Creating Physicists: Making reasoning explicit: If … and … then …

Lawson Primarily a hypothetico-deductive process Make an observation of a “strange” phenomenon, generates tentative theories, deduce specific predictions to test these theories through experimentation. Requires skills: properly identify and control variables, proportional, probabilistic, and correlation thinking. Allchin and others Inductive processes a major component Induction a useful tool for identifying regularities, patterns, and associations “proper” science reasoning can be done even when limited theory or prior concepts exist to guide initial observations.

Conservation Proportional Isolation and control of variables Probabilistic Correctional (correlation vs. causation) Hypothetico-deductive

Conservation Proportional Isolation and control of variables Probabilistic Correctional (correlation vs. causation) Hypothetico-deductive

Lawson’s Classroom Test of Scientific Reasoning

It’s true … because my passive observation fits my model

Science jargon

non-sequitur

IF … AND … THEN … hypothesis test result if hypothesis is true

IF … AND … THEN … current is used up in a bulb we measure current on both sides the current will be less on one side

1.Blood circulates continuously due in part to contractions of the heart. 2.The heart contains one-way valves. Thus, circulating blood passes from the heart’s lower-right chamber (right ventricle) to the lungs (via the pulmonary arteries), then back to the heart’s upper-left chamber (left atrium), via the pulmonary veins, and from there into the heart’s lower-left chamber (left ventricle). 3.From the left ventricle, blood is forced into the aorta and through its branches and subbranches to all parts of the body except the lungs. 4.From the arteries’ smallest branches, blood flows through tiny unseen vessels (capillaries) into the smallest veins. 5.The veins contain one-way valves to prevent backward blood flow. Thus, due to contractions of nearby muscles, blood is squeezed from the smallest veins into larger and larger veins into the largest veins and then into the heart’s upper-right chamber (right atrium). 6.The heart’s right atrium then periodically forces blood into the right ventricle Postulates of William Harvey’s Blood Circulation Theory

If... blood flows in veins only toward the heart because of the presence of one-way valves (Postulate 5), and... a tourniquet is tied around the upper arm, a finger is pressed on the vein at point G, and then slid toward the hand down to point H (planned test), then... the vein between points G and H should bulge only part way down toward H (expected result). The vein should bulge only down to a point at which the blood encounters a valve (i.e. at point O), which will presumably retard its flow back toward the hand (theoretical rationale). Anton E. Lawson, Am. Biol. Teach. 62, 482 (2000).

And … the result of Harvey’s experiment is the one expected on Postulate 5 (observed result). Therefore... Postulate 5 is supported, that is, it appears that blood flows in veins in only one direction (conclusion). Going further: and/but … therefore

Draw a picture of the sun

S cr e e n What do you see on the screen?

S cr e e n Now what do you see?

If …The Kindergarten Sun Ray Model of Light is a good predictive model And …We place the piece of paper over the bulb as shown Then …The light circle on the screen should narrow.

A more accurate model

S cr e e n Kindergarten Sun Ray Model of Light

If... light travels in straight lines (straight-line hypothesis), and … light shines through the first slit as described above (planned test), then...the light should pass through the slit, but should be blocked by the second screen, thus should not reach the third screen (expected result). Anton E. Lawson, Am. Biol. Teach. 62, 482 (2000).

BUT... light is seen on the screen in a pattern of bands Therefore … the experimental evidence does not provide support for the straight-line hypothesis Anton E. Lawson, Am. Biol. Teach. 62, 482 (2000).

Did TWA Flight 800 get shot down by a missile? After recovery of the wreckage and reconstruction, what should we find to support this hypothesis?

If... the gaping holes were caused by missile strikes (missile-strike hypothesis), and... the metal pieces around the holes are carefully examined (planned test), then...traces of explosives should be found (expected result). But... when the metal pieces were carefully examined, no traces of explosives were found (observed result). Therefore... most likely, the holes were not caused by missile strikes (conclusion).

IF … AND … THEN … current is used up in a bulb we measure current on both sides the current will be less on one side

Students have difficultly w/ structure S1: If we add another bulb to the circuit, and we observe the brightness to decrease, then current is used up in bulbs. S2: If Ohm’s Law is valid, and the current decreases when the voltage decreases, then current is used up in bulbs. S3:If we conduct a controlled experiment varying only the current, and the bulbs behavior does not depend on the current, then current is not used up in bulbs.

“A single bulb circuit consists of a battery, two wires and a bulb arranged such that the bulb is lit. A student wonders whether it matters which part of the bulb is connected to the ‘+’ sign on the battery. Describe an experiment that the student can do to make this determination.” Common problems: (1) Hypothesis construction (2) Developing good testing experiments

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness ??? ?

S1: The orientation of the battery does not matter, so rebuild the circuit in exactly the same way to show that the orientation does not matter. S2:Get a different battery, bulb and set of wires and build a second circuit with the battery in the opposite orientation. S3:Using the same components, turn the battery in the opposite orientation. S4:Conduct a controlled experiment, varying only the number of batteries to see if that makes a difference. S5:Redo the experiment, but make sure it is controlled by holding all independent variables constant; then measure the bulb brightness

Thursday, April 7 th … Making reasoning explicit: the relative value of evidence Getting students to think about how useful their data really is.