7 & 8- SOLAR COOKER, Nuclear physics UPSC Notes | EduRev

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7: SOLAR COOKER, THERMOFLASK, CAR ENGINE RADIATOR, AIR CONDITIONERS

SOLAR COOKER  

A solar cooker, or solar oven, is a device which uses the energy of direct sunlight to heat, cook or pasteurize food or drink. The vast majority of solar cookers presently in use are relatively cheap, low-tech devices. Because they use no fuel and cost nothing to operate, many non-profit organizations are promoting their use worldwide in order to help reduce fuel costs (for low- income people) and air pollution, and to slow down the deforestation and desertification caused by gathering firewood for cooking. Solar cooking is a form of outdoor cooking and is often used in situations where minimal fuel consumption is important, or the danger of accidental fires is high.

SIMPLE SOLAR COOKERS USE THE FOLLOWING BASIC PRINCIPLES

  • Concentrating sunlight: A reflective mirror of polished glass, metal or metallised film concentrates light and heat from the sun on a small cooking area, making the energy more concentrated and increasing its heating power.
  • Converting light to heat: A black or low reflectivity surface on a food container or the inside of a solar cooker improves the effectiveness of turning light into heat. Light absorption converts the sun's visible light into heat, substantially improving the effectiveness of the cooker.
  • Trapping heat: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker. A plastic bag or tightly sealed glass cover traps the hot air inside. This makes it possible to reach temperatures on cold and windy days similar to those possible on hot days.
  • Greenhouse effect: Glass transmits visible light but blocks infrared thermal radiation from escaping. This amplifies the heat trapping effect.
  • Solar cookers use no fuel, which means that their users do not need to fetch or pay for firewood, gas, electricity, or other fuels. Therefore, over time a solar cooker can pay for itself in reduced fuel costs. Since it reduces firewood use, the solar cooker reduces deforestation and habitat loss. Since there are about 2 billion people who are still cooking on open fires, widespread use of solar cookers could have large economic and environmental benefits.
  • Solar box cookers attain temperatures of up to about 165 °C (325 °F), so they can be used to sterilize water or prepare most foods that can be made in a conventional oven or stove, from baked bread to steamed vegetables to roasted meat. When solar ovens are placed outside, they do not contribute unwanted heat inside houses.

Is an insulating storage vessel that greatly lengthens the time over which its contents remain? hotter or cooler than the flask's surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which prevents heat transfer by conduction or convection.

AIR CONDITIONERS  

Air conditioning is the process of altering the properties of air (primarily temperature and humidity) to more favourable conditions. More generally, air conditioning can refer to any form of technological cooling, heating, ventilation, or disinfection that modifies the condition of air.

An air conditioner (often referred to as air con, AC or A/C, and not to be confused with the abbreviation for alternating current) is a major or home appliance, system, or mechanism designed to change the air temperature and humidity within an area (used for cooling and sometimes heating depending on the air properties at a given time). The cooling is typically done using a simple refrigeration cycle, but sometimes evaporation is used, commonly for comfort cooling in buildings and motor vehicles. In construction, a complete system of heating, ventilation and air conditioning is referred to as "HVAC".

 AIR CONDITIONER S HAVE THE FOLLOWING USES  

Air-conditioning engineers broadly divide air-conditioning applications into what they call comfort and process applications.

Comfort applications aim to provide a building indoor environment that remains relatively constant despite changes in external weather conditions or in internal heat loads.

Air conditioning makes deep plan buildings feasible, for otherwise they would have to be built narrower or with light wells so that inner spaces received sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller, since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications are quite different for various building types and may be categorized as:

  • Low-rise residential buildings, including single family houses, duplexes, and small apartment buildings
  • High-rise residential buildings, such as tall dormitories and apartment blocks
  • Commercial buildings, which are built for commerce, including offices, malls, shopping centres, restaurants, etc.
  • Institutional buildings, which includes government buildings, hospitals, schools, etc.
  • Industrial spaces where thermal comfort of workers is desired.
  • Sports stadiums: recently, stadiums have been built with air conditioning, such as the University of Phoenix Stadium and in Qatar for the 2022 FIFA World Cup.

HEAT  

Heat is energy transferred from one body to another by thermal interactions. The transfer of energy can occur in a variety of ways, among them conduction, radiation, and convection. Heat is not a property of a system or body, but instead is always associated with a process of some kind, and is synonymous with heat flow and heat transfer. The SI unit of heat is the joule. Heat can be measured by calorimetric, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics. In calorimetric, the concepts of latent heat and of sensible heat are used. Latent heat produces changes of state without temperature change, while sensible heat produces temperature change.

The transfer of heat from a warm object to a cooler one takes place by one of three methods or a combination thereof. These methods are: conduction, convection or radiation.

CONDUCTION  

Conduction of heat occurs when faster moving molecules pass on some of their energy to adjacent molecules which are slower-moving, i.e. at a lower temperature. This may occur within a solid or between a solid and an adjacent fluid such as air. In any heated building or enclosure, heat is conducted or transmitted from the warm inside air to the inside surfaces, then through the wall or roof, to the cooler outside surface and on to the outside air. 

CONVECTION  

Convection transfer of heat involves the mixing of warm and cool particles of fluid. The mixing may come as a result of density differences due to temperature differences which is natural convection, or, if the mixing is produced by mechanical means, forced convection. In a heated building, convection losses occur when cold outside air enters a building, mixes with the warmer inside air, and then exits through an exhauster or through doors, cracks, etc. 

RADIATION  

Heat transfer by radiation differs from the transfer of heat by conduction or convection in that it does not need matter to accomplish the transfer. Radiated heat is usually termed infra-red. This is just one of the several forms of radiation. Infra-red is transmitted at the speed of light, 186,000 miles per second, in a straight line with minimal loss to the air. It can be aimed, reflected or focused by materials that have a highly reflective surface, bright aluminium, for example. When infra-red strikes an absorptive object such as concrete, wood, water, paint, skin or clothing it is converted into heat at the surface. Surrounding air is then warmed by conduction and convection. The best example of this transfer of heat is from the sun to the earth without loss of heat to outer space.

Radiation or infra-red energy is emitted by all matter that is above absolute zero (-460°F). The net transfer of heat is from one object to a cooler object.

Warm objects, including people inside a heated building lose or radiate heat to the cooler inside surfaces of the walls. The walls conduct heat to the outside surface and then lose heat by radiation, conduction and convection to the outside. 

LATENT HEAT  

Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. 

8: Nuclear physics, nuclear fusion, nuclear fission

Nuclear physics

Nuclear physics is the field of physics that studies the constituents and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those in nuclear medicine and magnetic resonance imaging, ion implantation in materials engineering, and radiocarbon dating in geology and archaeology.

The field of particle physics evolved out of nuclear physics and is typically taught in close association with nuclear physics.

NUCLEAR FUSION

In nuclear fusion, two low mass nuclei come into very close contact with each other, so that the strong force fuses them. It requires a large amount of energy to overcome the repulsion between the nuclei for the strong or nuclear forces to produce this effect, therefore nuclear fusion can only take place at very high temperatures or high pressures. Once the process succeeds, a very large amount of energy is released and the combined nucleus assumes a lower energy level. The binding energy per nucleon increases with mass number up until nickel-62. Stars like the Sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos. The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. A frontier in current research at various institutions, for example the Joint European Torus (JET) and ITER, is the development of an economically viable method of using energy from a controlled fusion reaction. Natural nuclear fusion is the origin of the light and energy produced by the core of all stars including our own sun. 

NUCLEAR  FISSION  

Nuclear fission is the reverse process of fusion. For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones.

The process of alpha decay is in essence a special type of spontaneous nuclear fission. This process produces a highly asymmetrical fission because the four particles which make up the alpha particle are especially tightly bound to each other, making production of this nucleus in fission particularly likely.

For certain of the heaviest nuclei which produce neutrons on fission, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a so-called chain reaction. Chain reactions were known in chemistry before physics, and in fact many familiar processes like fires and chemical explosions are chemical chain reactions. The fission or "nuclear" chain-reaction, using fission-produced neutrons, is the source of energy for nuclear power plants and fission type nuclear bombs, such as those detonated by the United States in Hiroshima and Nagasaki, Japan, at the end of World War II. Heavy nuclei such as uranium and thorium may also undergo spontaneous fission, but they are much more likely to undergo decay by alpha decay.

For a neutron-initiated chain-reaction to occur, there must be a critical mass of the element present in a certain space under certain conditions. The conditions for the smallest critical mass require the conservation of the emitted neutrons and also their slowing or moderation so there is a greater cross-section or probability of them initiating another fission. In two regions of Okla., Gabon, Africa, natural nuclear fission reactors were active over 1.5 billion years ago. Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the Earth's core results from radioactive decay. However, it is not known if any of this results from fission chain-reactions.

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