Momentum definition, force or speed of movement; impetus, as of a physical object or course of events: The car gained momentum going downhill. Her career lost momentum . The rest mass of the electron is ? 10 ?31 kg, which is only 1 / 1, the mass of a proton. An electron is therefore considered nearly massless in comparison with a proton or a neutron, and the electron mass is not included in calculating the mass number of .
Electronlightest stable subatomic particle known. It carries a negative charge of 1. The rest mass of the electron is 9. An electron is therefore considered nearly massless in comparison with a proton or a neutronand the electron mass is not included in calculating the mass number of an atom. The electron was discovered in by the English physicist J. Thomson during investigations of cathode rays. His discovery of electrons, which he initially called corpuscles, played a pivotal role in revolutionizing knowledge of atomic structure.
Under ordinary conditions electrons are bound to the positively charged nuclei of atoms by the attraction between opposite electric charges. In a neutral atom the number of electrons is identical to the number of positive charges on the nucleus.
Any atom, however, may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole; these charged atoms are known as ions. Not all electrons are associated with atoms; some occur in a free state with ions in the form of matter known as plasma.
What is the purpose of a w- 4 form any given atom, electrons move about the nucleus in an orderly arrangement of orbitalsthe attraction between electrons and nucleus overcoming repulsion among the electrons that would otherwise cause them to fly apart.
These orbitals are what is the definition of mass movement in concentric shells proceeding outward from the nucleus with an increasing number of subshells. The electrons in orbitals closest to the nucleus are held most tightly; those in the outermost orbitals are shielded by intervening electrons and are the most loosely held by the nucleus. As the electrons move about within this structure, they form a diffuse cloud of negative charge that occupies nearly the entire volume of the atom.
The detailed structural arrangement of electrons within an atom is referred to as the electronic configuration of the atom. The electronic configuration determines not only the size of an individual atom but also the chemical nature of the atom. The classification of elements within groups of similar elements in the periodic tablefor example, is based on the similarity in their electron structures.
Within the field of particle physicsthere are two ways of classifying electrons. The electron is a fermiona type of particle named after the Fermi-Dirac statistics that describe its behaviour. All fermions are characterized by half-integer values of their spinwhere spin corresponds to the intrinsic angular momentum of the particle. The concept of spin is embodied in the wave equation for the electron formulated by P.
The Dirac wave equation also predicts the existence of the antimatter counterpart of the electron, the positron. Within the fermion group of subatomic particles, the electron can be further classified as a lepton. A lepton is a subatomic particle that reacts only by the electromagneticweakand gravitational forces; it does how to evaluate a speech toastmasters respond to the short-range strong force that acts between quarks and binds protons and neutrons in the atomic nucleus.
Videos Images. Additional Info. More About Contributors Article History. Print Cite verified Cite. While every effort has been made to follow citation style rules, there may be some discrepancies.
Please refer to the appropriate style manual or other sources if you have any questions. Facebook Twitter. Give Feedback External Websites. Let us know if you have suggestions to improve this article requires login. External Websites. Rader's Chem4Kids. The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History.
Explore an atom's interior to discover the layout of its nucleus, protons, and electrons. Description of the arrangement of electrons in atoms of various elements.
Read More on This Topic. Scientists have known since the late 19th century that the electron has a negative electric charge. The value of this charge was first measured Electrons fill in shell and subshell levels in a semiregular process, as indicated by the arrows above.
After filling the first shell level with just an s subshellelectrons move into the second-level s subshell and then into the p subshell before starting on another shell level.
Because of its lower energy state, the 4 s orbital fills before the 3 dand later s orbitals fill similarly for example, 6 s fills before 4 f. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Learn More in these related Britannica articles:. The value of this charge was first measured by the American physicist Robert Millikan between and Subatomic particles include electrons, the negatively charged, almost massless particles that nevertheless account for most of the size of the atom, and they include the heavier building blocks of the small but very dense what channel is rose bowl on of the atom, the positively charged protons and the electrically neutral neutrons….
In experiments at CERN and at the Brookhaven National Laboratory in Upton, New York, physicists collide heavy nuclei at high energies in order to achieve temperatures and densities that may be high…. History at your fingertips. Sign up here to see what happened On This Dayevery day in your inbox! Email address. By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
Meet Grammar Coach
Dec 14, · Solidarity, Polish trade union that in the early s became the first independent labor union in a country belonging to the Soviet Bloc. Lech Walesa helped found the organization in , and he became its influential leader, eventually serving as president of Poland. Learn more about Solidarity. No single definition of local food systems exists. The geographic distances between production and consumption varies within the movement. However, the general public recognizes that "local" describes the marketing arrangement (e.g. farmers selling directly to .
Local food is food that is produced within a short distance of where it is consumed, often accompanied by a social structure and supply chain different from the large-scale supermarket system.
Local food or "locavore" movements aim to connect food producers and consumers in the same geographic region, to develop more self-reliant and resilient food networks ; improve local economies; or to affect the health , environment, community, or society of a particular place. Local food represents an alternative to the global food model, which often sees food traveling long distances before it reaches the consumer. In the USA, the local food movement has been traced to the Agricultural Adjustment Act of , which spawned agricultural subsidies and price supports.
In , Chicago pop culture made local food a trend in the Midwest. These largely unsuccessful resolutions encouraged increased local production to slow farmland loss. The program described "sustainable diets" - a term then new to the American public. At the time, the resolutions were met with strong criticism from pro-business institutions, but have had a strong resurgence of backing since No single definition of local food systems exists.
The geographic distances between production and consumption varies within the movement. However, the general public recognizes that "local" describes the marketing arrangement e. I the locality or region in which the final product is marketed, so that the total distance that the product is transported is less than miles from the origin of the product; or II the State in which the product is produced. The concept of "local" is also seen in terms of ecology , where food production is considered from the perspective of a basic ecological unit defined by its climate, soil, watershed , species and local agrisystems , a unit also called an ecoregion or a food shed.
Similar to watersheds, food sheds follow the process of where food comes from and where it ends up. There were 5, farmers' markets in , compared to 2, in In , there were 1, community-supported agriculture organizations CSAs.
There were 2, farm to school programs in In the Index, the three top-ranking states were Vermont, Maine and Oregon, while the three lowest-ranking states were Nevada, Texas and Florida. Websites now exist that aim to connect people to local food growers.
Supermarket chains also participate in the local food scene. Local food campaigns have been successful in supporting small local farmers. In , a campaign was started in Virginia by the Common Grains Alliance mirroring many of the efforts of the North Carolina campaign. Motivations for eating local food include healthier food, environmental benefits, and economic or community benefits.
Many local farmers whom locavores turn to for their source of food use the crop rotation method when producing their organic crops. This method not only aids in reducing the use of pesticides and pollutants, but also keeps the soil in good condition rather than depleting it. Reducing the travel time makes it possible to transport the crops while they are still fresh, without using chemical preservatives.
Local eating can support public objectives. It can promote community interaction by fostering relationships between farmers and consumers. Farmers' markets can inspire more sociable behavior, encouraging shoppers to visit in groups. Local foods require less energy to store and transport, possibly reducing greenhouse gas emissions. Farmers' markets create local jobs. Critics of the local foods movement question the fundamental principles behind the push to eat locally. For example, the concept that fewer "food miles" translates to a more sustainable meal has not been supported by major scientific studies.
According to a study conducted at Lincoln University in New Zealand : "As a concept, food miles has gained some traction with the popular press and certain groups overseas. However, this debate which only includes the distance food travels is spurious as it does not consider total energy use especially in the production of the product. Food miles concept does not consider agriculture, which is having contributed the highest when it comes to greenhouse gas emissions. Plus, season and transportation medium also makes a difference.
The only study to date that directly focuses on whether or not a local diet is more helpful in reducing greenhouse gases was conducted by Christopher L. Weber and H. Scott Matthews at Carnegie-Mellon. Numerous studies have shown that locally and sustainably grown foods release more greenhouse gases than food made in factory farms.
The "Land Degradation" section of the United Nations report Livestock's Long Shadow concludes that " Intensification - in terms of increased productivity both in livestock production and in feed crop agriculture - can reduce greenhouse gas emissions from deforestation ". While locavorism has been promoted as a feasible alternative to modern food production, some believe it might negatively affect the efficiency of production.
These latter criticisms combine with deeper concerns of food safety, cited on the lines of the historical pattern of economic or food safety inefficiencies of subsistence farming which form the topic of the book The Locavore's Dilemma by geographer Pierre Desrochers and public policy scholar Hiroko Shimizu.
From Wikipedia, the free encyclopedia. For foods with protected geographical names, see Geographical indication. The examples and perspective in this article may not represent a worldwide view of the subject. You may improve this article , discuss the issue on the talk page , or create a new article , as appropriate. October Learn how and when to remove this template message. Theories and ideas. Notable works.
Organizations and groups. Adbusters Crass CrimethInc. Deep Green Resistance Democracy Now! Related social movements. See also. Food portal Agriculture and Agronomy portal Agriculture portal Gardening portal.
Local food systems : background and issues. Nova Science Publishers. ISBN OCLC Agriculture and Human Values. Economic Research Service.
Retrieved 17 June March Multiplicity in food retailers' perspectives on sourcing and marketing local foods". Renewable Agriculture and Food Systems. Cambridge University Press. Archived from the original on 6 September Retrieved 13 March Society for Nutrition Education. Making healthy places designing and building for health, well-being, and sustainability. Retrieved 11 December MSU Extension.
Retrieved 16 March The New York Times. Retrieved 20 July Amy Francis. Detroit: Greenhaven Press, At Issue. Is Reshaping Farms and Food Retailing. Opposing Viewpoints in Context. Center for Environmental Farming Systems. Retrieved 7 July NC Farm Bureau Magazine. January Archived from the original on 26 April General OneFile.
Retrieved 5 October Detroit: Gale, August American Heart Journal. Retrieved 5 June United Nations, n. Meats Environment Science News.
The Independent Institute. Retrieved 26 March Authority control NDL : Categories : Food politics Localism politics Rural community development Sustainable food system Ecology terminology Food security.