Problem Solving Strategies

user8718165

4:32 AM

Good morning sir :-) @JohnRennie

John Rennie

@user8718165 morning :-)

yuvraj singh

@JohnRennie morning sir. :)

John Rennie

@yuvrajsingh morning :-)

Aladdin

4:49 AM

@JohnRennie hi

John Rennie

@Aladdin hi :-)

user8718165

5:09 AM

@JohnRennie sir what happened? Is Yuvraj and everyone else fine :-)

psa

@JohnRennie hiya

hiya all

John Rennie

@psa hi :-)

psa

I have a question

My thinking was that the entirety of the system's photons is given by the area under the graph (I guess I was thinking of it like a probability distribution?) and so if you find the integral from about -3 to 3, that will give you the probability that the photon hits the screen in the central interference fringe. That's about 12%, so if you want two photons (I figured these are independent events) you'd have 0.12 x 0.12 = 0.0144 = 1.44%.

Seems somewhat unrealistically low though...

John Rennie

Yes, that's how I'd do it.

psa

5:24 AM

OK kewl

Thank you!

yuvraj singh

@JohnRennie hi

John Rennie

@yuvrajsingh hi

How are you this morning? Feeling better?

Yes,

Good :-)

I have a question

5:28 AM

Yes?

yuvraj singh

Sir if I need to accelerate the electron at the speed of light how much force should I need is it infinite.

@JohnRennie

John Rennie

If you apply a force $F$ for a time $t$ then the change in momentum is $\Delta p = Ft$. Yes?

yuvraj singh

Ok

John Rennie

So if your electron starts at rest, $p=0$, and ends up with some momentum $p$ then $Ft$ has to be equal to $p$.

This could be a large force for a short time or a small force for a long time, as long as the product $Ft$ is equal to the final momentum.

yuvraj singh

Have a doubt.

John Rennie

5:33 AM

Yes?

yuvraj singh

I think when I accelerate the electron

It velocity will change till it reaches the speed of light

John Rennie

It's a little more complicated than that.

yuvraj singh

@JohnRennie

John Rennie

The usual expression for momentum is $p=mv$ but this only applies at speeds much less than the speed of light.

For relativistic speeds the equation for the momentum is:

$$ p = \frac{mv}{\sqrt{1 - v^2/c^2}} $$

yuvraj singh

Can I use de broglie equation

John Rennie

5:37 AM

If you set $v=c$ you will find the denominator is zero and hence the momentum becomes infinite.

yuvraj singh

Lambda=h/mv

Will this hold for a particle, who is approaching at the speed of light.

@JohnRennie

John Rennie

No. The equation is $\lambda = h/p$ and for relativistic particles you need to use the relativistic equation for the momentum.

yuvraj singh

OK, I think it bit of more, complicated, sir I am asking this because previous year in interview exam they ask this question, @JohnRennie

John Rennie

@yuvrajsingh well as $v\to c$ the momentum goes to infinity. So that means $Ft \to \infty$ as well.

So to reach the speed of light you either need an infinite force or an infinite time.

yuvraj singh

Thanks, I got a some very interesting fact about my last night. Incident

John Rennie

5:43 AM

Yes?

yuvraj singh

Actually when I take the auto, auto driver changes the original root which goes to my home.

It old root which I have taken.

@JohnRennie

John Rennie

I'd guess he deliberately chose a route that went past the shop where his brother was.

yuvraj singh

Yes, today around 7A.M I went to the shop but it was closed.

Why would a shop be closed on day time. @JohnRennie

Till now I haven, t discuss about it with anyone.

John Rennie

Do shops normally open at 7 a.m.? In the UK shops usually open at 9 a.m.

yuvraj singh

It, s is village, so shop here journey open at 4A.m

John Rennie

5:53 AM

OK

It would be worth asking if anyone in the village has used the shop.

yuvraj singh

I am not sure I should discuss it or not,.

John Rennie

Talk to your parents and see what they think.

They will know everyone in the village better thn you do.

It could be that the driver was just hoping for some extra money i.e. you'd pay him to fix the the auto (even though it wasn't broken) and drive the last 5km.

yuvraj singh

Do you remember when I was at the shop, I said the were calling someone.

John Rennie

Yes?

yuvraj singh

But when I ask a shop owner, can make a call he denied

Saying he doesn't, t have balance. @JohnRennie

@user8718165 last night was a tragic night.

John Rennie

5:59 AM

To be fair it could have been he just didn't want you making a call on his phone that he would have to pay for.

But it does sound suspicious.

yuvraj singh

I can make a missed call, what was wrong in that.

John Rennie

After all, how likely is it that the auto would break down right outside his brother's shop? And 5km from your village.

But they may not have been intending to rob you. They might have just wanted extra money to go the last 5km.

yuvraj singh

That, s where my mind, goes, because before that auto work correctly, and we were smoothly moving, road was good before that.

John Rennie

You need to discuss it with your parents.

Does your village have a mayor or a head man?

If so discuss it with him.

yuvraj singh

And when I ask a auto man his name he said at station he says Ravi, and when I ask his name again on shop he said something else, I do not remover what he said but it was different because my brother friend had come

@JohnRennie

Yes we have a head man.

John Rennie

6:07 AM

@yuvrajsingh ask your parents if they think you should speak to the head man about it.

If the men at the shop are robbers then they may attempt to rob other people in the village as well, so the head man should know.

yuvraj singh

Should I explain them whole situation. @JohnRennie

John Rennie

Explain everything to your parents and see what they think.

yuvraj singh

But u do not have a conclusive proof @JohnRennie

OK I take your advice and I will have chat with them thanks.

John Rennie

You don't need proof. If you do speak to the head man just tell him what happened, not what you think about the men. The head man can decide for himself based on what you say.

But speak to your parents first and see what they say.

yuvraj singh

Yes, you correct let head man decide what he think about it. @JohnRennie

Sir I think when I was chatting with you do not thought that, the thing will goes like this do you. @JohnRennie

John Rennie

6:14 AM

I've heard stories before where taxi drivers will claim the car has broken down and you have to give them some money to repair it before they will continue the journey.

yuvraj singh

Seriously your advice in the night help me so much.

John Rennie

You're welcome :-)

yuvraj singh

and my father it self a policemen, he posted in metropolitan city.

John Rennie

In Bikaner?

yuvraj singh

No in jaipur.

I have a question just a small one. @JohnRennie

John Rennie

6:21 AM

@yuvrajsingh yes?

yuvraj singh

Sir I have two loud speaker who are facing each other, and I stand behind one of the speaker, will I hear the sound of the too speaker clearly, or the sound wave collides. @JohnRennie

John Rennie

Sound waves will interefe like light waves. You can even do a double slit experiment with sound waves.

yuvraj singh

Elaborate please

John Rennie

There isn't much to elabrate. If you Google "sound wave double slits" or something like that you'll find lots of articles and videos showing the double slit experiment with sound waves.

yuvraj singh

Sir if I hit a metal rod, it create same type of pressure wave.

John Rennie

6:32 AM

Yes, a sound wave in metal is a longitudinal wave just like a sound wave in air.

I need to answer a question in CodeClub now. I'll be a few minutes.

yuvraj singh

Sure.

psa

6:46 AM

@JohnRennie For this question, would the fact that the potential (0.5V) at which the photocurrent starts to decrease tell you that the least energetic electrons have an energy of $e(0.5V) = 0.5eV$?

John Rennie

I would guess that's what the question means, yes. The least energetic electrons would have come from the lowest energy part of the band i.e. 3eV below the vacuum energy. So their energy will be $hf - 3eV = 0.5eV$, immediately telling you that the photon energy is $3.5eV$.

For the record this is a crap question because photoelectrons are always emitted with a range of energies from zero up to $hf-\phi$. This is because the energy transfer from the photon to the photoelectron is not always 100% efficient.

But go with the question I guess.

psa

Wait, so why does that make it a bad question?

John Rennie

Because the question says the retarding potential has no effect as long as it is below 0.5V, and that implies the minimum photoelectron energy is 0.5eV i.e. no photoelectrons are emitted with energies in the range 0 to 0.5eV.

psa

Oh I see

That part isn't realistic then, yeah

John Rennie

And that's wrong. In a real experiment photoelectrons always have a continuous range of energies from zero up to $hf-\phi$.

psa

6:59 AM

Is there a point where it becomes significantly more noticeable?

Or I guess since it's continuous not really?

John Rennie

No. The photocurrent generally reduces smoothly with retarding voltage. If you Google you can find experimental data for this.

But you often find you have to go along with questions even when they're wrong :-)

psa

It would have been nice if it's clarified that it's wrong.

John Rennie

Oh well :-)

The point is obviously that the photon energy is 3.5eV so the most energetic electron has an energy of $3.5eV - \phi = 1.5eV$

psa

@JohnRennie so to completely stop the current we need the kinetic energy of the electron + the work function to be greater than or equal to the energy of the photon?

John Rennie

The kinetic energy of the electron (the maximum KE) is the photon energy minus the work function. Yes?

psa

7:08 AM

yeah

John Rennie

So in this case the maximum KE would be 1.5eV.

In this experiment you are stopping the electrons by using an external potential to apply a decelerating force to the electrons.

If you apply an external potential $V$ then the work done by the external potential is $eV$. And you'll stop the electron if the work done is equal to the initial KE of the electron.

psa

right, so $1.5eV = eV_{stop}$

John Rennie

Actually we've going to get mixed up between the unit eV and the work $e \times V$.

psa

no I know the difference

it's alright

John Rennie

@psa yes, so $V_{stop} = 1.5V$

psa

7:13 AM

so as soon as the voltage produced a field which produced a force on the electron such that the work done by the force = the initial KE of the electron, you've halted their travel between the plates?

John Rennie

Yes.

psa

sweet! finally have a conceptual understanding of what's going on in this experiment

wasn't fully understanding the photoelectric effect setup but I think I do now

yuvraj singh

7:34 AM

@JohnRennie hi

John Rennie

@yuvrajsingh hi

yuvraj singh

Can you come in this room for a while. Sir

Yesterday inertia chat room. @JohnRennie

user8718165

10:28 AM

@JohnRennie hello sir

Intellex

@JohnRennie, Hi Sir. Are you free now?

John Rennie

@user8718165 @Intellex hi

user8718165

@JohnRennie sir can you please have a look at my answer...?

Intellex

@JohnRennie, Sir is fire plasma?

After @user8718165, I will as my doubt sir.

John Rennie

5

Q: Confused about fire?

Im confused about fire. The way I see it : Heat creates (kinetic) energy in mass and this creates stronger vibrations of atoms. When those vibrations are strong enough the electrons interact stronger due to electromagnetic forces. This causes the electrons to fly away. This is the creation of a...

user8718165

10:32 AM

@Intellex I think it has ions...because it gets deflected due to E-field.

John Rennie

3

Q: To which state of matter does the flame belong to?

I had this question from the day(9 years old, now 16) that i learned about states of matter. I have asked so many of my teachers some of them told me gas some as plasma etc. can anyone answer my question? Recently I've learnt that plasma state is obtained when all the electrons from the atom are ...

states-of-matter

user8718165

@JohnRennie sir,will you please have a look?

John Rennie

@user8718165 at what?

user8718165

@JohnRennie sir my answer...just once

John Rennie

@user8718165 which answer?

@user8718165 your answer looks fine.

user8718165

10:37 AM

@JohnRennie yeah :)

@JohnRennie thank you very much sir...for telling

Intellex

@JohnRennie, Thank you sir, I read that :)

user8718165

@JohnRennie sir can I ask you a question?

Intellex

I read an article on Medium on Why Is a Candle Flame in Zero Gravity So Different than One on Earth? but I cant understand why its spherical. Could you please explain why its spherical, sir?

Is that the same reason water bubbles are spherical there? I now know that fire is not plasma, so no surface tension right, sir?

John Rennie

On Earth flames have the shape they have because hot air rises.

Intellex

Ok sir, convection.

John Rennie

10:46 AM

The flame heats the air, the heated air rises and cool air is pulled into the bottom of the flame.

So we get candle flames that are tall and thin.

OK so far?

Intellex

Yes sir. I don't understand why its tear shaped on earth, I will refer PSE if you are busy now.

If cold air exerts more pressure at the bottom, I think it must be an inverted tear shape.

John Rennie

You don't need to know the exact shape of the candle flame. Just that the flame goes upwards from the burning part because hot air rises and the flame is basically hot air.

Intellex

@JohnRennie I understand this sir, but not the shape. I think we can proceed.

John Rennie

@Intellex but in space hot air doesn't rise because there is no gravity. The air heated by the flame just forms a ball of hot air near the flame.

Intellex

@JohnRennie Why should it be spherical sir? I understand all directions are equivalent. But for the flame, the candle side is different than the non-wax facing side, right sir?

This is what confusing me sir.

John Rennie

10:53 AM

Well OK, it won't be exactly spherical.

But it will be a lot closer to a sphere than it would be on Earth.

Intellex

@JohnRennie Can we explain the distorted sphere as something analogous to surface tension sir?

@JohnRennie Understood this sir. I've seen them.

John Rennie

@Intellex there is no surface tension in gases. I suspect calculating the exact shape would be really hard. I certainly don't know how to do it.

Intellex

@JohnRennie Sir, I don't need any quantitative study, just an intuition for the qualitative analysis is enough.

John Rennie

To be honest I don't know.

Intellex

@JohnRennie I am surprised sir. How can you say like that? Are you lying to me :) ?

There is a question on PSE, but doesn't cover this aspect really well and I suspect asking the same will make my question get closed as duplicate :(

John Rennie

11:00 AM

I've never seen a candle burn in zero g so I have no idea what the shape is and how the presence of the candle affects it.

Intellex

@JohnRennie,Thank you sir. No problem. I tried to upload an image. But I don't know what happened, everything in my comp. froze sir.

@JohnRennie, Bye sir :)

John Rennie

@Intellex bye :-)

yuvraj singh

11:41 AM

@JohnRennie hi

John Rennie

@yuvrajsingh hi :-)

yuvraj singh

Sir I have a small question

John Rennie

@yuvrajsingh yes?

@yuvrajsingh I need to go now. I'll be back later.

yuvraj singh

11:59 AM

OK, sir actually connection was lost. I will wait sir.

Aladdin

3:03 PM

@JohnRennie hi

John Rennie

@Aladdin hi :-)

Aladdin

Do u know about corrosion chemistry

yuvraj singh

@JohnRennie hi

John Rennie

@Aladdin a bit

@yuvrajsingh hi

yuvraj singh

I solve my problem by my own, it was a calculation error @JohnRennie

John Rennie

3:06 PM

@yuvrajsingh :-)

yuvraj singh

Yes sir @JohnRennie

Aladdin

@JohnRennie during Galvanic corrosion, why does metal with higher electrode potential is oxidized

John Rennie

The electrode potential is directly related to the energy of the reaction $M \to M^+ + e$

I'd have to look it up to be sure, but I think it's the Nernst equation that relates the EP to the free energy change.

So when you have two metals with different electrode potentials what this means is that they have different free energies for the reaction $M \to M^+ + e$.

OK so far?

Aladdin

Ok

@JohnRennie

John Rennie

Call our metals $M_1$ and $M_2$ and the free energies $\Delta G_1$ and $\Delta G_2$. Then the total free energy for the reaction $M_1 + M_2^+ \to M_1^+ + M_2$ is $\Delta G_1 - \Delta G_2$.

Aladdin

3:18 PM

Yes

John Rennie

And conversely the total free energy for the reaction $M_2 + M_1^+ \to M_2^+ + M_1$ is $\Delta G_2 - \Delta G_1$.

And the reaction that happens will be the one with the negative total free energy change.

Aladdin

Ok

John Rennie

Now I can't remember exactly how the SEP and free energy are related, but if you know the SEP then you know the free energy, and if you know the two free energies you can tell which way the reaction goes.

And now I'm afraid I'm starting lunch!

yuvraj singh

delta g =nfe @JohnRennie

John Rennie

@yuvrajsingh thanks :-)

yuvraj singh

3:32 PM

It, s OK

user8718165

4:20 PM

hello sir :-) @JohnRennie

John Rennie

@user8718165 hi :-)

yuvraj singh

4:41 PM

@JohnRennie hi

John Rennie

@yuvrajsingh hi :-)

yuvraj singh

Thief are x posed

John Rennie

Aha!

What happened?

yuvraj singh

Now you will be surprised, what they are.

John Rennie

Go on ...

yuvraj singh

4:43 PM

They are, professional robber use to rob, in the near village

John Rennie

Was it the two men in the shop?

yuvraj singh

I have doubt in them from starting.

Yes

Gang is of three member.

John Rennie

So even though it was a scary experience you helped catch the robbers!

yuvraj singh

Yes let me tell you the whole incident what I did today, after I post something can you say OK so that I can confirm you are listening.

user8718165

@yuvrajsingh @JohnRennie I've no idea what you people are talking 'bout. :-/

John Rennie

4:46 PM

@yuvrajsingh OK

yuvraj singh

@user8718165 rennie is will tell you.

user8718165

@JohnRennie sir could you please tell me

John Rennie

Yuvrah nearly got robbed by a pair of dacoits last night.

yuvraj singh

So sir I went to the shop, today around 7 o clock till then.

user8718165

@yuvrajsingh you caught a thief bro ?

yuvraj singh

4:47 PM

Yes

John Rennie

@yuvrajsingh OK

user8718165

@JohnRennie sir, that's scary :(

John Rennie

Damn right!

user8718165

@yuvrajsingh Did you fight them?

@JohnRennie I thought it was a ghost story

yuvraj singh

So I saw the shop men there, so I stop my bike near by the shop so that they can, t here, it was dark here, and road is as usual.

user8718165

4:49 PM

@yuvrajsingh I'm very happy you're totally safe. I was worried from the morning... :-)

yuvraj singh

Silent.

John Rennie

OK

yuvraj singh

Thanks @user8718165 @JohnRennie OK sir

user8718165

@yuvrajsingh You're welcome 😊

yuvraj singh

So after few minutes auto man reached there, and with one more man and @JohnRennie you won, t believe they are talking about me.

John Rennie

4:51 PM

Wow. What did they say?

yuvraj singh

'auto man to shop owner:) We missed the chance last night that chap doesn't look hard to fight enough.

They are blaming each other.

John Rennie

OK

yuvraj singh

Now it is clear to my why they are saying to me about. Coffee.

Sir that, s why I ask you what do at that time.

John Rennie

You think they had drugged the tea they offered you?

yuvraj singh

, otherwise why would they request me,

John Rennie

4:55 PM

So this was about three hours ago? What happened next?

yuvraj singh

OK

So I recorded I try to go close as I can to record their voice.

John Rennie

OK

yuvraj singh

So I recorded the voice and won, t belive they are trying for same. Plan which does.n,t work for me. To rob someone

John Rennie

OK

yuvraj singh

Like auto will break down, and then

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