question: How does a fridge produce cold?
comment: What a fridge does can best be explained by "removing heat" rather than by "producing cold" Some fundamental knowledge of thermodynamics and properties of gasses is needed to understand in full, but the basic principle is this: Compressed (cold) freon is circulated into the fridge and through an evaporator coil, which allows the freon to expand and absorb heat from the inside of the fridge. The hot freon is then circulated outside of the fridge, where it is compressed in the condenser. When compressed, the heat in the freon is released outside the fridge. Insulation in the fridge walls & doors prevents the heat from migrating back into the fridge. Edit: the electricity used is to run the compressor, which is a small motor & pump to help compress the freon on the condenser. Wiki the Ideal Gas Law for a better understanding of what happens here, but we're basically using an energy input to move heat from one place to another. The heat is removed from the inside of the fridge faster than it can return (insulation) so the inside of the fridge gets cooler.
passages: Section::::Refrigerator mother theory.
   Refrigerators and freezers may be free-standing, or built into a kitchen.
  
  Three distinct classes of refrigerator are common:
   Regardless of who provides it, the chilled water (between 4° and 7°C (39-44 °F)) is pumped through an air handler, which captures the heat from the air, then disperses the air throughout the area to be cooled.
  
  Section::::Site generated.
   To produce yogurt, milk is first heated, usually to about 85 °C (185 °F), to denature the milk proteins so that they do not form curds. After heating, the milk is allowed to cool to about 45 °C (113 °F). The bacterial culture is mixed in, and that temperature of 45 °C is maintained for 4 to 12 hours to allow fermentation to occur.
  
  Section::::Etymology and spelling.
   The history of artificial refrigeration began when Scottish professor William Cullen designed a small refrigerating machine in 1755. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.
   "Ultra-cold" or "ultra-low temperature (ULT)" (typically -80C) freezers, as used for storing biological samples, also generally employ two stages of cooling, but in cascade. The lower temperature stage uses methane, or a similar gas, as a refrigerant, with its condenser kept at around -40C by a second stage which uses a more conventional refrigerant. Well known brands include Forma and Revco (both now Thermo Scientific) and Thermoline. For much lower temperatures (around -196C), laboratories usually purchase liquid nitrogen, kept in a Dewar flask, into which the samples are suspended.
  
  Alternatives to the vapor-compression cycle not in current use include:
   BULLET::::- Fruit Growers Express number 38374 was equipped with an experimental aluminum body in the Indiana Harbor, Indiana shops.
  
  Section::::History.:Experimentation.:"Depression Baby".
   Another variant, uses air, water, and a salt water solution. The intake of warm, moist air is passed through a sprayed solution of salt water. The spray lowers the humidity but does not significantly change the temperature. The less humid, warm air is then passed through an evaporative cooler, consisting of a spray of fresh water, which cools and re-humidifies the air. Humidity is removed from the cooled air with another spray of salt solution, providing the outlet of cool, dry air.
   In Kanner's original paper, however, only one set of parents were described as "cold", with many family members appearing to be from one neurological minority or another upon close reading of the text.
   BULLET::::- From d to a. The lp valve is closed and the hp valve opened with fixed position of the displacer. The gas, now in the hot end of the cold head, is compressed and heat is released to the surroundings. In the end of this step we are back in position a.
  
  Section::::Pulse-tube refrigerators.
   The film is similar to the Filipino horror film Shake Rattle and Roll's episode: Pridyider, which was also remade in 2012, also known as "Fridge".
  
  Section::::Plot.
  
  The film opens with a couple drunk driving through the streets of New York. Upon arriving at their apartment, they make love in front of the kitchen. Afterwards, the wife walks into the kitchen whereupon the refrigerator opens up and sucks her in.
   The thermal mass used in Michael Reynolds' design is a combination of a liquid (i.e. water or beer) together with concrete mass. Concrete's temperature can be decreased quickly, while a liquid's (such as beer or water) with its higher thermal mass requires more energy to change temperature, holding the cold for longer. In Michael Reynolds' design, the liquid is added in the form of beer cans, placed in the back of the refrigerator.
   Section::::Methods of refrigeration.:Passive systems.
  
  MIT researchers have devised a new way of providing cooling on a hot sunny day, using inexpensive materials and requiring no fossil fuel-generated power. The passive system, which could be used to supplement other cooling systems to preserve food and medications in hot, off-grid locations, is essentially a high-tech version of a parasol.
  
  Section::::Capacity ratings.
   Section::::Commercial and domestic refrigerators.
   BULLET::::- 1887: Parker Earle joined F.A. Thomas of Chicago in the fruit shipping business. The company owned 60 ice-cooled railcars by 1888, and 600 by 1891.
  
  BULLET::::- 1888: Armour & Co. shipped beef from Chicago to Florida in a car cooled by ethyl chloride-compression machinery. Florida oranges were shipped to New York under refrigeration for the first time.
  
  BULLET::::- 1889: The first cooled shipment of fruit from California was sold on the New York market.
   BULLET::::2. Generator : After exiting the heat exchanger, the dilute solution moves into the upper shell. The solution surrounds a bundle of tubes which carries either steam or hot water. The steam or hot water transfers heat into the pool of dilute lithium bromide solution. The solution boils, sending refrigerant vapor upward into the condenser and leaving behind concentrated lithium bromide. The concentrated lithium bromide solution moves down to the heat exchanger, where it is cooled by the weak solution being pumped up to the generator.
   Section::::History.:Cryogenic refrigeration.
   In 1876, German engineer Carl von Linde patented the process of liquefying gas that would later become an important part of basic refrigeration technology (U.S. Patent 1027862). In 1879 and 1891, two African American inventors patented improved refrigerator designs in the United States (Thomas Elkins – U.S. patent #221222 and respectively John Standard – U.S. patent #455891).
   In 2013 the technology that uses an ice-mass to maintain refrigerator temperature without power won a $100,000 research award from the Bill and Melinda Gates Foundation. In 2014, the company was awarded a further $1.4 Million from the Gates Foundation to develop a life-saving vaccine cooler as part of the foundation’s goal to eliminate preventable diseases worldwide. 
  
  Section::::The Inventor.
   Section::::Principles.:Simple salt and water system.
  
  A simple absorption refrigeration system common in large commercial plants uses a solution of lithium bromide and lithium chloride salt and water. Water under low pressure is evaporated from the coils that are to be chilled. The water is absorbed by a lithium bromide/water solution. The system drives the water off the lithium bromide solution with heat.
  
  Section::::Principles.:Water spray absorption refrigeration.
   Section::::Technology.
   In the tube the gas is thermally isolated (adiabatic), so the temperature of the gas in the tube vary with the pressure.
  
  At the cold end of the tube, the gas enters the tube via X when the pressure is high with temperature "T" and return when the pressure is low with a temperature below "T", hence taking up heat from X : this gives the desired cooling effect at X.
   Section::::Specialized applications.:Tropicana "Juice Train".
   Section::::Overview.:Charge temperature.
   Section::::Principle of operation.
  
  Figure 1 represents the Stirling-type single-orifice Pulse-Tube Refrigerator (PTR), which is filled with a gas, typically helium at a pressure varying from 10 to 30 bar. From left to right the components are:
  
  BULLET::::- a compressor, with a piston moving back and forth at room temperature "T"
  
  BULLET::::- a heat exchanger X where heat is released to the surroundings at room temperature
  
raw_presuppositions: A fridge produces cold.
raw_corrections: A refrigerator does not produce cold, it removes heat.