[Bogie_smiles]

In the simplest terms, I have always understood energy as the ability to do work.

[Eternal Student]

Hi.

In the simplest terms, I have always understood energy as the ability to do work.

   Yes that's excellent.    This is exactly how it's often defined at school level.   It will take you a long way.

 The remainder of this post is complicated and just for general interest.  Don't add it to your answers for school level physics UNLESS it's a long answer question (like an essay) and you're confident you undertsand it.

   By the above definition,  if you had a bottle that contained some really hot stuff then you have some energy in that bottle.   We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.
   The bottle has energy in it and while we are here on planet earth then we can see this easily.   We could open the bottle a little and allow some of that heat to flow from the inside of the bottle to the outside world and while that's happening drive a piston with it.   I won't draw the diagrams but you could just let the hot stuff heat some water in a tank and drive an old fashioned steam engine with the steam from the hot water,  there's lots of options.   The main thing is that it can be done.  We can get useful work from the bottle of hot stuff,  so it's a store of energy as described in the definition you gave.


   Now we're going to travel to another region of the universe,   let's assume we can get to the sun.   It'll be very hot there but this is theoretical physics not engineering.   Now the problem is that the surrounding might be as hot as the stuff you had in the bottle.   So the heat just won't flow out of the bottle.  Heat only flows from a higher temperature to a lower temperature.   Thermodynamics states that you can't get any useful work out of the stuff in that bottle while you are here in this hot place.   In fact you'd be a lot better of with a bottle of cold stuff in this place.   If you had a bottle of cold stuff then you can get heat to flow from the surroundings into the bottle and extract useful work while that is happening.    You see, in the sun,  that bottle of hot stuff is useless,  it offers you no ability to do useful work.    If ytou were a being that lived in the sun then what you want, what you would naturally think of as a valuable commodity is some cold stuff.   In the sun, a bottle of cold stuff gives you the ability to do useful work,  so by the above definition, the colder something is the better:   It has more energy since it has the ability to allow you to perform more useful work while heat is flowing into that cold stuff from your surroundings.   You see how backwards this is? 

   Anyway, if you've understood this, you'll see that "the ability to do work" isn't a property that an energy source can have on its own.   It's actually a property of BOTH the fuel substance  AND also the environment it is in.   So the simple definition of energy as "the ability to do work" falls down slightly.    You have to ask does something stop being a form of energy when you move it to a new envronment?   
   We have a load of physics based on the idea that a hot thing has internal kinetic energy.... there really should be some "energy" in there.   However there is no ability to extract useful work from this energy source at all when its in some environments.

Best Wishes.

[alancalverd]

And "work" is defined as what? Destroying the British economy with the stroke of a pen, or force x distance? If you are going to use the term at school level, and particularly for a 5 year old, I think you need to get the pupil to do some mechanical work to put the term into context.

[Eternal Student]

Hi.
     

If you are going to use the term at school level, and particularly for a 5 year old, I think you need to get the pupil to do some mechanical work to put the term into context.

      That does sound a bit like slave labour.    We're going to see how soap reduces the friction of a scrubbing brush on this floor... etc.

Best Wishes.

[alancalverd]

Of course, mea culpa. You must first carry out a risk assessment then
obtain parental permission to ask the child to kneel
provide appropriate kneeling pads
instruct the child on the correct use thereof
check the child's health record for allergies and intolerances
provide hypoallergenic gloves
review the child's psychiatric profile for claustrophobia
provide goggles and a face mask
provide alternative and equivalent learning ("teaching" implies liability, so we don't do it) facilities for any
    claustrophobic children who cannot wear masks and goggles
obtain oxygen, ventolin and sugar in case of asthma or diabetic coma induced by exercise
install an ECG and infant-size defibrillator
assess the environmental hazard of disposal of  contaminated water
obtain an Environment Agency dispensation
obtain culturally appropriate brushes and buckets, including at least 10% lefthanded brushes
sensitively identify the non-swimmers to your first-aider (kids can drown in an inch of water)
review the practical housekeeping training records and CPD of the supervising teachers
provide counselling for the kids who come last (every activity turns into a competition - that's childhood)

Clearly it is not practicable to teach physics nowadays, so we have to resort to religion. Say "Energy is the ability to do useful work. Remember these words and repeat them to the examiner or you will rot in Hell for eternity."
"Please, sir, my dad is a priest/politican/philosopher. Does that mean he has no energy?"



 

[paul cotter]

Wow!, Alancalverd, that's some outburst. You are becoming more cynical than me, if that is possible. Seriously, you have some good points here: life is inherently hazardous and efforts to eliminate these hazards will cripple true education, leading to stunted personal development. I hold personal responsibility to be of utmost importance-if I come to grief through my own actions I don't seek someone to blame. The "blame" culture is now so strong that practical science in school has been dumbed down to the point of futility. All off-topic, I apologise. 

[alancalverd]

My dad used to say that satire should be like a dermatome - cutting just below the surface to expose the rottenness beneath. But now and again it's fun to use a machine gun!

[Bored chemist]

Hi.

In the simplest terms, I have always understood energy as the ability to do work.

   Yes that's excellent.    This is exactly how it's often defined at school level.   It will take you a long way.

 The remainder of this post is complicated and just for general interest.  Don't add it to your answers for school level physics UNLESS it's a long answer question (like an essay) and you're confident you undertsand it.

   By the above definition,  if you had a bottle that contained some really hot stuff then you have some energy in that bottle.   We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.
   The bottle has energy in it and while we are here on planet earth then we can see this easily.   We could open the bottle a little and allow some of that heat to flow from the inside of the bottle to the outside world and while that's happening drive a piston with it.   I won't draw the diagrams but you could just let the hot stuff heat some water in a tank and drive an old fashioned steam engine with the steam from the hot water,  there's lots of options.   The main thing is that it can be done.  We can get useful work from the bottle of hot stuff,  so it's a store of energy as described in the definition you gave.


   Now we're going to travel to another region of the universe,   let's assume we can get to the sun.   It'll be very hot there but this is theoretical physics not engineering.   Now the problem is that the surrounding might be as hot as the stuff you had in the bottle.   So the heat just won't flow out of the bottle.  Heat only flows from a higher temperature to a lower temperature.   Thermodynamics states that you can't get any useful work out of the stuff in that bottle while you are here in this hot place.   In fact you'd be a lot better of with a bottle of cold stuff in this place.   If you had a bottle of cold stuff then you can get heat to flow from the surroundings into the bottle and extract useful work while that is happening.    You see, in the sun,  that bottle of hot stuff is useless,  it offers you no ability to do useful work.    If ytou were a being that lived in the sun then what you want, what you would naturally think of as a valuable commodity is some cold stuff.   In the sun, a bottle of cold stuff gives you the ability to do useful work,  so by the above definition, the colder something is the better:   It has more energy since it has the ability to allow you to perform more useful work while heat is flowing into that cold stuff from your surroundings.   You see how backwards this is? 

   Anyway, if you've understood this, you'll see that "the ability to do work" isn't a property that an energy source can have on its own.   It's actually a property of BOTH the fuel substance  AND also the environment it is in.   So the simple definition of energy as "the ability to do work" falls down slightly.    You have to ask does something stop being a form of energy when you move it to a new envronment?   
   We have a load of physics based on the idea that a hot thing has internal kinetic energy.... there really should be some "energy" in there.   However there is no ability to extract useful work from this energy source at all when its in some environments.

Best Wishes.

That's just an engineering problem. If you happen to be stuck in the middle of the sun where the "hot" stuff in the bottle is not any warmer than your surroundings, you simply have to get a well lagged metal bar that reaches far outside the sun then put a radiator on the end of it and let the heat flow along that.

efforts to eliminate these hazards

Is anyone trying to do that?
or are they just trying to reduce unnecessary hazards?

[paul cotter]

I do believe there is a move to eliminate all hazards, however impossible this is, in many areas. Even if one stays in bed to avoid external hazards a meteorite might come crashing through the roof. School practical chemistry has been particularly hit. I would not allow students to use, for instance white phosphorus or thallium salts but reducing practical chemistry to mixing vinegar with bread-soda is pathetic and I have heard of this( in aus ). By all means obvious high risk activities should be avoided. Our chemistry teacher did the sodium and potassium in water demo and wanted to obtain rubidium and caesium to show the trend of increased reactivity: he wasn't able to obtain these metals and I doubt if he realised the expense involved. Increased litigation has caused society in general to become increasingly risk averse. Or else i'm just too old!

[Bored chemist]

The big problem is "ambulance chasing lawyers".

[paul cotter]

Absolutely, BC.

[Eternal Student]

Hi.

That's just an engineering problem. If you happen to be stuck in the middle of the sun where the "hot" stuff in the bottle is not any warmer than your surroundings, you simply have to get a well lagged metal bar that reaches far outside the sun then put a radiator on the end of it and let the heat flow along that.

    I like it.   I mean it's cheating but I do like it.   It took me a moment to think about, so thanks for that.  It's better than watching TV for an evening.

    Just for the sake of theory, we can dismiss it:    What you've said was only possible because you could get to a place that was cooler.   We have to assume that you cannot do that.     Also we could argue that you have only generated useful work that can be harnessed at the far end (where the radiator was),  although engineers are deceitful and they could argue that they will build a piston with a long shaft.  Long enough that it will bring the useful work back to the point in the star where the bottle of hot stuff was.   As people have mentioned enough times, you can't trust an engineer they're a devious bunch.

   Anyway, the idea of not being able to find a cooler place isn't just an unrealistic or "in theory only" condition.   The entropy of the universe is increasing.   Eventually there will be a uniform temperature everywhere.   Under those conditions, your conduction bar or heat pipe doesn't work.   Heat won't get out of the bottle into the bar without a small temperature gradient, it won't travel along the bar without a slight temperature gradient and it won't get out at the radiator end either.   If the whole universe is at, let's say 10 deg C, then a bottle of stuff at 10 deg C offers you no ability to do any useful work.   You can refrigerate or heat up some region of the universe BUT that will take at least as much useful work as that which you can extract later by letting heat flow in or out of the bottle.   That's it,  end of the line on an entire universe-wide scale, no useful work can be extracted from that bottle of stuff (well just from its natural temperature anyway - you could still try to make it hotter by annihilating some particles within it or something like that but this is beside the point - the thermal energy of the stuff cannot be used to generate useful work and we only need to find one form of energy that fails to be identified by the definition  "Energy is the ability to do useful work").    So, with only one more essay worth of writing space,  we could re-assert the earlier statement:   The ability to do useful work isn't just a property of the substance you are considering as an energy source.   It is also a property of the environment it is in.

    Obviously none of the above is something I would recommend writing in someone's school work unless they do have a whole essay or two to fit this in.

Best Wishes and thanks again for making me pause to consider another situation.

[alancalverd]

Gunpowder, or a watch spring, can do useful work in any environment, surely?

Having the requisite number of limbs and eyes, I have the ability to play cricket. But I'd be more of a liability than an asset at County level. I think we need to distinguish between capacity and realisability.

[Bored chemist]

Gunpowder, or a watch spring, can do useful work in any environment, surely?

Seriously?
You think they would work in the middle of the sun?

 We have to assume that you cannot do that.

No, we don't.
We just need the universe to cool down until it is cooler.

[Bored chemist]

Also we could argue that you have only generated useful work that can be harnessed at the far end (where the radiator was), 

No
I plan to install a perfectly lagged, very good conductor out to the radiator so the end of it that is in the sun is cold.
Then I can run a steam engine using the temperature difference between the hot thing thing and the cold sink.

[Bored chemist]

Eventually there will be a uniform temperature everywhere. 

Not quite everywhere
Remember, we were provided in the first instance with some "hot stuff" in a perfectly insulated container.

if you had a bottle that contained some really hot stuff then you have some energy in that bottle.   We're going to make that bottle a perfect Dewar flask so that we can carry it around and no heat escapes.

I think  "imparts" is better than "carries" since mp = 0

I think carries is the right word because the momentum starts from one place and ends up in another place.

[alancalverd]

Quote from: alancalverd on Today at 09:30:57
Gunpowder, or a watch spring, can do useful work in any environment, surely?
Seriously?
You think they would work in the middle of the sun?

OK, delete "useful". Fact is that neither depends on a cold sink. We have discussed elsewhere what happens when you dissolcve a compressed spring in acid, and the same applies here. The gunpowder, or the watch spring, would locally increase the temperature of the sun.

[Eternal Student]

Hi.

Not quite everywhere
Remember, we were provided in the first instance with some "hot stuff" in a perfectly insulated container.

   No, we're not having any of that.   Post # 31 said...

If the whole universe is at, let's say 10 deg C, then a bottle of stuff at 10 deg C offers you no ability to do any useful work.

   The hot stuff was deliberately replaced with stuff at 10 deg C.   That is still "hot" in comparison to absolute zero (0 k) specifically it should have thermal energy due to its temperature.   We need only find ONE example where the definition of energy as the ability to do work fails.   This is the example we are choosing.    A bottle of stuff at 10 deg C  offers no ability to do work when the whole universe is at 10 deg C.

Best Wishes.

[Bored chemist]

If the whole universe is at, let's say 10 deg C

We just need the universe to cool down until it is cooler.

[Eternal Student]

Hi.

We just need the universe to cool down until it is cooler.

   It won't cool down.   It's all at 10 deg C.  It is not in contact with anything cooler.
You'd need an expanding universe to stand any chance of the temperature continuing to fall (things like molecules slow down in an expanding universe and settle to a static co-moving co-ordinate position).
   Before you say, "we'll allow an expanding universe because it is", if you allowed yourself an expanding universe then there are other examples we can find where something doesn't give the right ability to do work.   
    In an expanding universe there isn't the time symmetry we want for Noether's theorem,  so it's all a bit of a moot point worrying about what "Energy" is.   It isn't a conserved quantity and we can find examples where something gave us the ability to do a lot of useful work at t=early time but almost no no ability to do useful work at t = later time.  Changing to a different time is just changing the "environment" (in the wider sense).

Best Wishes.