vol I chap 2 sect 3
Previous: 2.2. Physics Nobel Lectures by Thomson, Millikan, Franck, Hertz, and Compton.
2.3. Knowledge domains for understanding.¶
Description of knowledge domains for understanding.¶
Galileo Galilei (1564 -1642) once wrote that “mathematics is the language in which the book of nature is written”. To read that book as any other one, we need to follow at least three steps: to open the book with an intention of learning and understanding (to be prepared), to read it with interest and delight (to be attentive and have had some experience in reading) and then to think about what has been read and understood (to reflect on the knowledge derived from the information that has been read).
Reading information and understanding the knowledge it implies is a meaningful process consisting of four knowledge domains:
In this Section 2.3, the three regions considered in the experiments discussed in the Chapter (Preparation, Transformation and Detection and Measurement) are related in the following way to the previously described knowledge domains: the Preparation region involves the Factual and Analytic knowledge domains, the Transformation region refers to the Analytic and Conceptual knowledge domains, and the Detection and Measurement region concerns the Conceptual and Operational domains.
The knowledge obtained from analyzing the information described previously in Section 2.1 is now concentrated in two Tables where the rows correspond to the knowledge domains and the columns refers to the experiments considered in this Chapter. Table 2.1 concerns the calculation of electronic properties and refers to the experiments made by Thomson and Millikan. Table 2.2 deals with the description of electronic interactions and is about the experiments made by Franck-Hertz and Compton.
Experiments for the calculation of electronic properties.¶
| Table 2.1. Basic properties of the electron. | ||
|---|---|---|
| Communication domains | Experiments by Thomson | Experiments by Millikan |
|
Factual domain (Past information) |
Laws formulated by Coulomb, Örsted, Ampere, Faraday and Maxwell. Experiments concerning electrical discharges in gases. Description of trajectories of charged particles. Discovery of X rays by Roentgen. Experiment by Rutherford showing that atoms have a positive nucleus and a cloud of negative particles. |
Laws formulated by Coulomb, Örsted, Ampere, Faraday and Maxwell. Experiments concerning electrical discharges in gases. Description of trajectories of charged particles. Experiments for determining the value of the charge of an electron by measuring the charge of oil drops. The electronic charge is an elementary unit of charge. |
|
Analytic domain (Current information) |
Experiments showing that heated atoms emit charged particles. Experiment for determining that emitted particles have negative electric charges. Experiment for calculating the ratio charge/mass for the emitted particle. |
A drop of water or oil experiments three forces inside a chamber: its weight, the buoyancy force produced by friction with the air inside the contained and the electric force exerted by an external electric field. |
|
Conceptual domain (Accepted or debatable knowledge) |
Atoms have a structure and are not indivisible. The electron is a fundamental particle. ß-rays emitted from radioactive substances are just electrons. |
The drops attain different terminal velocities when the external field is absent ($v_a$) or is present ($v_p$). The electric charges of drops are multiples of the charge of an electron. |
|
Operational domain (critical contributions) |
Calculation of the ratio $(q/m)= El/(B^2 ds)$ by measuring three distances ($l$, $s$, and $d$) and two field intensities (E and B). | Changes in the charge of the drops are expressed as $Δq = (C)(Δv_a)$ where the constant $C = [(6πrη)/E]$. |
Experiments for the description of interactions of radiation with electrons.¶
| Table 2.2. Interacting electrons. | ||
|---|---|---|
| Communication domains | Experiments by Franck and Hertz | Experiments by Condon |
|
Factual domain (Past information) |
Behavior of electrons as particles and electromagnetic radiations as waves. Rules for explaining atomic spectra. |
Behavior of electrons as particles and electromagnetic radiations as waves. Roentgen’s discovery of X rays. |
|
Analytic domain (Current information) |
Bohr atomic model hypothesizes the existence of quantized electronic levels. | Matter exposed to X-rays emits electrons and secondary X-radiation. |
|
Conceptual domain (Accepted or debatable knowledge) |
The gap between the basic level and the first excited level corresponds to the wavelength of the spectroscopic transition. Experimental evidence of the existence of stationary electronic energy levels without considering any incident radiation as the cause of such behavior. |
Experiments indicated that an incoming radiation (photon) was dispersed by a relativistic collision with an electron. X-ray photons behave as particles of zero mass and show corpuscular properties. |
|
Operational domain (critical contributions) |
As for photons $E = hν = hc/λ$. If $E = qV_c$ where $q$ is the charge of the electron, then $λ = (hc)/(qV_c)$. | From the relativistic conservation of energy and momentum conditions it follows $Δλ= (Λ_C )(1- cosθ)$. |
REFERENCES¶
- FEYNMAN, R. P., LEIGHTON, R. B., and SANDS, M. The Feynman Lectures on Physics, Mainly Mechanics, Radiation and Heat. Volume I, Chapter 2. Reading, Massachusetts, Addisson Wesley. (1963).