[alancalverd]

This is the second, and longer, "solution" video to my "new brainteaser" video about compressing air, losing ALL the energy used to compress it, and still having compressed air that can be used to run my pneumatic tools.

Written by someone who has no concept of physics.

Anyone who had actually built a perpetual motion machine wouldn't waste his time making "educational" videos about it.

[hamdani yusuf (OP)]

Anyone who had actually built a perpetual motion machine wouldn't waste his time making "educational" videos about it.

What perpetual motion machine are you referring to?

[Bored chemist]

Anyone who had actually built a perpetual motion machine wouldn't waste his time making "educational" videos about it.

What perpetual motion machine are you referring to?

The hypothetical one.
Did you not recognise that?

[hamdani yusuf (OP)]

Anyone who had actually built a perpetual motion machine wouldn't waste his time making "educational" videos about it.

What perpetual motion machine are you referring to?

The hypothetical one.
Did you not recognise that?

No. Can you help me out?

[Bored chemist]

"would
/wʊd/
verb
1.
past of will1, in various senses.
"he said he would be away for a couple of days"
2.
(expressing the conditional mood) indicating the consequence of an imagined event or situation.
"he would lose his job if he were identified" "


Alan didn't say "the person who did make  a PM machine  didn't  waste his time making "educational" videos about it." as he would have done if the event was actually real.

[alancalverd]

What perpetual motion machine are you referring to?

The supposed system that allows you to compress air, remove all the energy you used to compress it, and still have compressed air. You could use the removed energy to heat something, and use the remaining compressed air to compress more air, ad infinitum.

[hamdani yusuf (OP)]

What perpetual motion machine are you referring to?

The supposed system that allows you to compress air, remove all the energy you used to compress it, and still have compressed air. You could use the removed energy to heat something, and use the remaining compressed air to compress more air, ad infinitum.

I think I know where he got it wrong. When the compressed air has cooled down to ambient temperature, there's still potential energy left in it in the form of higher pressure than initial condition.

[alancalverd]

What a surprise!

 

losing ALL the energy used to compress it, and still having compressed air that can be used to run my pneumatic tools.

What an idiot!

[hamdani yusuf (OP)]

What a surprise!

 

losing ALL the energy used to compress it, and still having compressed air that can be used to run my pneumatic tools.

What an idiot!

Everyone makes mistakes sometimes.
He came to prominence as physicist after the Blackbird project, and popularized by Veritasium later on.

CNN Early Start - Blackbird upwind cart

CNN aired this segment on our wind powered cart.  Nice segment, but it seems they forgot to mention that the vehicle was designed to go directly upwind (at about 2X wind speed) and directly downwind (at about 3X wind speed).

Wind powered direct upwind vehicle - DUWFTTW

This is a wind powered vehicle that goes directly into the wind at more than 2X wind speed.  This video was taken on 16 June 2012 at the New Jerusalem airstrip in Northern California.  The vehicle was originally designed and built to go directly downwind faster than the wind, and set a world record by doing so on 2 July 2010, when it went 2.8X wind-speed directly downwind.  The data from one of the runs on this video will be submitted to the North American Land Sailing Association for consideration for a direct upwind record.  The vehicle is called the "Blackbird".

[Bored chemist]

Would anyone care to answer this?

Here's another, similar brain teaser.
I come home and I think my room (roughly 3m by 3m by 2.5 m ; I'm ignoring the volume of the fireplace) is too cold; it's only 10 C in here (283K) , so I switch on the heating- a 1 KW electric fire.
After 1000 seconds it is warm enough and I switch the heater off.

How much more energy is there in the air in the room?

(Making the usual (unrealistic) assumption that there's no energy loss to the walls and furniture or any gain from me being in there metabolising food.
We are  also assuming air is an ideal gas)

[alancalverd]

Nothing "defies physics" because physics is the mathematical description of what actually happens.

Lots of things defy arrogant folk who call themselves physicists.

The answer to BC's question is obviously 1 megajoule. Knowing the density (about 1.23 kg/m³ )and specific heat capacity (about 0.72 kJ/kg.K) of air, you can calculate the final temperature.

[Bored chemist]

It's "obvious" that it's 1MJ, but it's wrong.
The correct answer is zero.
Now work out why.

[alancalverd]

Either
(a) because your electric fire was powered by a battery that was "in the air"  or
(b) it was powered from an external source (the mains) and the laws of  physics do not apply in your part of the world or
(c) when you say "it is warm enough", you actually mean "I am warm enough" because you just heated yourself with a radiant rather than a convective heater. But you will eventually equilibrate by convection.

[Bored chemist]

Either
(a) because your electric fire was powered by a battery that was "in the air"  or
(b) it was powered from an external source (the mains) and the laws of  physics do not apply in your part of the world or
(c) when you say "it is warm enough", you actually mean "I am warm enough" because you just heated yourself with a radiant rather than a convective heater. But you will eventually equilibrate by convection.

Nope.
It's simpler than that and it's related to the observation that was made in the video about energy and compressed air.
It's also related to why this is certainty not correct.

Knowing the density (about 1.23 kg/m3 )

[paul cotter]

If the room is not sealed we are dealing with a constant pressure process and the air will expand leading to a loss to the external environment. Whether it all adds to zero would involve too much calculation for me.

[hamdani yusuf (OP)]

The heater has converted 1MJ of electrical energy into heat. It must go somewhere in the room. If it's not absorbed by the air, it must be absorbed by some other objects.

[alancalverd]

the observation that was made in the video about energy and compressed air.

See my reply #865 above.

Having just landed from an "interesting" flight, I can assure you that on Planet East Anglia the air density is indeed around 1.23 kg/m³, which is why the wings and engine worked exactly as calculated.

And I can't believe that a sensible scientist like BC would not have draught-sealed doors and windows.

[Bored chemist]

If the room is not sealed we are dealing with a constant pressure process and the air will expand leading to a loss to the external environment. Whether it all adds to zero would involve too much calculation for me.

The calculation is that PV is a constant.

And I can't believe that a sensible scientist like BC would not have draught-sealed doors and windows.

I'm ignoring the volume of the fireplace

Buildings leak; they have to or atmospheric pressure variations would make them unworkable.

It must go somewhere in the room.

No. It leaves the room and raises the atmosphere by a tiny amount, doing work against gravity.

The air expands to an extent which is proportional to the change in absolute temperature. So the amount of air left falls by exactly the same ratio.
Each molecule has an energy that is proportional to the absolute temperature so the effects cancel out and the energy remains the same.

[Bored chemist]

the air density is indeed around 1.23 kg/m³

Whatever the  air density was at the start, it was different at the end.

[alancalverd]

Only if you let some of it escape. And a sensible chap with electric heating will have blocked up the fireplace - no point in blowing expensive electricity up the chimney.

Now if all the heat had been absorbed by the air, you would have raised your 27.7 kg of air by about 50K  - dangerously unbreathable, whether or not you allowed some to escape.