This physics professor is becoming a TikTok star for her cool demonstrations

[Paul_Townsend]

To answer your question, were I to teach basic chemistry I wouldn’t start by stating claims as scientific fact that are demonstrably false. So, no Phlogiston theory, Caloric theory, “Classical elements” (i.e. earth, air, fire, and water) or Dalton’s “Plum pudding” model of the atom.

The claim that the air exiting the hair dryer in the demonstration has lower pressure is more easily and conclusively disproved than any of those archaic theories. Just stick a pressure meter on the output. QED.

Ok, it might be reasonable to mention these superseded models as a historical note. And perhaps starting with the Bohr model of the atom with quantized states would be ok as a simplified description of the atom. I don’t teach chemistry, but I recall that my high school chemistry class started with a very simplified version of quantum mechanics to describe how electrons “orbit” the nucleus. I’d probably start there. Not Schrodinger’s equation, but the concept of fixed, quantized orbitals and shells.

I wouldn’t start by making claims that are simply false.

You teach biology/genomics. How would you react to a video that presented Lamarkian evolution or Lysenkoism as scientific fact? Or that claimed that women have more ribs than men? Would it be ok because it’s just aimed at kids doesn’t need to be rigorously scientifically correct?

[BradC]

I would argue that Lamark and the other models you mention are wrong in a different way. They don’t simplify even a small part of a real concept but rather run in the wrong direction.

In my opinion some topics are more OK than others to oversimplify. For example many people believe a p-value of 0.05 means there is a 1/20 chance that the hypothesis was false. In reality the meaning of a p-value (and the meaning of significance is far more complex). However the incorrect interpretation (and related incorrect assumptions) are close enough to allow some insight. (Inappropriate frequentist statistics could be a whole discussion on it’s own)

(I am very much not a physicist, so I am happy to be corrected and learn)
Is the difference between pressure differentials acelerate fluid and fluid speed reduces pressure within a flow really important for the most simplistic understanding of the bernouli equation itself (which is primarily a conservation of energy situation as I understand it)? Can we build from this to get to the true model? (for e.g. I found bernoulli helped me understand Coanda) Like the way we first teach how airplane wings work, which is very incomplete. How would you (I assume you are an actual physicist) explain the ball in the air flow experiment to a kid?

(edited for a plethora of typos and clarity)

[Paul_Townsend]

Here’s how I would explain it to a kid:

I’d start with another associated principle which was derived at the same time as Bernoulli’s principle in the mid 1700s. It didn’t have a name in English until recently, but it’s now called the “streamline curvature theorem”. It says that any time a fluid follows a path that is curved there are pressure differences perpendicular to the fluid flow with lower pressure on the inside and higher pressure on the outside. Think of a tornado or a hurricane or water circulating around the drain in the bathtub. It’s easy to demonstrate by stirring a bowl of water and observing that the water on the outside of the curve rises due to the increased pressure and the water in the middle drops due to the lower pressure.

Mathematically, this theorem is expressed as a differential equation, but you don’t need to understand calculus to grasp it’s meaning, and depending on the audience not necessary to even mention that it’s a differential equation.

Applying this idea to the balls in a jet stream: the air flows around he ball and follows a path that is curved. There’s lower pressure on the inside of the curve adjacent to the surface of the ball. Further out, the pressure is higher and this higher pressure pushes toward the ball holding it more or less stationary.

Is the Bernoulli principle involved? Yes! As the air flows from normal pressure through the region of low pressure adjacent to the ball it speeds up in accordance with Bernoulli’s equation. But this is not at all apparent from the demonstration. So, while there’s nothing wrong with Bernoulli’s principle, and the air obeys the formula it’s not a good explanation of the behavior.

The usual explanation based on the air having lower pressure because it’s moving faster coming out of the hair dryer is just plain wrong.

More broadly, Bernoulli’s equation addresses pressure differences parallel to the fluid flow. The streamline curvature theorem addresses pressure differences perpendicular to the flow. For any situation where the force is perpendicular to the flow (e.g. this lift on an airplane wing) it’s more intuitive to use the idea of curved flow causing pressure differences than trying to use Bernoulli.

Of course, the cool thing about Physics is that there are multiple ways to explain things or arrive at results. And Bernoulli’s law is certainly part of a comprehensive explanation of anything involving fluid mechanics. But all too often, in their eagerness to cite Bernoulli, false premises are cited, for instance “the air has lower pressure when it exits the hair dryer”.

Another example is the all too common Equal Transit Time Fallacy, which is the way most of us were taught how airplane wings work back in the day. For the better part of the 20th century this piece of very bad physics predominated; it’s much rarer today since it’s been recognized widely to be incorrect. NASA refers to it as the “Incorrect Theory of Flight #1”. There’s a great paper on this called “How do Wings Work” by Holgar Babinsky. Well worth a read.

[Paul_Townsend]

@BradC

Is the difference between pressure differentials acelerate fluid and fluid speed reduces pressure within a flow really important for the most simplistic understanding of the bernouli equation itself (which is primarily a conservation of energy situation as I understand it)?

Yes, I think so. More importantly, the former idea makes Bernoulli much easier to understand.

• When a fluid flows from a region of high pressure to a region of low pressure, there is more pressure behind than in front. More pressure behind than in front pushes the fluid and it speeds up.

That, in essence, is Bernoulli’s principle. Simple and easy to understand by anyone.

So, why isn’t it taught that way? Well, if you want to actually derive the equation from that idea, you’d need to do it the hard way like Leonard Euler did (Historical note: Bernoulli’s equation does not appear anywhere in Bernoulli’s writings, his longtime colleague, Euler, published the formula some years later).

Imagine a small parcel of fluid, say a cubic millimeter or smaller. Assume that the pressure is changing at some rate per unit distance. That gives the pressure difference between the front and back faces of the cube. Multiply that by the area of the face and you get the force. Multiply the volume of the cube by the density of the fluid and you get the mass. With mass and force known, calculate the acceleration according to F=ma. Apply calculus to convert acceleration into velocity. And if you do all this correctly, you’ll get a nice tidy algebraic expression relating pressure, density, and speed. That’s Bernoulli’s formula.

The idea is simple: more pressure behind pushes the air and it speeds up. The mathematical analysis is not so easy. But the advantage of looking at it this way is that it’s clear that the fluid speeds up due to a force pushing on it.

So, what about conservation of energy? Well, the concept of conservation of energy wouldn’t be defined/discovered/invented for several decades, so neither Bernoulli nor Euler would have done it that way. But it’s a great way to derive the formula without doing all that hard mathematical work:

Just write down the kinetic energy per unit volume (½ ρv^2) , add it to the pressure, set that to a constant value, and you get the familiar formula for Bernoulli’s equation.

(eliding over some details here - we usually think of pressure as “force per unit area” but pressure is also energy per unit volume)

It’s that simple. A nice tidy algebraic equation that you can derive in one or two steps by just adding up the total energy using no more than elementary school mathematics. The problem with this approach is that it obscures the physics. Students are left wondering why, exactly, a fluid has to drop in pressure just because it is moving faster.

A further problem is that it’s often stated as “faster moving air causes lower pressure” which, intuitively at least, gets cause and effect backwards. Intuitively, we think of forces causing motion. If something is to accelerate, there must be a force to cause that acceleration. When we explain Bernoulli by “pressure differences push the fluid and it accelerates”, that’s simple and easy to understand. When we start with the speed change it’s not so clear.

Ok, I’ve written enough for now. Thanks for asking.