Problem Solving Strategies

OK sir, I have a circular plate dipped in a liquid of density k, plate has a mass M, and radius R, disc is allowed to rotate vertically, I need to find the resistive torque offered by liquid, if the disc is rotating with constant angular velocity $w$, by a constant internal mechanical force $F$. @JohnRennie

John Rennie

Can you post the original question so I can see exactly what is being asked?

yuvraj singh

5:38 AM

Container height is $H$, and disc is dipped completely, actually sir I do not have the pic, because I got the same language question through whatsapp @JohnRennie

I have no idea how to start with this. Sorry I forgot to give liquid mass it is $m$,

Aladdin

@yuvrajsingh If you use mobile,download imgur and post pictures using it

it's there in playstore

yuvraj singh

Actually I have no pic, this is the actual question in written way. @JohnRennie

John Rennie

It isn't clear how the disk is rotating. Is the disk horizontal and rotating about a vertical axis through its centre?

yuvraj singh

It look like this @JohnRennie i.stack.imgur.com/W4qf1.jpg

John Rennie

5:54 AM

@yuvrajsingh so the disk is acting like a stirrer i.e. as it turns it is stirring the liquid and making it rotate?

yuvraj singh

Yes @JohnRennie

John Rennie

There is no simple way to answer that question.

yuvraj singh

I started with torque, since moving with the constant angular velocity so net torque is zero, hence net external torque should be zero, . @JohnRennie

John Rennie

The torque will be because the moving disk has to accelerate the liquid it is pushing out of the way.

Just like if you wave your hand in water you feel a force because your hand is accelerating the water.

But the force will depend on the details of exactly how the water flows around the disk, and that will be complicated.

yuvraj singh

So at last, can you explain who will provide this resistive torque. @JohnRennie

John Rennie

6:06 AM

When the disc rotates in the water it has to move the water. Yes?

yuvraj singh

Ok

John Rennie

And the water has a density, so the water being moved has a mass. And to move anything that has a mass you have to apply a force.

That force is what creates the torque.

yuvraj singh

I mean what kind of nature of this force has, viscous @JohnRennie

John Rennie

Whenever you are making a liquid flow there are two forces:
1. viscous
2. inertial

Generally speaking viscous forces dominate at low flow rates and inertial forces dominate at high flow rates

The viscous forces come from the thickness of the liquid i.e. how gooey it is. It takes more force to make a thick liquid like honey flow than to make a thin liquid like water flow.

yuvraj singh

What, thing which make the question more complicated according to you. @JohnRennie

John Rennie

6:13 AM

the inertial forces come from the fact the liquid has a mass, and to make it flow you have to accelerate it. The force is due to Newton's second law F=ma, where m is the mass of the bit of the liquid you are accelerating and a is how much you accelerate the liquid.

@yuvrajsingh in this problem we aren't given the viscosity, so that implies we are supposed to consider just the inertial forces.

But we don't know how much the liquid is being accelerated by the rotating disk because it depends on exactly how the liquid floes around the disk.

yuvraj singh

Did, the viscous force remain constant through the rotation. @JohnRennie

John Rennie

@yuvrajsingh the viscous force will also depend on exactly how the liquid flows around the disk, so we cant calculate the viscous force either.

yuvraj singh

@JohnRennie'how the liquid flows around the disk' what you mean by this.

John Rennie

I need to work now for a while. I'll be back later but it won't be for an hour or two.

Aladdin

6:47 AM

@JohnRennie When you come back,ping me

Aladdin

7:00 AM

@Mr.Xcoder Hi

Mr. Xcoder

Hi. I'd help you if I were free but today is a really busy day for me, unfortunately

Aladdin

okay

user8718165

hello sir @JohnRennie are you working now?

John Rennie

8:29 AM

@Aladdin @user8718165 I'm back. Work is finished for today :-)

user8718165

@JohnRennie hello sir

John Rennie

@user8718165 hi

user8718165

@JohnRennie I want to ask you some questions sir

John Rennie

@user8718165 OK ... ?

user8718165

@JohnRennie sir in order of stability ethane>ethene then why does ethene have a hotter flame?

John Rennie

8:31 AM

Don't know.

I guess that would be because ethene reacts faster.

i.e. you get more moles per second reacting and this makes up for the lower enthalpy per mole.

user8718165

@JohnRennie also sir, ethane makes better fuel than ethene...then ethane should have hotter flame and stuff...but ethene has hotter flame :-/

@JohnRennie okay sir...

John Rennie

I don't know what constitutes a better fuel.

If you need a hot flame then clearly ethene is better.

user8718165

@JohnRennie okay sir...if ethene has hotter flame then it should also have a higher enthalpy of combustion than ethane...but it doesn't sir...

@JohnRennie sir could you please expand this a bit?

@JohnRennie hello

John Rennie

The temperature of the flame will depend on the rate of heat production i.e. joules per second. Yes?

user8718165

@JohnRennie yes sir...

John Rennie

8:44 AM

And joules per second = (joules per mole) x (moles per second)

user8718165

@JohnRennie yeah sir

@JohnRennie got it sir...you mean ethene combusts faster than ethane...right

John Rennie

ethene has a lower joules per mole than ethane, so it if has a hotter flame that must mean it burns more moles per second.

user8718165

@JohnRennie got it sir...also its more reactive hence must react faster

@JohnRennie thank you very much sir...can I ask one more question?

John Rennie

@user8718165 yes, I've finished work now so I'm just messing around.

user8718165

@JohnRennie okay sir...ethane is already saturated...but after combustion it releases more energy than ethene which is unsaturated...I got the reason...but isn't it a bit confusing sir?

John Rennie

8:53 AM

When you burn ethane you form 6 OH bonds and two CO double bonds. The formation of each bond releases energy.

When you burn ethene you form only 4 OH bonds and the same two CO double bonds. So you get two OH bonds less energy released.

user8718165

@JohnRennie sir thank you sooo much...one last question sir

John Rennie

@user8718165 yes?

user8718165

@JohnRennie sir, since hydrogen is highly combustible, can we say that since ethane has 6 H's while ethene only has 4 hence ethane releases more energy...sorry if that's wrong...I made this reason

John Rennie

Basically yes.

user8718165

@JohnRennie thank you so much sir for helping. Now you can take rest :-)

yuvraj singh

9:28 AM

@JohnRennie irony but it won't be for an hour or two.

@yuvrajsingh hi :-)

Hi sir @JohnRennie

@yuvrajsingh hi

Is time reversal is possible @JohnRennie

Sorry to ask this again

John Rennie

That depends what you're asking.

If you're asking if we can go back in time then as far as we can tell the aswer is no.

yuvraj singh

9:35 AM

@JohnRennie I am having some difficulty in trying to understand your answer. Will be thankful for any help. Actually time reversal can be written as a product of unitary operator and complex congugation operator.In the coordinate space the unitary operator is just an identity operator so one just takes the complex congugation.

I mean In h bar

John Rennie

I think this is too hard for someone who hasn't done a physics degree.

yuvraj singh

Can you give simple explanation, without using the complicated, in short @JohnRennie

John Rennie

No

user8718165

10:02 AM

@JohnRennie hello sir :-)

John Rennie

@user8718165 hi :-)

user8718165

@JohnRennie I have a question sir...

Intellex

@JohnRennie, Hi Sir. After user8718165's question, if possible could you please clarify this question on Physics.SE?

0

Q: What will happen if we Boil Water in the Absence of Gravity?

We know that, Convection is the mode of heat transfer due to the motion of individual molecules of a fluid (liquid or gas). The motion of the constituent particles arises due to difference in densities. In the absence of gravity, water takes a spherical shape in order to minimize the energy due ...

John Rennie

@Intellex well what do you think will happen?

Intellex

@JohnRennie, I think, upon heating water gradually from the centre, the heat must travel to the surface by conduction. When the entire sphere's temperature reaches 100 degree Celsius, bubbles start to form at the centre. After that I am clueless.

I don't know whether it will explode or not.

I don't know how will convection behave in the absence of gravity.

John Rennie

10:08 AM

How do you plan to heat the ball of water? Put a heating element in the middle? Or just heat the air around the drop?

Intellex

@JohnRennie I assume a heat source which perfectly gives heat only at the centre of the water sphere.

It gives heat gradually to the system.

John Rennie

Ok, so we could put a heating element on the end of a wire, push the element gently into the middle of the drop then turn on the electricity to heat it.

Intellex

And we must neglect the cord which connects the heating element and the power supply.

Else, Let us consider it to be battery powered

John Rennie

Yes, we can insulate them so they don't transfer any heat to or from the drop.

Or as you say just use a self contained heater.

Intellex

@JohnRennie No sir, wire causes a lot of trouble. It alters the shape of water due to surface tension.

John Rennie

10:12 AM

OK, we'll use a self contained heater.

Intellex

@JohnRennie Perfect.

John Rennie

Water is a very poor conductor of heat. The reason water heats up fast in a pan on the oven is because we get convention currents that mix the hot water at the bottom of the pan with the cold water above it.

In zero g there are no convection currents, so what happens is that the water in contact with the element stays there and heats up fast while the rest of the water stays cold.

3

Eventually the water in contact with the element boils and forms a bubble of steam around the element.

3

Intellex

Ok sir, understood till this.

So once the bubble is formed, does it stay at the centre or just EXPLODE !@##$%^^&

John Rennie

There are two possibilities:

1. a bubble of steam nucleates quickly and then just grows steadily. All quite boring really.

2. the water superheats. Do you know what I mean by superheating?

Intellex

@JohnRennie Guessing: Heating under increased pressure

@JohnRennie How can this happen sir? Once bubble is formed, the contact with water and heating element is lost right?

John Rennie

10:20 AM

@Intellex yes, but if we keep pumping in heat that heat has to go somewhere. What it will do is heat the steam in contact with the element, and that steam will heat the water at the edge of the bubble of steam and make it evaporate. So the bubble of steam will grow steadily.

Intellex

@JohnRennie Now clear sir. Lets move to superheating.

John Rennie

Have a look at:

4

A: Why droplets of water under oil explode when heated

The bubbles explode because they superheat. The oil prevents the nucleation of steam bubbles you get when heating water in a pan, and the water drops turn to steam all at once and of course the steam expands explosively. Even though water and steam are in equilbrium at 100°C there is a potential...

That describes what superheating is, and actually it's quite relevant to what we're discussing here.

Intellex

Ok sir. Please wait.

user8718165

@JohnRennie sir, I solved my question. Still thanks :-)

John Rennie

@user8718165 OK :-)

Intellex

10:26 AM

@JohnRennie, Reading in progress, but a small doubt. Why should steam bubbles form at scratch points or defects? Is that something to do with surface tension sir?

user8718165

nucleation sites?

John Rennie

@Intellex water molecules generally interact strongly with the molecules at solid surfaces, and this can lower the activation neergy for formation of a bubble.

As @user8718165 says, this process is called nucleation.

Exactly how this occurs is going to depend on the exact interaction going on and is generally complicated.

Intellex

Thank you @JohnRennie and @user8718165

I am ready @JohnRennie sir

user8718165

@Intellex You're very welcome :-)

Intellex

Is superheating because of increased pressure due to the presence of oil layer?

John Rennie

10:30 AM

@Intellex no. You get superheating because there is a barrier to nucleation of a steam bubble. You need to get the temperature above 100°C to generate enough energy to get over this barrier and form the bubble.

So this will probably happen with your droplet floating in zero g.

Intellex

@JohnRennie We are actually heating a liquid above its boiling point but in the liquid state itself. Is this correct sir?

John Rennie

Correct.

Intellex

Why not we consider this as elevation of boiling point due to increased pressure then sir?

John Rennie

If you take very pure water and heat it carefully you can get it to well above 100°C and still stay liquid.

@Intellex The sort of superheating I'm talking about happens at ambient pressure i.e. 1 atm.

Intellex

@JohnRennie, Ok sir. I am again surprised. New concept!

John Rennie

10:36 AM

Of course you can put the water into a pressure cooker and heat it at increased pressure, but that needs some form of pressure vessel. Superheating happens at ambient pressure.

Intellex

Thank you sir. @JohnRennie. Now understood the difference.

John Rennie

Anyhow, if superheating happens with your water drop in zero g then when the bubble does nucleate you'll get an explosive growth in the bubble size as the superheated water changes rapidly to steam.

The drop will explode and spatter boiling water all over you :-)

Intellex

Now some notifications in my brain: I have seen supercooling - where one shakes some water after taking it from refrigerator when it immediately freezes to form ice. Is this the analogue for superheating?

@JohnRennie So sad, I thought the story must be a good-ending one :)

John Rennie

@Intellex yes, supercooling happens for exactly the same reason. There is a barrier to nucleation of an ice crystal just like there is a barrier to nucleation of a steam bubble.

Intellex

I have seen that in Discovery channel - You have been warned

@JohnRennie, Did you see Neil Armstrong and Buzz Aldrin set foot on the moon in 1969? In which grade were you in at school?

John Rennie

10:46 AM

@Intellex I was eight - I was born in 1961. Yes, I watched it live on TV.

Intellex

@JohnRennie Wonderful!!!!!!!!!!!

Really happy that you saw it, even though I didn't see that live. :)

John Rennie

I don't remember being that impressed to be honest. Possibly I was a bit too young to appreciate it.

Intellex

@JohnRennie yes agree. But now I am happy to chat with a person who has seen that live.

Sir, could you please help me with this question?

0

Q: Do Electric Charges in the Van Allen Radiation belt move in Opposite Directions?

The Wikipedia Article on Van Allen Radiation Belt gives the following explanation: A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetic field. The term "energ...

radiation solar-wind

John Rennie

I think I answered a question on that. Let me do a quick search ...

Intellex

Ok @JohnRennie sir. Thank you

John Rennie

10:51 AM

6

A: Analogy between magnetic bottle and Van Allen's radiation belt

In the Magnetic Bottle 1 YouTube video the charged particles are reflected from the ends of the bottle (where the ring are) because the magnetic field in non-uniform there. The particles tend to stay in the middle of the bottle where the field lines are roughly parallel. The same thing happens i...

Intellex

@JohnRennie, Thank you for sharing that sir. I will read that and ask if I have any doubts. I thought I discovered something new based on my high school physics and some little about astronomy. You know I was on pins and needles after this idea came into my mind :)

John Rennie

It's always good fun thinking about things like this. I used to do it all the time as an enthusiastic teenage physicist. 99.9% of the time you won't find anything new, but it's fun anyway :-)

Intellex

@JohnRennie Wow!! Nothing has changed even though many years have passed! Yes it's fun.

@JohnRennie Did you invent anything new sir?

John Rennie

@Intellex yes, I have a few patents from my time at Unilever. Nthing that has made the world a better place though :-)

Intellex

@JohnRennie Great sir. Are you aiming for the Nobel Prize, sir?

John Rennie

11:00 AM

Antiperspirant material and compositions containing it

Underarm composition

And there are several others that I can't easily find. But as a general rule they don't give out Nobel prizes for inventing new anti-perspirants.

user8718165

@JohnRennie sir can I ask you a question?

John Rennie

@user8718165 yes

Intellex

@JohnRennie, Thanks for sharing sir. But I think currently I won't understand anything.

Bye @JohnRennie sir. Thanks a lot for your help.

John Rennie

Bye :-)

user8718165

@JohnRennie sir, do you have an FRS degree because you made these things?

John Rennie

11:03 AM

FRS degree?

In the UK FRS generally means Fellow of the Royal Society, and only a few of the top scientists get that.

user8718165

@JohnRennie I'm not sure if that's a degree or not. I meant are you an FRS sir?

@JohnRennie you also made these things which is also very difficult

John Rennie

@user8718165 no, I'm not a Fellow of the Royal Society.

user8718165

@JohnRennie no worries sir...that's like catching a star. do you know Martyn Poliakoff

John Rennie

After my PhD I became an industrial scientist working for Unilever. As a general rule you only get to be an FRS if you stay in university and become a professor.

@user8718165 no

user8718165

@JohnRennie sir why didn't you try to be one at Cambridge? You know a lot of physics

John Rennie

11:08 AM

Life is hard for scientists trying to stay at university. Only a very small percentage end up with a senior academic post. I decided to go into industry and make lots of money instead :-)

user8718165

@JohnRennie sir...uncertain right?

John Rennie

@user8718165 very much so.

user8718165

@JohnRennie you did the correct thing sir...well you are no less than a physics teacher for kids here XDXD

John Rennie

Well going into industry meant I made enough money that I could give up full time work when I was 50. That's why I have the time to answer questions here.

user8718165

@JohnRennie did you plan all these things beforehand...like answering qn's here and working part time?

John Rennie

11:13 AM

No. Making plans is fun but basically pointless. You can never tell how things are going to work out. As it happens two of the companies I have worked for no longer exist, so if I had tried to stay with them I'd have lost my job.

As it is I had left for a new job before they shut down.

user8718165

@JohnRennie yeah sir...got it...that's a very practical and reasonable choice...sir 1 last qn

John Rennie

Yes?

user8718165

@JohnRennie did you ever go to tuition classes to qualify the Cambridge exam or you studied on your own sir?

John Rennie

I studied on my own.

It's been a long time since I sat the exam, but I don't remember it being as hard as the JEE questions.

user8718165

@JohnRennie God Level...actually I'm trying to do that with chemistry

John Rennie

11:19 AM

With the JEE I think it's not really about learning physics or chemistry or whatever. It's about learning how to answer JEE questions fast and accurately.

user8718165

@JohnRennie I thought they were much much harder than the JEE :-(

@JohnRennie agreed sir...

John Rennie

I suspect the toppers are just people who have really good memories and manage to memorise enough previous JEE questions that they can recognise the questions in their years exam.

2

user8718165

@JohnRennie yeah sir...even the teachers suggest doing these...and sir...they really help for the exams.

@JohnRennie you have an answer to almost every question we ask you sir... I don't think teachers here can do that...what's the secret behind sir?

John Rennie

@user8718165 I'm good at Googling :-)

user8718165

@JohnRennie no sir...you tell stuff from your memory...I'm pretty sure...I also google questions before asking you. I hardly find what I need there :-(

John Rennie

11:27 AM

I'm very good at Googling :-)

user8718165

@JohnRennie LOL!

@JohnRennie sir well...'twas a nice chat...BTW will you return in the evening sir?

John Rennie

@user8718165 properly not. I'll be bck tomorrow.

user8718165

@JohnRennie okay sir...no worries....cya ;-)

Transcript for

Problem Solving Strategies