Crappy science instruction

This is what a pitting by a pedantic, overeducated mechanical engineer looks like.

My wife found a YouTube video last night that purports to demostrate the cooling associated with adiabatic expansion of gases. The problem is that it doesn’t actually demonstrate what the instructor in the video claims it demonstrates.

Video is here. I’m guessing it was made perhaps 20-30 years ago, because the instructor - a middle-aged Paul G. Hewitt - is now about 82 years old. That means a lot of people have seen it by now, and have probably shared these misconceptions with their friends.

:smack: :smack::smack::smack:

What’s the problem?

First, he demonstrates that pursing your lips and blowing forcefully on your hand feels cooler than an open-mouthed exhale. Then he claims that this is because pursing your lips elevates the pressure in your mouth/lungs, so that when this air leaves your mouth, it expands to atmospheric pressure and cools. While this is technically true, the pressure rise is tiny - a fraction of a PSI if you’re blowing lightly, perhaps as much as 1.5 psi if you’re really pushing as hard as you can - and so the temperature drop when you exhale is correspondingly small, just a few degrees. The cooling effect you feel when blowing on your hand is almost entirely due to your breath mixing with cooler ambient air, and the increased convective cooling due to the higher air velocity.

Next he turns to a water-filled pressure cooker operating at full temperature, as evidenced by the jiggling relief valve - probably 15 psi, and a temperature of 250 degrees F. He removes the weight from the valve, and the pressure cooker produces a steam jet. He claims that adiabatic expansion of the 250-degree steam as it exits the pressure cooker results in a temperature drop. Then he demonstrates this by holding his bare hand in the steam jet - about 12 inches above the pressure cooker. He refuses to hold his hand lower for fear of burning it, because yes, that steam jet is damn hot right when it comes out of the pressure cooker. In fact, it’s at exactly 212 degrees. Yes, it’s undergone adiabatic expansion/cooling at that point, but most of the temperature drop (from 212 to whatever temp his hand can tolerate) happens after that expansion, and is because of mixing of the 212-degree steam with cool ambient air.

If you don’t have a background in the physical sciences, maybe this all seems trivial to you. But it bugs the shit out of me to think that people - a lot of people - are being miseducated about the phenomena seen in the video. We all hold our heads in frustration at the state of science education in this country, and this guy is part of the problem. No doubt people believe him because he’s in a position of authority (by golly, he’s a physics instructor!). Goddamit, my wife watched the video, and now I’m struggling to convince her that he’s wrong. If I stick to my guns, it’s a fair bet that she’ll get pissed off at me and accuse me of thinking she’s stupid. If it comes to that, well then I’m blaming Hewitt.

I agree. Those are largely incorrect examples.

Showing how a small window air conditioner works would be the best example, I would think.

I agree, you are a pedantic, overeducated mechanical engineer.

But I assume you are also right. So it’s a pretty stupid video. A lot of science demonstrations are like that. They try to show simple examples based on the idea that it will get students interested. Instead you get a student who thinks, “Cool, steam”, and never learns anything. Then in this case you get those who are actually interested, and they learn the wrong thing.

So are you saying he’s full of hot air? Maybe if he’d become a blowhard then he’d be cooler? :dubious:

Hey, man, everybody’s entitled to their own opinion.

: d&r :

My dad’s favorite joke:

Q: What’s the difference between cowboy boots and engineer boots?

A: With cowboy boots, the shit’s on the outside.

[sub]He was an engineer, too.[/sub]

I was just telling my friend, nothing makes me fly into a rage like an improper demonstration of adiabatic expansion.

Jeez. Congenital engineering. Maybe they’ll find a cure someday.

One day at the local science museum they were doing a laser demonstration where they had a series of powered mirrors spin and reflect a laser beam onto a wall in a nice neat pattern. One of the kids attending the demo asked how the beam could be in every place at once and the person doing the demo(who I can only assume was a student at a local school or university) said the laser made the wall glow because it was so high energy.

Another time a professor was doing a laser and water show, demonstrating how light can be refracted through a stream of water pouring from one bucket(on a podium) into another bucket) down below. Then he turned off the laser and still the upper bucket glowed and light continued “pouring” down into the lower bucket. He ended the demo there, as if that were normal for lasers. Turns out he had a little waterproof LED rig in the upper bucket that he turned on and left on after he turned the laser off.

Just plain wrong in the first example, and misleading in the name of showmanship in the second. I’d rather have an instructor who can take the real wonders of the world around us and make them interesting instead of someone who makes it up or fakes it.


Wanna really get pissed off at bad science? Here’s’s attempt to explain how to stop a soda can from fizzing over. How friggin’ hard is it to see that the biggest variable in effect here is most likely the time that passes between opening the first and the second can?!

I’ve been there so many times I purchased a timeshare. It’s really cheaper than renting.

It’s times like this I really miss Carl Sagan. There was an episode of Cosmos back in the day which dealt with Kepler and how he devised his laws of planetary motion. Sagan kept talking about how Kepler was a brilliant man and how important his discoveries and theories were, but he showed Kepler’s completely wrong-headed attempt to prove the planets of the solar system fit into a series of nested perfect polygons. At no time did he denigrate Kepler, and he went out of his way to praise him by saying that Kepler never rejected solid evidence that disproved his model, he just kept trying to refine his model to fit the evidence. Kepler was completely wrong, as we now know, but Kepler was an honest enough man to admit the holes in his theories and attempt to refine them instead of just using his intuition and saying his theory was correct irrespective of the evidence. Kepler went to his grave thinking the motion of the planets of the solar system was ruled by ratios of polygons, but never being able to prove it. Sagan continued to praise Kepler as one of the first true scientists, who put evidence above theory, and how important Kepler was in the arc of the development of the scientific method.

We know that humans love heuristics and a simple, incorrect, explanation is more likely to stick in the collective consciousness than a complex, correct, explanation, but wouldn’t it be nice if everyone accepted this flaw in themselves and stuck with the evidence despite what it did to their own pet theory?


Google “How a rocket works”. You’ll likely get a pile of links that say it’s because of Newton’s Third Law. Even NASA sites claim this.

In reality Newton’s Third Law has little to do with how a rocket works. In fact, there are ion thrusters whose exhaust doesn’t even obey Newton’s Third Law and they work just fine.

The operation of a rocket is explained by Newton’s Second Law generalized to open systems.

I thought this thread was going to be about the horseshit that gets passed off as science by creationists…at least the OP’s example was well-intentioned crappy science instruction.

You, too? Man, if I had a nickel …


At least the kids who saw this demonstration might now have a general idea of what adiabatic expansion is. They might have picked up a few misconceptions about the particulars of it too, but if they do go on to studying the relevant physics, those misconceptions can be cleared up at the appropriate time. At least the video gave them something they can build on.

Education, like science itself, is a process of getting a closer and closer approximation to the Truth, or the Full Story, and, if you try to jump to the full story straight away, well, it turns out you just can’t do it. If Hewitt had explained all the wrinkles mentioned in the OP, the kids would be confused and turned off, and nobody would have learned anything. Likewise, there would have been no Einstein without Newton, and no Newton without Aristotle. Although Newton and Aristotle were both “wrong” in important ways (like almost every other scientist before last week), we could not have had modern science with having had their work for later investigators to both build upon and react against.

The real question is, can you Machine Elf, come up with a better way of explaining adiabatic expansion to kids, that will catch their imaginations at least as well (that bit is important), but will remain as strictly accurate as you would wish? If you can, fine. It will, I suppose, be marginally better in educational terms. If you can’t then you need to shut up and let educators get on with effective educating, instead of trying to hobble them with pettifogging pedantry.

We really should just form a club.

Paul G Hewitt isn’t normally that bad, I promise.

In fact, his video lectures are normally pretty good.

I will freely give it to you that he majorly flubbed up explaining those principles though.

If it makes you feel any better, I never showed the video on adiabatic expansion to *my *students.

(former high school physics teacher who showed an occasional Hewitt video)

Absolutely everything in the Universe, from subatomic particles to supermassive black holes, obeys Newton’s Third Law, with no exceptions. And it is in fact one of a number of principles that is quite relevant to how a rocket works.

Can you say more?