From @JohnDenker: You may consider this to be somewhat nitpicky, but it
is correct ... and the bug is easy to fix ... and it
can be parlayed into a lesson about how science is done.
On page 597, it says "There are two and only two types of charge."
Alas, that is not the correct physics. As far as anybody
knows, there is only one kind of electrical charge.
Specifically, there is only one variable (Q) that you
need in order to keep track of the charge. The value
of this variable might be positive or negative, but it's
still only one variable.
The behavior of electrical charge stands in
contrast to quantum chromodynamics, where there
are three different kinds of color charge (in
addition to one kind of plain old electrical
charge). Three variables are needed to keep track
of color charge.
It seems to be an article of faith in the PER community
that there are two kinds of charge, but that is only
because they have failed in the fundamental duty to
consider all the plausible hypotheses. The experiments
with plastic tape are /consistent/ with the two-fluid
model ... but they are also entirely consistent with
the one-fluid model.
You will recall that I made a big fuss about
hypotheses (plural) instead of hypothesis (singular)
in connection with page 2 of the book.
There is no experiment you can do that favors either
model over the other. You could rewrite all the laws
of physics to include two charge-like variables, if
you're enough of a masochist, but I don't recommend it.
The one-fluid model wins on the basis of simplicity
and convenience. This is not a new idea; Ben Franklin
is credited with coming up with the idea, and coining
the words "positive" and "negative".
In chemistry and biochemistry, there are hundreds of
different charge carriers, such as sodium ions, calcium
ions, chloride ions, et cetera. Some might claim that
physics says there are fundamentally only two charge
carriers, namely electrons and protons, but that's not
right either; physics knows about muons and quarks
and all sorts of other "fundamental" charge-carrying
particles.
I mention this because if you want to keep track of
the /particles/ you might need two or more variables,
but still you need only one variable to keep track
of the charge itself.
Charge is an abstraction, not to be confused with the
details of this-or-that charge-carrying particle.
When balancing a chemical reaction equation, you
need to balance all of the chemical species and
also balance the electrical charge.
From a pedagogical and tactical point of of view, on
page 597 it costs nothing to say that we need only one
variable to keep track of the charge, and the value
thereof can range from positive to negative.
Tangentially, you might (or might not) wish to
remark that the experiments with tape cannot tell
you how many types of charge-carrying particles
there are ... but in any case, only one variable
is needed for keeping track of the charge itself.
Again: Charge is an abstraction, not to be
confused with the details of this-or-that charge-
carrying particle.
This illustrates the limits of guided inquiry. It
is super-easy to guide beginners to exactly the wrong
conclusion, especially when they haven't seen all of
the data. Fragmentary data is very often open to
multiple interpretations.
This problem is so common that it even has a name. It
is considered /sophomoric/ to assume that the first theory
that fits the data must be correct, without considering
the full range of data, and without considering the full
range of competing hypotheses.
From @JohnDenker: You may consider this to be somewhat nitpicky, but it is correct ... and the bug is easy to fix ... and it can be parlayed into a lesson about how science is done.
On page 597, it says "There are two and only two types of charge."
Alas, that is not the correct physics. As far as anybody knows, there is only one kind of electrical charge.
Specifically, there is only one variable (Q) that you need in order to keep track of the charge. The value of this variable might be positive or negative, but it's still only one variable.
The behavior of electrical charge stands in contrast to quantum chromodynamics, where there are three different kinds of color charge (in addition to one kind of plain old electrical charge). Three variables are needed to keep track of color charge.
It seems to be an article of faith in the PER community that there are two kinds of charge, but that is only because they have failed in the fundamental duty to consider all the plausible hypotheses. The experiments with plastic tape are /consistent/ with the two-fluid model ... but they are also entirely consistent with the one-fluid model.
You will recall that I made a big fuss about hypotheses (plural) instead of hypothesis (singular) in connection with page 2 of the book.
There is no experiment you can do that favors either model over the other. You could rewrite all the laws of physics to include two charge-like variables, if you're enough of a masochist, but I don't recommend it. The one-fluid model wins on the basis of simplicity and convenience. This is not a new idea; Ben Franklin is credited with coming up with the idea, and coining the words "positive" and "negative".
In chemistry and biochemistry, there are hundreds of different charge carriers, such as sodium ions, calcium ions, chloride ions, et cetera. Some might claim that physics says there are fundamentally only two charge carriers, namely electrons and protons, but that's not right either; physics knows about muons and quarks and all sorts of other "fundamental" charge-carrying particles.
I mention this because if you want to keep track of the /particles/ you might need two or more variables, but still you need only one variable to keep track of the charge itself.
Charge is an abstraction, not to be confused with the details of this-or-that charge-carrying particle. When balancing a chemical reaction equation, you need to balance all of the chemical species and also balance the electrical charge.
From a pedagogical and tactical point of of view, on page 597 it costs nothing to say that we need only one variable to keep track of the charge, and the value thereof can range from positive to negative.
Tangentially, you might (or might not) wish to remark that the experiments with tape cannot tell you how many types of charge-carrying particles there are ... but in any case, only one variable is needed for keeping track of the charge itself. Again: Charge is an abstraction, not to be confused with the details of this-or-that charge- carrying particle.
This illustrates the limits of guided inquiry. It is super-easy to guide beginners to exactly the wrong conclusion, especially when they haven't seen all of the data. Fragmentary data is very often open to multiple interpretations.
This problem is so common that it even has a name. It is considered /sophomoric/ to assume that the first theory that fits the data must be correct, without considering the full range of data, and without considering the full range of competing hypotheses.