I just mentioned above will creep in

@JohnRennie Am I correct?

John Rennie

@McSuperbX1 yes.

The system is an analogy for compressing a gas.

McSuperbX1

7:08 AM

Hmm

Thank you so much!

John Rennie

If you think of the particle as a gas molecule and the tube as a container than as you compress the gas the temperature goes up, and the temperature is just the KE of the gas molecules.

McSuperbX1

True.

Jasmine

7:20 AM

@JohnRennie Morning :-)

John Rennie

@Jasmine hi :-)

Jasmine

@JohnRennie how long are you available

John Rennie

@Jasmine I need to work now for about half an hour. After that I'll be around for several hours.

Jasmine

7:45 AM

@JohnRennie ok :-)

Advil Sell

8:46 AM

@JohnRennie Hi !

are you here ?

I need to discuss a circuit

John Rennie

@AdvilSell hi :-)

What's the circuit?

Advil Sell

Just a min , will upload

@JohnRennie here it is

John Rennie

Hmm, I think this may be stretching my knowledge of circuits a bit ...

Advil Sell

Hm..okay , I will try in electronic rooms , Thanks !

John Rennie

Actually it's not that complicated.

You're using two photocouplers and mosfets aranged so you can drive the motor in either direction.

Advil Sell

8:52 AM

Yeah , it's not , it's just MOSFET as a Low side switch

@JohnRennie No theres no photocoupler

John Rennie

Ah, is that just a zener in parallel with the mosfets?

Advil Sell

@JohnRennie Yeah

John Rennie

What did you want to ask?

Advil Sell

I have circled the 2 voltage divider I used to bring the voltage down

so that I can measure it using ADC in arduino

So basically I want a feedback mechanism such that I would get the info if the MOS is on or off

@JohnRennie You here ?

John Rennie

Hi. I think this is beyond my knowledge of circuits. Sorry :-(

Advil Sell

9:06 AM

@JohnRennie No problem , Enjoy your day :-) bbye !

Yuvraj Singh...

12:38 PM

@JohnRennie hi.

John Rennie

@YuvrajSingh... hi

Yuvraj Singh...

Do watch videos on YouTube. I mean physics related videos.

John Rennie

@YuvrajSingh... sometimes, though only for entertainment. If I want to learn about a subject I prefer reading a book.

With a book I can go at the speed that suits me, and easily flip back and reread parts I find hard. With a video you have to go at the pace of the video.

Yuvraj Singh...

Can you suggest some. @JohnRennie

John Rennie

What do you want to learn about?

Yuvraj Singh...

12:46 PM

Quantum,string theory,

@JohnRennie

And lot, just topics which I do not know.

John Rennie

I don't know any good videos for those.

To be honest I don't have any recommendations. I would just search for YouTube videos as and when I want to watch one.

Yuvraj Singh...

OK, because I do not know how to judge.

John Rennie

I don't really recommend YouTube as a general resource. It's good if you're looking for the solution to a particular problem e.g. how to find a centre of mass.

M. Guru Vishnu

2:08 PM

Is there any physical meaning of "Electric displacement vector" - $D=\epsilon_0\vec E+\vec P$?

where, $\vec E$ is the electric field inside a dielectric

and, $\vec P$ is the polarization vector

M. Guru Vishnu

2:24 PM

The following didn't help:

2

Q: Electric Displacement Vector

How do I interpret what electric displacement vector is? I know that it exists and I know it's an equation but I'm not able to really understand or interpret what it is. $$\oint_A \mathbf{D} \cdot \mathrm{d}\mathbf{A} = Q_\text{free}$$

My thoughts on this: It has the dimensions of length^(-2) X charge i.e., similar to surface charge density. So, I think $\vec D$ must be something related to surface charge density, but which one is it referring to?

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Problem Solving Strategies