The h Bar

A fluorescent lamp or a fluorescent tube is a low pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than incandescent lamps. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. Fluorescent lamp...

Its written here in detail

Compact fluorescent lamp

A compact fluorescent lamp (CFL), also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent light bulb; some types fit into light fixtures designed for incandescent bulbs. The lamps use a tube which is curved or folded to fit into the space of an incandescent bulb, and a compact electronic ballast in the base of the lamp. Compared to general-service incandescent lamps giving the same amount of visible light, CFLs use one-fifth to one-third the electric power, and last eight to fifteen times longer. A CFL has...

Yashas Samaga

Since when has fluorescent lamps entered high school syllabus?

anonymous

@YashasSamaga Modern Physics chapter I guess

Photoelectric effect

Yashas Samaga

:|

CBSE?

anonymous

Yeah

It is given in NCERT

Some chapter

Read NCERT thoroughly

Yashas Samaga

It isn't there for JEE, right?

anonymous

7:46 AM

It is there

Obviously

Yashas Samaga

.

anonymous

Everything in NCERT is there in JEE

Ramanujan

@anonymous, @YashasSamaga, It is in the chapter Current Electricity

anonymous

Yeah right

I remember now

Even I need to revise the book

Yashas Samaga

I am pretty sure it isn't there in JEE -,-

Ramanujan

7:47 AM

@anonymous, which country do you belong?

anonymous

@YashasSamaga Live with your misconceptions :P

Every year they ask short questions from that book

Yashas Samaga

NCERT will work for JEE Main but you'll get screwed in JEE Advanced

anonymous

@YashasSamaga I never said study "only" NCERT.

@Ramanujan Indian living in Kuwait.

Yashas Samaga

It isn't there in JEE adv syllabus so idc.

anonymous

It just takes 30 min to go through one chapter of that book

Better not to take risk

Anyway, your choice

Is a black hole a black body ?

Yashas Samaga

8:01 AM

6

Q: Is a black hole a perfect black body?

A black body absorbs all light/radiation in its reach. According to basic laws of physics, the more energy a body absorbs the more it can emit. Therefore, a black body absorbs all energy directed at it and also emits all energy that's been absorbed. A black hole is known to absorb all sorts of ...

anonymous

Some people say so. However, I feel that black bodies should be good emitters along with being good absorbers. Black holes might satisfy the latter condition but not the former.

So, are black holes good emitters of light ?

That part isn't answered there.

Yashas Samaga

black bodies are both good emitters and good absorbers

there is even a law stating that

anonymous

@YashasSamaga Hmm, so is a black hole a good emitter?

In case you say "Yes, black holes are supposedly near-perfect black bodies."

Yashas Samaga

I guess so.

user228700

When did this place become the IIT JEE prep. room?

anonymous

8:04 AM

@YashasSamaga Oh really? Black holes are good emitters of light? I sort of disagree.

Ya, there is Hawking radiation but that does not make it a good emitter.

I will conclude that the sun (or star) is a better qualification for black body than a black hole

@JohnRennie Can black holes be called near-perfect black bodies? Are they good emitters alongside being good absorbers?

Yashas Samaga

I am not sure about black holes but there is a rule which states that the spectral emission coefficient must be equal to the spectral absorption coefficient.

anonymous

@YashasSamaga That rule works only for classical objects.

Yashas Samaga

yea

John Rennie

@anonymous Yes and yes. They are perfect emitters but their temperature is exceedingly small so the amount of black body (i.e. Hawking) radiation they emit is very small.

anonymous

@JohnRennie Could you explain why they are perfect emitters? Do they really emit light?

Till now I thought of it as something that only absorbs light

John Rennie

8:11 AM

@anonymous The light they emit is the Hawking radiation. The spectrum of the Hawking radiation is a black body spectrum corresponding to the Hawking temperature.

I suspect you're getting mixed up with reflection as opposed to emission. A black hole dioes not reflect any light because it is a perfect absorber (all perfect black bodies are perfect absorbers).

Yashas Samaga

@JohnRennie does this makes sense for a black body? "spectral emission coefficient must be equal to the spectral absorption coefficient"

John Rennie

@YashasSamaga Yes

Yashas Samaga

Does it make sense for a black hole too?

John Rennie

Yes

Yashas Samaga

So a black hole spews out what it previously sucked?

without any change?

anonymous

8:16 AM

Hehe

Then our examiners don't know about hawking radiation

The answer given in my book is (A)

Haha :'D

@JohnRennie Thanks. I realize now that HR is the emission

Yashas Samaga

black holes aren't in the IITJEE syllabus, I'd eliminate that option immediately.

Such questions won't come in the actual test.

anonymous

Hey man, this is from a jee paper

@YashasSamaga

You say "won't come" for everything

:P

Yashas Samaga

That must be from AIEEE?

anonymous

Yeah

Yashas Samaga

AIEEE is not JEE

JEE replaced AIEEE

JEE Advanced papers are set by IITs

anonymous

8:19 AM

JEE Main is set by CBSE

Yashas Samaga

This year's paper is set by IIT Madras, so prepare well in math.

user228700

I happen to personally know some of the profs. setting the paper, if that's of any interest to either of u :-P

anonymous

@Kaumudi.H As if they will tell you the questions ;)

user228700

Of course they won't :-) They're my friends' parents so I know them is all.

anonymous

They won't or else they will lose their jobs

@YashasSamaga You too :)

user228700

8:23 AM

@anonymous Probably.

Slereah

I am making some waffles

I think I poured in too much batter

anonymous

I hate the dough in waffles

Rather I hate dough itself

Slereah

Send help

anonymous

Let it be. Clean the mess later :P

Kenshin

yo

anonymous

8:36 AM

ho ho ho

Slereah

8:49 AM

Hm

what book to get

I should get Penrose and Haag

Haag is 40€

ouch

But still reasonable by physics standards

Oh wait, I already ordered Penrose

But it's still not there

It's been two weeks dammit

hm, what else to get thne

anonymous

@Slereah Ebook ?

Or hard copy?

Slereah

Real books

anonymous

Ebooks are way cheaper

Slereah

Yeah but they are not very practical to read

amazon.com/Philosophy-Space-Dover-Books-Physics/dp/0486604438

I should get this

it's cheap and it's referenced in Geroch's paper

Hm, what else

choquet bruhat still too expensive

Jost, maybe

I could use a good analysis book on the shelf

there we go

All done

Ramanujan

9:26 AM

Hello

skill patrol

9:41 AM

hi

Slereah

hey

anonymous

Hi. Hello. Hey. How are you? How is the weather ? :'D

Ramanujan

It's sunny

Slereah

So anyway

How did meteors work, in Aristotle's physics

Meteors were supposed to be below the ether

But how did they get up there?

Obviously they can't go up naturally because only air and fire go up in those layers of the world

So it had to be a forced motion

But which one

anonymous

10:00 AM

Is there any way to calculate the number of field lines a magnet (say bar magnet) produce, from one pole to another ?

Slereah

"number"?

anonymous

In the lab I always found that iron filings arrange in a certain way along field lines

Slereah

yes, but

anonymous

And there are finite number of them

Slereah

There are infinitely many field lines

anonymous

10:02 AM

@Slereah In that case field lines should not be visible when iron fillings are arranged isn't it? They should be everywhere around the bar magnet

I am badly confused

Slereah

Well, I don't know if iron fillings would arrange themselves along all field lines

Yashas Samaga

number of field lines is not a legit physics question

Slereah

They might bunch up together due to things

Yashas Samaga

field lines are just a visual construct to make understanding fields easier

anonymous

Yashas Samaga

10:03 AM

higher density of field lines = larger the strength of the magnetic field

anonymous

There should not be any white space left there

Slereah

Let's find out

anonymous

As there are infinite field lines'

Yashas Samaga

Field lines are not real.

Slereah

My guess is that some process makes it so that the filling bunch together due to some process

Yashas Samaga

10:04 AM

If you had smaller filinings, you would find less white space.

Slereah

Maybe they get slightly magnetized?

Yashas Samaga

The more smaller you make, the less white space you get.

Slereah

"when exposed to a magnetic field, the filings are themselves temporarily magnetized along their long axis"

yeah

Probably why

anonymous

Its seems more to do with shape and size of iron fillings

I am not sure though

Yashas Samaga

so they might try to pull adjacent filings to themselves

but field lines are just a mathematical construct

Slereah

10:06 AM

That will form clusters of a certain width depending on the field strength

anonymous

@YashasSamaga What makes them leave white spaces nearby and still form patterns farther away? The iron fillings are small enough to fit in the nearby white spaces.

Slereah

See what I said

Yashas Samaga

Firstly, you need to give away the idea that you can "measure" the number of field lines.

anonymous

@Slereah "when exposed to a magnetic field, the filings are themselves temporarily magnetized along their long axis" Agreed. They will form clusters of certain width ? Why? Why not a continuous patterns rather than leaving white spaces between ?

Yashas Samaga

Either accept it as infinite or accept it as just a mathematical construct to understand fields.

anonymous

10:09 AM

@YashasSamaga I get that.

Yashas Samaga

I am guessing that if you put extremly tiny filings, you will have all the white space filled.

Slereah

Because if you have a continuous cover of iron filings, the magnetic force exerted by nearby iron filings will attract the nearby ones

Up until there are no more filings in proximity

Yashas Samaga

And the magnetization near the poles are stronger.