Problem Solving Strategies

@JohnRennie Alright, but can you tell me the name of board or the national senior secondary books publisher, in my case its NCERT as publisher and CBSE board (India).

And In India we have 'Classes' 1 to 12 instead of 'Grades' in US/UK. We call classes 11 & 12 : Senior Secondary in which we study (physics) vectors introduction, newtonian mechanics, Work Energy Theorem, Rotation (Inertia, Center of Mass), and Gravitation (Escape Velocity), etc.

Akash. B

hi

Ravi Prakash

Can you compare and tell in which grades these things taught is US/UK?

Akash. B

Okay let me check

Ravi Prakash

To see what exactly is in India's class 11 & 12, I would refer you to our Class 11 Physics textbook : ncert.nic.in/NCERTS/l/keph1dd.zip

John Rennie

@RaviPrakash I don't think it works that way in the UK. The examination boards don't publish books or have set books.

The examination boards publish a syllabus, but it's up to schools which books they choose to teach the subject.

Ravi Prakash

7:32 AM

Oh, I got it.

Akash. B

I have sold my books

John Rennie

Examination boards in the United Kingdom

Examination boards in the United Kingdom (sometimes called awarding bodies or "awarding organisations") are the examination boards responsible for setting and awarding secondary education level qualifications, such as GCSEs, Standard Grades, A Levels, Highers and vocational qualifications, to students in the United Kingdom. Until the mid-1990s, academic exam boards and vocational accreditors were run very much as separate organisations. In more recent times, this distinction has been removed, with all the term 'awarding bodies' now being used. This article focuses on the contemporary and historical...

Akash. B

@JohnRennie What are the forces that can influence light other than gravity?

John Rennie

If you search the web sites of the current UK examination boards I'm sure you'll find the syllabusus.

@Akash.B light has an associated electric and magnetic field, and it interacts with everything else using these fields.

The electric field is much stronger than the magnetic field so light interacts primarily through its electric field. So basically light interacts with anything that carries an electric charge.

Akash. B

@JohnRennie How electrons are able to influence light?

John Rennie

7:37 AM

@Akash.B electrons are charged

Akash. B

okay 1 min

what are the changes brought to the light when it interacts with charges?

sir @JohnRennie?

John Rennie

That's a surprisingly complicated question.

Akash. B

okay so no more answers!

John Rennie

If the interaction is weak then all the light does is make the charged object oscillate a bit. This is how a radio aerial picks up the radio wave. The radio wave makes the electrons in aerial oscillate and that generates an electrical signal that is amplified in the radio.

If the interaction is strong then the light and the electrons merge to form a quasiparticle called a polariton. However this only happens in extreme circumstances e.g. in Bose Einstein condensates.

Akash. B

7:53 AM

Is there any changes in speed of light?

sir @JohnRennie

John Rennie

@Akash.B yes, and in fact the interaction I've just described is exactly why light slows down in things like glass. The light interacts with the electrons in the glass and the reslting interacting system behaves like it has a non-zero mass.

In effect the interaction gives the photon a mass, so it travels slower than light.

Akash. B

sir can you explain me Einstein's equation E=mc^2

John Rennie

I'm not sure what there is to explain about it ...

Akash. B

Is mass is converted into energy or energy is converted into mass?

John Rennie

I've answered a question on this. Hang on and I'll find the link ...

Akash. B

8:07 AM

4

Q: Meaning of Einstein's equation $E=mc^2$?

Meaning of Einstein's equation $E=mc^2$? How can a 1g mass possess energy equal to 9* 10 ^ 13 J? What does it actually mean?

mass-energy

I got it

John Rennie

52

A: What keeps mass from turning into energy?

This is inevitably going to be an unsatisfactory answer because your question is vastly more complicated than you (probably) realise. I'll attempt an answer in general terms, but you have to appreciate this is a pale shadow of the physics that describes this area. Anyhow, Einstein was the first ...

Akash. B

from mass to energy

John Rennie

The point is that quantum field theory explains what actually happens when mass changes to energy and when energy changes to mass

Tanuj

Sorry , to interrupt , but how old are you again @Akash.B

Akash. B

why this question?

Tanuj

8:11 AM

nvm

Akash. B

In the interaction it gives the photon a mass ,so it travels slower than mass .Is there a situation in which photons loose its mass?

@Tanuj nvm?

sir @JohnRennie

nevermind

check my profile

nah nvm

how old are you?

8:19 AM

@Akash.B photons have zero mass (in a vacuum) so they don't have any mass to lose. That's why they travel at the speed of light. All massless particles travel at the speed of light.

Akash. B

okay

how mass is produced in that interaction?

John Rennie

@Akash.B electrons have a mass, and the light gets tangled up with the electrons. So in effect the light gets a bit of mass from the electrons.

Akash. B

How electrons are able to transfer its mass?

John Rennie

I don't think there's an easy way to explain that. In quantum mechanics when two particles interact they become entangled and behave as a single system.

The electron doesn't really transfer its mass in the sense of snipping off a bit of its mass and transferring it to the photon. When the electron and photon become entangled they start to share each other's properties.

Akash. B

Can it happen in the case of protons and neutrons ?

@JohnRennie sir?

John Rennie

8:29 AM

@Akash.B sort of. Protons and neutrons are related by isospin transformations so they do get mixed up in the nuclei.

Akash. B

@JohnRennie I didn't understand isospin,sir

John Rennie

Neither do I :-)

It's a symmetry related to the weak force and it is very complicated.

Akash. B

@JohnRennie Sir ,as a theoretical physicist in which field you are conducting your research?

John Rennie

I'm not a theoretical physicist. I was a physical chemist, specifically a colloid scientist, and my work was almost entirely experimental. I retired a while back and now just do physics for fun.

Akash. B

@JohnRennie sir,any theoretical physicist in this site?

John Rennie

8:43 AM

@Akash.B Loads!

Akash. B

Loads?

John Rennie

Though the guys who work as theoretical physicists tend to be quite busy so they don't post that often.

2

I'm only around so much because I'm retired and don't have that much to do.

Akash. B

okay pleasure chatting with you

Ravi Prakash

10:23 AM

Books are quite difficult to get! So I would request you to please recommend me a good online resource for Electrostatics, and Current Electricity @JohnRennie

Madhuchhanda Mandal

11:04 AM

@RaviPrakash I know a offline source though

John Rennie

11:14 AM

@RaviPrakash I don't know what the good books are because I'm well out of touch with the educational system.

You could have a look at this though:

280

Q: Book recommendations

Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this is probably the place to start. All the question...

resource-recommendations faq

Akash. B

11:51 AM

@JohnRennie sir from which online site I can get Feynman's autobiography

Sid

1:26 PM

@Akash.B does googling not help?

John Rennie

1:59 PM

@Akash.B do you mean Surely you're joking Mr. Feynman, and if so what format? PDF? Epub? Kindle?

Ravi Prakash

2:20 PM

@JohnRennie An electric field acts from z axis to x-y plane, then the electric flux through a rectangle (length L and breadth B) is?

John Rennie

If the field is everywhere normal to the area, which it is in this case, then it's just field strength time area isn't it?

Ravi Prakash

Field strength time area? I don't know. Can you explain a little more?

Sid

@JohnRennie how do you know that? The question doesn't mention where the rectangle lies, no?

Ravi Prakash

I agree that field is everywhere normal

John Rennie

@Sid Hmm, I assumed the rectangle was in the xy plane ..

Ravi Prakash

2:29 PM

Rectangle is centered at origin with z = √3/2 i.e. (0,0,√3/2)

John Rennie

@RaviPrakash see here:

Electric flux

In electromagnetism, electric flux is the measure of flow of the electric field through a given area. == Overview == An electric "charge," such as a single electron in space, has an electric field surrounding it. In pictorial form, this electric field is shown as a dot, the charge, radiating "lines of flux." These are called Gauss lines. Electric Flux Density is the amount of electric flux, the number of "lines," passing through a given area. Units are Gauss/square meter. Electric flux is proportional to the number of electric field lines going through a normally perpendicular surface. If...

Ravi Prakash

yes,

I understand the concept. Flux = |E|•|A|•cos(x)

If x=90° the flux vanishes

But the field originates from +z to -z and it passes through Rectangular Area

Also, its isn't a Gaussian Surface so $ Q_{in}/ \varepsilon_0 $

doesn't applies.

Now, see this question

@JohnRennie What about this question?

John Rennie

2:49 PM

Suppose you look at this from the side i.e. so you're looking along the line charge. It will look like (diagram to follow):

Sid

@RaviPrakash that's IIT-JEE 2013 (?) Question, no?

Akash. B

@JohnRennie I want it as book,sir

John Rennie

@RaviPrakash so the angle that the field lines hit the rectangle changes across the rectangle.

@Akash.B buy it from Amazon then

You can't download paper :-)

Akash. B

okay thank you

John Rennie

3:06 PM

@Akash.B available for ₹260 plus postage

Ravi Prakash

@JohnRennie Yes, absolutely.

Then, What's next?

@Sid Are you in class 11 / 12?

Sid

@RaviPrakash college

Or Uni. Whatever you like to call those things

Tanuj

@JohnRennie Hi , have to ask you a question.

John Rennie

@Tanuj Yes?

Tanuj

3:21 PM

Okay , so at the moment it strikes the bump at O , the translational motion stops and rotational motion begins.

John Rennie

So presumably the bottom edge of the block hits the bump and stops, and the block pivots about $O$ ?

Tanuj

Now , I can conserve angular momentum as net torque about O is essentially 0 ?

John Rennie

Yes, that's the way to approach the problem.

Tanuj

@JohnRennie I'm definitely doing something silly then.

@JohnRennie initially , angular momentum about O would be $mvr$ , where $r=30cm$

John Rennie

The centre of mass isn't 30cm above the pivot ...

Tanuj

3:25 PM

@JohnRennie Oh yea , Scheiße

I've just turned Nazi

Sid

@Tanuj why would r be 30?

Tanuj

@Sid It won't be , I got it wrong.

@JohnRennie Also , for writing $I\omega$ , do I take $I$ to be about point $O$ or COM ?

John Rennie

@Tanuj the cube is rotating about $O$, so you want the MOI for a cube pivotting about an edge

Tanuj

@JohnRennie I mean how can I decide that if I'm not able to visualize it ?

John Rennie

@Tanuj it's kind of obvious isn't it. The leading edge hits the bump and stops. If the leading edge is stationary the rest of the cube must be rotating about it.

Tanuj

3:31 PM

@JohnRennie yea , I see it now.

Thanks :)

@JohnRennie one more question , if you're free.

John Rennie

Yes?

Tanuj

Okay , so I might have done something stupid but I assumed the two equations for the two SHMs to be

$x=A\sin(\omega t)$ and $x=A\sin\left(\omega t - \dfrac{\pi}{6}\right)$

Gaurang Tandon

@Tanuj i know you aren't looking for this, but trying out the options seems to be the easiest

Tanuj

@GaurangTandon How would you try them ? I tried and honestly the correct answer didn't match the expectation :p

John Rennie

At time $t=0$ the position of the first particle is $A$, so the equation for the first particle is $A\sin(\omega t + \pi/2)$

Gaurang Tandon

3:37 PM

@Tanuj i have mugged up the time an SHM takes between A to A/2 and A/2 to 0, so that's how I do that :P

Ravi Prakash

@JohnRennie What to do next to find the flux?

I got that $ \lambda = Q / L $ So that $ Q = \lambda L $

Tanuj

@GaurangTandon Hey ! I mugged up something too , but I'm not sure it applies to this. Time object undergoing SHM takes from going mean position $(x=0)$ to half the amplitude $(x=A/2)$ is $T/12$ and for a particle going from $(x=A)$ to $(x=A/2)$ is $T/6$

John Rennie

@RaviPrakash you divide the rectangle into strips of width $dx$ parallel to the $y$ axis. The position of the strip is $x$ where $-a/2 \le x \le a/2$. At each strip the angle of the field lines is constant along the length of the strip.

Gaurang Tandon

@Tanuj that's right

Tanuj

@GaurangTandon I didn't get the answer when I applied this.What answer are you getting ?

Gaurang Tandon

3:41 PM

hmm let me try then

Ravi Prakash

@JohnRennie Thanks so much. I'll try , currently @ train

John Rennie

@RaviPrakash write the angle as a function of $x$ and work out the field strength, which is also a function of $x$. The area of the strip is $Ldx$ so you can work out an expression for the flux through the strip. Then integrate it to get the total flux.

Tanuj

@JohnRennie Wow , I messed up big time. Thanks for pointing that out , I do get the correct answer now !

Gaurang Tandon

@Tanuj T/6?

Tanuj

@GaurangTandon yea , how did you do it ?

Gaurang Tandon

3:44 PM

@Tanuj in T/12, the second particle will go from -A/2 to 0. Correct?

Ravi Prakash

@JohnRennie .Yeah

Tanuj

@GaurangTandon Why the fuck do I keep messing things up ! Thanks anyways ! :)

@GaurangTandon I took it to be $T/6$ for some reason.

Sid

@Tanuj because you are worrying too much about messing up.

Chill bro. When is Mains exam this year?

Ravi Prakash

What do you think the difficulty level of this question @JohnRennie

Gaurang Tandon

no worries @Tanuj. relax and you're good to go

Tanuj

3:46 PM

@Sid I know. These mocks are killing me.

Not getting above 150 in any !

Ravi Prakash

4 Apr - JEE Main

Sid

@RaviPrakash lots of time then

John Rennie

@RaviPrakash it seems like a fairly standard JEE question, though I'm not an expert on JEE questions

Tanuj

@RaviPrakash Are you giving it this year ?

Gaurang Tandon

@JohnRennie i think you've already seen enough JEE questions ;)

John Rennie

3:48 PM

@GaurangTandon true :-)

Tanuj

John would be like 360/360 in mains and full marks in advanced and be like , yea I'm surprised I remember all of that from 80s and stuff.

Sid

Mains usually just lifts questions from previous year papers.

Gaurang Tandon

@Tanuj John will probably need to mug up some chemistry as well ;-)

Tanuj

@GaurangTandon I'm sure chemistry is already up his sleeve.

Gaurang Tandon

@Tanuj ah yes, he was a colloid scientist. Then he's gonna get 360/360 no doubt!

Tanuj

3:53 PM

@GaurangTandon And he'll be so modest about it you wouldn't even believe he achieved such a feat

Gaurang Tandon

yeah right

Sid

In any case, someone help me with a kinematics problem which I seem to be blanking for sometime now...

Tanuj

Tbh , John teaches me a lot , about life as well. He is a true role model , but let's not get into that , or he'll again be like there are much better things to be impressed by than me.

4

Sid

I am sure I had solved such questions before but I can't remember what exactly I was doing on such questions

How do you solve questions of planar motion when some part of the track is horizontal and then it is inclined?

I should probably provide a picture..

Gaurang Tandon

4:08 PM

post a picture

(though I got to leave now)

Sid

We have to find s and time of motion

(assuming of course that after striking B, ball sticks to it and comes to rest)

Ravi Prakash

4:48 PM

@Tanuj Yes

John is the AIR 1 with 100% score in JEE (Advanced) 2018

@JohnRennie How do you get to know about JEE? Its a bit surprising!

John Rennie

@RaviPrakash because I've helped students with several million JEE questions here (slight exaggeration :-)

Tanuj

@Sid you there ?

Sid

Yeah.

Tanuj

@Sid The angles are a bit nasty but you could use equation of trajectory for a projectile

Ravi Prakash

@JohnRennie So if you know about JEE, can you recommend me some tips as well as the difficulty level of JEE and how deep one topic to be studied?

Sid

4:57 PM

@RaviPrakash JEE is difficult for a high-school student. It isn't very hard for a college student.

John Rennie

@RaviPrakash from what students have said here there are loads and loads of books that specialise in preparing students for the JEE.

Ravi Prakash

@JohnRennie Glad you're helping Indian students.

Tanuj

@RaviPrakash He helps everyone !

Sid

@Tanuj I don't care about the equation of trajectory. I just need to know whether I have to calculate the velocity at the point the inclination starts.

John Rennie

@RaviPrakash I'm happy to help all students, but the nature of the Indian education system means it's mostly Indian students who ask.

Tanuj

4:59 PM

@Sid Idk man , I see no reason why you'd wanna do that , if you don't have to.

Sid

And then, I guess I have to do something like calculating the acceleration it experiences?

@JohnRennie The Indian education system lacks only two things. 1. Education. 2. System.

Ravi Prakash

@JohnRennie Yes they (book authors) just want their books to be sold more and more

Tanuj

equation of trajectory basically does the job , requiring you to solve just for $s$ .

John Rennie

Incidently, if I sat the JEE right now I'd run out of time and fail. You all underestimate just how amazingly good you get at doing these sorts of problems. Yes it's a long and painful process, and I wouldn't wish it on my worst enemy, but it does leave you with remarkable skills when you get through it.

Tanuj

@Sid Well said. Did you hear about the CBSE paper leaks ?

Sid

5:01 PM

@Tanuj No one cares about CBSE except your relatives.

Tanuj

@Sid nope , not even my relatives give a shit about it. :p

Ravi Prakash

@JohnRennie Yes, It is.... Our education system needs to evolve.

John Rennie

@RaviPrakash I guess it's because there are so few places at colleges - good colleges anyway. In the UK pretty much anyone can go to university because so many universty places are available.

Though competition for top universities like Cambridge is still very fierce.

As India gets wealthier it will build more colleges. Your children will have a much saner education system to deal with.

Though I guess that's not much comfort to you right now :-)

Sid

@JohnRennie "As India gets wealthier" - I wouldn't count on that

John Rennie

@Sid We'll see :-)

Ravi Prakash

5:17 PM

@JohnRennie Exactly. But then also, what can we do right now? My current focus is to give my best at JEE. So I was asking about that

John Rennie

@RaviPrakash yes, just work like hell and get to a good college. It's going to be a bit rubbish for the next few years but it is worth it in the long run.

But I'm not the person to ask about the best way to prepare for the JEE because I don't know enough about it.

Ravi Prakash

@JohnRennie Hmmm. :-) True said.

@JohnRennie Yes, but you are topic's expertise. Like of electrostatics,....etc

Now its time to say Good Night, as its 22:51:00:00 IST !

John Rennie

Goodnight all. I'm going too. Time to sit in my armchair and read a book! :-)

Ravi Prakash

Yes! Njoy!

I'm in train and nearby to The Taj Mahal, Agra (UP), if you know, one of The Seven Wonders of the World !

The train has acquired velocity $ v $, such that $ 150 Km/Hr << v $ !

Good Night !

Sid

@RaviPrakash you are travelling close to the speed of light? :o

King Tut

5:37 PM

Hello can I show a physics question i have

Sid

@KingTut Yeah. That's why this site (and this chatroom) exists.

If we can help you, we will/

King Tut

Thank you! My questoin is that why we can apply gauss law for finding charge distribution when we know it may not be inverse square law obedient, please excuse my english is not upto the mark

Supposing that we find distribution of charge in some volume $dV$. Potential may be given as say $f(r)$ or some other variable. If we apply gauss law here to find charge distribution then we will impose that electric field is obeying inverse square relation!! But we do not know that those charge distribution create inverse square field or not, so is it valid assumption

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Problem Solving Strategies