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Re: [Phys-l] glassware needed

Well if you look at the misconceptions of students which are addressed by
the RTP and ILDs, the important factor for students to understand is how
relative factors change the temperature-heating relationships. Simple
experiments which just measure how much the temperature rises for a given
input of energy do not address the misconceptions. Students need to
understand that the relationship is one of direct proportionality. Then
they need to appreciate how the specific heat relationship works by such
things as adding 100g of a hot metal to 100g of room temperature water after
predicting what the t vs time curves will look like. They are generally
surprised that the final temperature is much closer to room temperature than
to the higher temperature of the metal. This is true even when they have
the information about the specific heat of each substance.

So in the end the stated wattage is more than adequate for the kinds of
experiments the students need to do. But since they are making measurements
over time, a good time controller is much more important, and makes the
experiments much easier to do. A 20% error in the wattage will still result
in excellent straight line graphs of T vs energy. Students generally will
draw prediction curves rather than straight lines for T vs the number of
equal energy pulses, or they will draw lines with different slopes for T vs
time if there is an intervening section of time with no energy input. They
do not understand the relationship involved.

So knowing the number of pulses or the amount of energy are both equally as
good, and knowing the absolutely precise amount of energy is not that
important to exposing and helping students to deal with misconceptions.

Notice that this discussion is predicated on the idea that exact
measurements of things are vital for student learning. The type of lab
where students try to get precise measurements of quantities such as
specific heats of materials do not generally address common misconceptions.
The RTP and ILDs do address the misconceptions and help students develop
better understanding of the physics. Oh, and knowing the exact wattage, is
not very useful if you can't control the heating with precise pulses of
energy. Manually turning switches on and off will result in higher error.

While it certainly is a good academic exercise to have a way to calibrate an
experiment of this type, doing experiments that actually result in better
student learning is much more important.

John M. Clement
Houston, TX

At 04:41 PM 1/1/2009, you wrote:
More important than a wattmeter is a precise method of turning the heater
and off. RTP uses the Vernier pulser module. You really want students
be able to compare change in temperature with the number of pulses of
From the point of understanding, knowing the exact amount of energy is
less important. And as mentioned before, just using Styrofoam cups makes
the process almost adiabatic.

John M. Clement
Houston, TX

Forgive me for wondering how knowing that there were x pulses of
unspecified length involved in heating a sample is superior to knowing
w watts were supplied for t seconds. Is this a case of less is more?

Brian Whatcott Altus OK Eureka!

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