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23 April 2018

Do I need to know about pressure/displacement nodes in AP Physics 1?

I was asked about standing waves in pipes on the AP Physics 1 exam.  Specifically, is it important for students to understand the difference between a variation in air pressure, and a variation in the amplitude of particle displacement?

On one hand, it is certainly important to understand nodes and antinodes in closed and open pipes.  When the boundary conditions are the same (closed at both ends, open at both ends) the fundamental frequency is v/2L and all multiples of the fundamental frequency can be played; when the boundary conditions are different (open at one end and closed at the other), the fundamental frequency is v/4L with only odd multiples available.  

Then, we have to understand the WHY behind these facts.  Students must be able to draw pictures of standing waves, must be able to identify the wavelength as it relates to a picture of standing waves and as it relates to a pipe length, understand why the speed of waves is constant, how the wave speed relates to the speed of particles in the material, how the particles actually move in a transverse wave and how that relates to the pictures and to the wave's amplitude.  (Whew.)

But, when drawing standing waves, are we drawing a representation of the air pressure or the particle displacement?  And, does it matter?

Any time you're wondering about what will be tested on an AP exam, be as a biblical fundamentalist: ignore peoples' pronouncements and go straight to the source text.

I've just been through four released AP Physics 1 exams.  Not a single question addressed the difference between air pressure and particle displacement.  Now, that doesn't mean a question next year or the year after couldn't do so, because the AP Physics 1 exam is only a few years old.  

Nevertheless, I'm taking a Bayesian approach.  The difference between particle displacement- and air pressure- representation is extraordinarily abstract, and difficult to understand for a student who hasn't studied fluids, anyway.  The mathematics and representations of standing waves work fine even if students don't know what exactly they're representing.  Therefore, I don't address this issue of pressure vs. particle displacement.  

If that means my students have to guess on one multiple choice question every half-decade, that's a price I'm definitely willing to pay for simplifying their understanding of standing waves in a pipe.

1 comment:

  1. For the record, from the AP Physics 1 Course and Exam Description:

    b) b. For standing sound waves, pressure nodes correspond to displacement antinodes, and vice versa. For example, the open end of a tube is a pressure node because the pressure equalizes with the surrounding air pressure and therefore does not oscillate. The closed end of a tube is a displacement node because the air adjacent to the closed end is blocked from oscillating.

    I don't really think they'll ask about the *names* "pressure node" and "displacement node." What I do think could happen is that they could ask some conceptual question about the air being "free to move/oscillate" at certain points, and they could certainly have Student A draw a pressure standing wave representation and describe it as a displacement one, and make the student pick apart what they got right and what they got wrong.

    What's tricky about this (beyond the concept, anyway) is that there are a LOT of textbooks that only present one or the other--the pressure wave or the displacement wave, but not both. And whichever one they include, they often just call it a "standing wave" and leave it at that. So the inconsistency among official sources may (or may not, for all I know) make this particular topic not the fairest to ask about; any question on it will favor students who had one brand of text or e-text instead of another.