18. /20 мин./ а/Прочтите следующий текст и найдите ответы на вопросы:
1. What did Greek philosopher Thales find out? /1/ 2. Why was the direction of the current flow chosen by Franklin turned to be opposite to the direction of electron flow? /2/. 3. What is the nature of electrons? /2/ 4. What are consequences of the Bohr propositions about atoms? /3/
Positive & Negative Charges - the Bohr Atom Model
1. History reveals that more than 2000 years ago the Greek philosopher, Thales of Miletus, found that a piece of amber would attract light objects after he had rubbed it with his-fingers. Nothing further was made of this until A.D. 1600 when William Gilbert, an English scientist, experimented & found that other materials act the same way.
2. In the 18th century Benjamin Franklin /1706-1790/ experimented with static-electricity machines. Unfortunate as it is he could not have known about electrons at the time /1749/. It appeared logical to Franklin, & to most experimenters of his day, no doubt, to choose the direction of current flow as it seemed to be when he observed electrical discharge between the terminals of his machine. This caused him to conclude that the flow was from the positive terminal of the machine through the external path - air in this instance - to the negative terminal. About 150 years later Sir Joseph J.Thompson /England, 1856-1940/, & others, convinced most scientists of the granular nature of electric charges & of the nature of electron flow. Franklin’s chosen direction of current flow turned out-to be opposite to the direction of electron flow. But meanwhile so much work had been done„ principles established, & papers & books written employing the positive-to-negative direction for electric current that most researchers & writers adhere to this convention.
3. Early in this century it was proposed by Niels Bohr that the atoms of every kind of material are made up of a hard core called a nucleus, carrying a positive electric charge & one or more negative electric charges called electrons. The electrons rotate in orbits around the nucleus in the manner in which our earth & the other planets rotate around the sun. The Bohr model of the atom made possible a vast amount of progress by researchers working to uncover reasons for the differences & chemical properties of solids, liquids, & gasses. It helped to define the chemical element which was earlier regarded as the ultimate form of each kind of matter.
