From @JohnDenker: In the book, figures 31.7 and 31.11 project a strong image
of an incompressible fluid ... which is not the right physics.
The text on page 814 explicitly exacerbates the misconception
when it says «because the tube is filled».
The text on page 816 avoids this particular misconception.
It's a step in the right direction, but it's not big enough
to make up for multiple steps in the wrong direction. I
guarantee that many students will end up with the wrong
idea ... especially given that they may have been exposed
to it previously, e.g.
http://faculty.wwu.edu/vawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html
The text on page 816 has other problems; it says «Because
the charge inside the system is not changing» -- but that
is just proof by tacit assumption. The fundamental laws
of physics /permit/ charge to accumulate, and indeed the
model of "steady" circuit that we are considering demands
that batteries accumulate unlimited amounts of charge.
So, if you want to argue that such accumulation elsewhere
is negligible you have to say something about timescales
and self-capacitances and such.
Possibly constructive suggestion: There is a video showing
how you can have steady, conservative flow of a fluid with
not constant density. Situations like this are observed
in the real world all the time. The movie and a few hundred
words of explanation can be found at
https://www.av8n.com/physics/conservation-continuity.htm#sec-speedup
There have been scattered reports of trouble with the inline
video player. If necessary, you can see a backup copy of
the video here:
https://www.youtube.com/watch?v=OvVfThgQXY8
From @JohnDenker: In the book, figures 31.7 and 31.11 project a strong image of an incompressible fluid ... which is not the right physics. The text on page 814 explicitly exacerbates the misconception when it says «because the tube is filled».
The text on page 816 avoids this particular misconception. It's a step in the right direction, but it's not big enough to make up for multiple steps in the wrong direction. I guarantee that many students will end up with the wrong idea ... especially given that they may have been exposed to it previously, e.g. http://faculty.wwu.edu/vawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html
The text on page 816 has other problems; it says «Because the charge inside the system is not changing» -- but that is just proof by tacit assumption. The fundamental laws of physics /permit/ charge to accumulate, and indeed the model of "steady" circuit that we are considering demands that batteries accumulate unlimited amounts of charge. So, if you want to argue that such accumulation elsewhere is negligible you have to say something about timescales and self-capacitances and such.
Possibly constructive suggestion: There is a video showing how you can have steady, conservative flow of a fluid with not constant density. Situations like this are observed in the real world all the time. The movie and a few hundred words of explanation can be found at https://www.av8n.com/physics/conservation-continuity.htm#sec-speedup
There have been scattered reports of trouble with the inline video player. If necessary, you can see a backup copy of the video here: https://www.youtube.com/watch?v=OvVfThgQXY8