CZ:Featured article/Current: Difference between revisions

From Citizendium
Jump to navigation Jump to search
imported>Chunbum Park
imported>John Stephenson
(template)
 
(97 intermediate revisions by 5 users not shown)
Line 1: Line 1:
== '''[[Jet engine]]''' ==
{{:{{FeaturedArticleTitle}}}}
----
<small>
{{Image|Jet engine.png|center|260px|A typical gas turbine jet engine (also known as a "turbojet"). Air is compressed by the fan blades as it enters the engine, and it is mixed and burned with fuel in the combustion section. The hot exhaust gases provide forward thrust and turn the turbines which drive the compressor fan blades.}}  
==Footnotes==
A '''jet engine''' is a reaction engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets. In general, most jet engines are [[internal combustion engine]]s but non-combusting forms also exist.
 
In some common usage, the term ''jet engine'' generally refers to an internal combustion duct engine, which typically consists of an engine with a rotary (rotating) air compressor powered by a turbine ("Brayton cycle"), with the leftover power providing thrust via a propelling nozzle. These types of jet engines are primarily used by jet aircraft for long distance travel. The early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines which help give high speeds as well as, over long distances, giving better fuel efficiency than many other forms of transport.
===History===
Jet engines can be dated back to the first century AD, when [[Hero of Alexandria]] (a [[Greek]] mathematician) invented the aeolipile. This used steam power directed through two jet nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was little used for supplying mechanical power, and the potential practical applications of Hero's invention of the jet engine were not recognized. It was simply considered a curiosity.
 
 
''[[Jet engine|.... (read more)]]''
 
{| class="wikitable collapsible collapsed" style="width: 90%; float: center; margin: 0.5em 1em 0.8em 0px;"
|-
! style="text-align: center;" | &nbsp;[[Jet engine#References|notes]]
|-
|
{{reflist|2}}
{{reflist|2}}
|}
</small>

Latest revision as of 09:19, 11 September 2020

The Mathare Valley slum near Nairobi, Kenya, in 2009.

Poverty is deprivation based on lack of material resources. The concept is value-based and political. Hence its definition, causes and remedies (and the possibility of remedies) are highly contentious.[1] The word poverty may also be used figuratively to indicate a lack, instead of material goods or money, of any kind of quality, as in a poverty of imagination.

Definitions

Primary and secondary poverty

The use of the terms primary and secondary poverty dates back to Seebohm Rowntree, who conducted the second British survey to calculate the extent of poverty. This was carried out in York and was published in 1899. He defined primary poverty as having insufficient income to “obtain the minimum necessaries for the maintenance of merely physical efficiency”. In secondary poverty, the income “would be sufficient for the maintenance of merely physical efficiency were it not that some portion of it is absorbed by some other expenditure.” Even with these rigorous criteria he found that 9.9% of the population was in primary poverty and a further 17.9% in secondary.[2]

Absolute and comparative poverty

More recent definitions tend to use the terms absolute and comparative poverty. Absolute is in line with Rowntree's primary poverty, but comparative poverty is usually expressed in terms of ability to play a part in the society in which a person lives. Comparative poverty will thus vary from one country to another.[3] The difficulty of definition is illustrated by the fact that a recession can actually reduce "poverty".

Causes of poverty

The causes of poverty most often considered are:

  • Character defects
  • An established “culture of poverty”, with low expectations handed down from one generation to another
  • Unemployment
  • Irregular employment, and/or low pay
  • Position in the life cycle (see below) and household size
  • Disability
  • Structural inequality, both within countries and between countries. (R H Tawney: “What thoughtful rich people call the problem of poverty, thoughtful poor people call with equal justice a problem of riches”)[4]

As noted above, most of these, or the extent to which they can be, or should be changed, are matters of heated controversy.

Footnotes

  1. Alcock, P. Understanding poverty. Macmillan. 1997. ch 1.
  2. Harris, B. The origins of the British welfare state. Palgrave Macmillan. 2004. Also, Oxford Dictionary of National Biography.
  3. Alcock, Pt II
  4. Alcock, Preface to 1st edition and pt III.