聆听学习科学/热力学
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Heat is always understood, as a process of transfer. It is absorbed or produced, as an energy exchange. Heat always flows, from a hotter body, to a colder body. For heat to flow, it needs a medium. The medium through which heat flows, can be a solid, liquid, or gas. Heat cannot flow, in a vacuum. In other words, there can be heat, only when there is a substance. When heat flows through a solid, it is called conduction. When heat flows through a liquid or gas, it is called convection.
The relative hotness, or coldness, of a body, is the temperature. This temperature can be measured, with a standard scale.
The most commonly used scale, for measuring temperature, is centigrade or celsius. The boiling point, and the freezing point, of water, is used to define this scale. The freezing point of water, is defined as, 0 degree centigrade. The boiling point of water, is defined as, 100 degree centigrade. The temperature difference, between the boiling point and freezing point, is divided into 100 equal degrees. That is why, it was called as the centigrade scale. Now it is called, as the ‘Celsius scale’. The symbol for Celsius, is ‘c’. For example, we can say, that the body temperature, is 37 degrees ‘c’.
Another temperature scale, is the kelvin scale. Scientists have determined, the lowest temperature, that can be achieved. This they found out to be, as minus 273.15 degrees celsius. This is called the absolute 0, of temperature. That is temperature can never go, below minus 273.15 degrees ‘c’. This temperature is defined as 0 degrees kelvin, in the kelvin scale. All temperatures are measured, relative to 0 degree kelvin. The kelvin scale, is widely used by scientists. The symbol for kelvin is ‘k’. Kelvin is the S I unit, for temperature. One degree kelvin is the same as 1 degree celsius.
An older temperature scale, that was used, was Fahrenheit. In the Fahrenheit scale, the freezing point of water, is 32 degrees Fahrenheit. The boiling point of water, is 212 degrees Fahrenheit. The difference between the boiling point and the freezing point, is divided into 180 degrees. The symbol for Fahrenheit, is ‘F’. Though this scale is obsolete, some countries like U S, still use it.
We can easily convert temperature, from one scale to another. Converting from celsius to kelvin. Temperature in celsius, + 273.15 is temperature in kelvin. So, in the kelvin scale, freezing point of water, is 273.15 degree kelvin, boiling point of water, is 373.15 degree kelvin.
Converting from Fahrenheit to celsius. Degrees celsius = open bracket, degree fahrenheit minus 32, close bracket, multiplied by 5 by 9. Examples. Open bracket, 32 degree fahrenheit minus 32 Close bracket, multiplied by, 5 by 9 . = 0 degrees ‘c’. So, 32 degree fahrenheit = 0 degree celsius.
Open bracket, 212 degree fahrenheit minus 32 Close bracket, multiplied by, 5 by 9. = 100 degrees ‘c’. So, 212 degree fahrenheit = 100 degrees celsius.
The normal human body temperature, is 98.6 degrees fahrenheit. Open bracket, 98.6 degree fahrenheit minus 32 Close bracket, multiplied by, 5 by 9. = 37 degrees ‘c’. So, 98.6 degree fahrenheit = 37 degrees celsius.
A thermometer, is used to measure temperature. A thermometer, can measure temperature, within a given range. A clinical thermometer, is used to measure body temperature. This is normally, about 37 degrees celsius, or, 98.6 degrees fahrenheit. The clinical thermometer, has mercury enclosed in a thin glass tube. When the temperature increases, the mercury expands. The thermometer is calibrated, to show the expanded mercury level, as degrees of temperature.
The temperatures, on earth varies depending on location, season, etc. The average temperature is about 15 degrees celsius. One of the hottest temperature recorded, was about 57 degrees celsius, in death valley California. The coldest temperature recorded, was about minus 89 degrees celsius, in Antarctica. The temperature in an incandescent lamp, is about 2200 degrees celsius. The temperature in the sun, is many million degrees. Surprisingly, the temperature in outer space, is about 2.7 degrees kelvin, which is about, minus 270 degrees celsius. This is close to absolute 0. We are aware that absolute 0, is the lowest temperature possible. When the temperature in the sun is many million degrees, It may cause us to wonder, why outer space is so ultra cold. This is because, outer space is a vacuum. It has no substances. The sun’s rays travel in outer space, without altering the temperature, because, there is no substance to heat up, in outer space. When the sun’s rays reach the earth, they heat up the atmosphere, the oceans, and the earth. The atmosphere, the oceans, and the earth, are substances, which are amenable of getting heated. This is how, the heat from the sun, is transferred to earth.
When a thermo nuclear weapon explodes, the temperature can reach, many million degrees. At this temperature, most substances evaporate. When the first nuclear bomb, was exploded over Hiroshima, thousands of people just evaporated.
Heat affects all matter, in a basic way. This happens at the molecular level. When heat is applied to a substance, the molecules get excited. That is, kinetic energy of the molecules increase. This happens to all substances. But the effect of heat, on a substance, varies from substance to substance. So scientists, have studied the effect of heat, of all commonly used substances. Every substance has unique thermal properties. The Thermal properties, of each substance, is well documented. We will discuss, some of the important thermal properties, of a substance.
The amount of introduced heat, to effect a given temperature change, is called the heat capacity of the material. If delta Q, is the increase in heat, and delta T, is the increase in temperature, then heat capacity, C, = delta Q by delta T. Heat capacity is expressed, in Joules per celsius.
The specific heat of a substance, is the quantity of heat, required to raise the temperature, of a unit mass of the substance, by a unit of temperature. Specific heat capacity = heat capacity of the body, divided by the mass of the body.
Specific heat is expressed as joules per kg degree kelvin. For example, the specific heat of water, is 4186 joules, per kg degree kelvin. This means, that it requires 4186 joules of heat, to raise the temperature, of 1 kg of water, by 1 degree kelvin or celsius.
The specific heat of Aluminium, is 900 joules per kg degree kelvin. This means that Aluminium, will heat up faster than Water. The specific heat of Gold, is 134 joules per kg degree kelvin. This means that Gold, will heat up much faster than Aluminium.
The property of a material, to conduct heat, is called thermal conductivity. Some such materials, are good conductors of heat. For example, Copper, Iron, Aluminium are good conductors of heat. Some material resists conduction of heat. They are also called as insulators. Examples are, porcelain, glass, wool, etc. We wear woollen clothing in winter, to help retain body warmth.
A substance can exists as a solid, liquid, or a gas. For example, water can exists as ice, water, or as water vapour. These are called, states of matter. When matter changes from one state to another, it is called a phase transition. For example, when water becomes water vapour, it is a phase transition. When water becomes ice, it is a phase transition. This process of phase transition, always involves heat. But there is a interesting, and an important difference, of the effect of heat. Normally heat results, in a change of temperature. In a phase transition, heat is transferred, without a change of temperature. For example, when water is boiled, to 100 degree celsius, water undergoes, phase transition to water vapour, at 100 degree celsius. There is no change in temperature, but the water vapour carries the heat. This heat is called the latent heat, of vaporisation. Similarly, when ice at 0 degree celsius melts, it undergoes a phase transition to water, at 0 degree celsius. There is no change of temperature, but heat is absorbed, from the water. This is called the Latent heat of fusion. Every substance will have, a unique latent heat of vaporisation, and fusion. This is measured by the specific latent heat, L. The specific latent heat, is the amount of heat, Q. required to effect a phase change, of a unit of mass, m, usually in kg. Latent heat, L = heat, Q, divided by mass,m. It is expressed in kilo joules, per kg. or, Q is = to m into L. Every substance will have a unique specific latent heat of fusion, and a specific latent heat of vaporisation. For example, the latent heat of water fusion, is 334 kilo joules, per kg. Latent heat of water vaporisation is 2260 kilo joules, per kg. In other words 2260 kilo joules of heat, is required, to convert 1 kg of water, at 100 degree celsius, to water vapour at 100 degree celsius.
Thermal expansion, is the tendency of matter, to change in volume, in response to change in temperature, through heat transfer.
Thermal expansion properties, vary from substance to substance. Every substance will have, a unique thermal expansion property. This property is measured, by the coefficient of thermal expansion. The coefficient of thermal expansion, of a given substance, is the degree of expansion, divided by the change in temperature.
A solid is characterised by structural rigidity, and resistance, to change of shape, or volume. The atoms in a solid, are tightly bound to each other. The atoms in the solid, vibrate. When a solid is heated, this vibration increases. Since this vibration is at the atomic level, we cannot see it. We can only feel the heat, of the solid.
Solids conduct heat by thermal conduction. Heat diffuses from one part of a solid, to other parts, in a linear way. If we heat one end, of a metal wire, the heat will diffuse to the other end. Capacity to conduct heat, is called as thermal conductivity.
Solids expand on heating. When this expansion, is measured in one direction, it is called as, the coefficient of linear expansion. The coefficient of linear expansion, is the increase in length, divided by original length, per degree kelvin. If a rod, of length L1 is heated, by one degree kelvin, expands to length L2, the coefficient of linear expansion, Alpha is =, L2 minus L1, divided by L1, per degree kelvin. For example, the coefficient of linear expansion of steel, is about 12, divided by 10 power 6, per degree kelvin. Let us take a railway track, 100 meters in length, and heat it, by 2 degree kelvin. So, the expansion will be 12 multiplied by 100, multiplied by 2, divided by 10 power 6 meters. To convert to millimetre, we can multiply by 1000, which will be 2.4 millimetres. So, a hundred meter railway track, if heated by 2 degree kelvin, will expand by 2.4 millimetres. This is the reason, engineers leave a small gap in the railway track, to allow for thermal expansion. You would have also noticed, electric wires sag in summer, due to thermal expansion.
A solid plate has a length and breadth, and an area, it also expands by heating. If the plate expands, when heated by 1 degree kelvin, from area A1 to A2, coefficient of area expansion, Beta, will be A2 minus A1, divided by A1, per degree kelvin. Coefficient of area expansion, Beta, is always twice, coefficient of linear expansion, Alpha. Beta is = 2 Alpha.
A solid object has volume. It also expands by heating. If a solid object expands, when heated by one degree kelvin, from volume V1 to V2, the coefficient of volume expansion, Gamma, Is V2 minus V1, divided by V1, per degree kelvin. Coefficient of volume expansion, Gamma, is always thrice, coefficient of Linear expansion, Alpha. Gamma is = 3 Alpha.
In liquid and gases, heat is conducted, through thermal convection. If we heat a vessel containing water, the temperature of the water, at the bottom of the vessel increases. These heated water molecules move up, and colder water on top, moves down. This process continues, till all the water reaches the same temperature. The process of thermal convection can be observed. There is a physical movement of water molecules. This kind of movement, is not possible in solids.
A similar process happens in gases also. When a gas is heated, its volume increases. Its density decreases. It becomes lighter. The heated gas, moves up. The colder gas, moves down.
Heat plays a major role on life on Planet Earth. Many factors are involved in this.
The sun is the source of energy, for all the planets in the solar system. The Earth is one of the planets, in the solar system. The Earth’s orbit around the Sun, is not too close to the Sun. That would have made the Earth, unbearably hot. It is also not too far away from the Sun. That would have made the Earth too cold. The Earth is just about the right distance from the Sun. It is said to be in the Goldilock Zone. Life does not exist, in any other planet in the solar system. Earth being in the Goldilock Zone, is blessed with life.
The largest source of heat, to planet earth, is sunlight. The earth receives, about 120 watts of solar radiation, per square meter. The total solar energy absorbed by the earth’s atmosphere, oceans, and land, is about, 385 thousand exajoules, per year. One exa joule is equal to 10 power 18 joules. More energy comes from sunlight in one hour, than the energy, used by the whole world, in one year. Photosynthesis captures about 3000 exajoules, per year, in bio mass. That is, plants and trees, absorb this energy, and convert it into biomass. This is a very small fraction, of the energy we receive from the sun. Most of the absorbed energy from the sunlight, is converted to heat energy. This plays a major role in life on the planet.
The atmosphere absorbs a significant portion of the sun’s energy. The earth rotates, on its axis, every 24 hours. So, we have day time and night time. During day time, we receive sunlight, or solar energy. During night time this is missing. Normally this would cause, a wild fluctuation, between day and night temperatures. Fortunately, the earth is surrounded, by a blanket of air, we call the atmosphere. The atmosphere acts as a giant temperature regulator, for earth. It helps in retaining the heat, that is absorbed. This is called the green house effect. Without the atmosphere, the earth will become extremely hot during the day, and extremely cold during the night. That is, without the atmosphere, we will boil during day time, and freeze during night time.
Maximum sunlight is received at the Equator. The North pole and the South pole, receive very little sunlight. The arctic region, around the North pole, is very cold through out the year. The antarctic region, around the South pole, is also very cold through out the year. The region between 33.5 degree North latitude, and 33.5 degree South latitude, is called the tropics. The tropics are close to the equator. Since this region, receives a lot of sunlight, It is relatively warm through out the year. Summer can be very warm in the tropics.
At higher altitudes, air pressure reduces. The temperature also drops, at higher altitudes. The temperature decreases, by about 6.4 degree centigrade, for every kilometre, of increase in altitude. That is why it is relatively cool, in hill stations. Mount Everest is more than 8 km in height. The peak of Everest is so cold, that it is always snow capped, through out the year.
The Earth orbits the sun, once in a year. The axis of the Earth is tilted. Due to this, the northern and southern hemisphere, receive differential sunlight, or solar energy. During the northern summer, the northern hemisphere, receives more sunlight. During the same time, the southern hemisphere, receives less sunlight. It experiences winter. The opposite happens, during the northern winter. In northern hemisphere, receives less sunlight, and experiences winter. During the same time, the southern hemisphere, experiences summer.
The weather keeps changing, from place to place, and from season to season. Local weather conditions, depend on many factors. Heat plays a major role, directly or indirectly, in determining weather conditions.
Due to the tilt in the axis of the earth, the air in the atmosphere, gets differentially heated, in the northern and southern hemisphere. This difference in temperature, causes air currents. Warm air moves to cooler regions, and vice versa. When the Air crosses, the oceans, it picks up moisture. This can precipitate as rain or snow. The air in the atmosphere, is constantly on the move, due to differences in temperature. These air currents have a pattern, which can be studied.
70% of the earths surface, comprises of oceans. The water in the oceans, act as a vast reservoir, of heat. It absorbs sunlight, and solar energy. This warms up the water in the ocean. This warming up also happens differentially. Some parts of the oceans warm up more, and some less. This causes ocean currents. Warm water moves towards cooler areas, and vice versa. The water in the oceans are constantly on the move. These are called ocean currents, and follow a pattern. Ocean current influences, weather conditions.
Weather is a resultant of many factors, which are interrelated, in a complex way. The study of weather, is a science by itself. It is called Meteorology. Meteorological scientists, use super computers to study weather. But even then, we still cannot predict weather accurately.
To sustain our life style, we need a lot of man made energy. Energy is required, by Households, and by Industry. We derive this energy, directly or indirectly, from fossil fuels. Examples of fossil fuels are coal, petroleum, and natural gas. Fossil fuels were created by nature, millions of years of ago. They are not renewable. They have potential thermal energy. When fossil fuels under go combustion, they release heat, or thermal energy. This thermal energy, can be converted, and used in a many ways. In vehicles like trains or cars, we burn diesel or petrol, to provide locomotive energy. Thermal power plants use coal or natural gas. This is burnt to heat water, and produce steam. Steam is used to drive turbines. These turbines produce electrical energy. Electrical energy, is the most widely used man made energy. Fossil Fuels are used to produce this energy. More than 86% of our man made energy requirements, comes from fossil fuels.
When fossil fuel is burnt, it releases polluting gases. Chief among this is carbon dioxide. Gases like carbon dioxide, and methane are called green house gases. The earth’s atmosphere, reflects about 30% of the energy it receives, back into space. Man made green house gases, accumulate in the atmosphere. These green house gases, reflect the heat, from the earth’s surface, back to earth. This effect is called as the green house phenomena.
Since the Industrial revolution, green house gases have been accumulating, in the atmosphere. This is causing increased intension of heat, in the earth. This is causing the Earth, to warm up, more than normal. Global warming, as this called, has many negative impacts, on the Earth’s environment. Global warming can cause extreme weather incidents. It can cause floods and drought. It can result in the rising of the sea levels of the oceans, which can cause Submersion of land, in coastal areas. Only now, countries have began to wake up, to the seriousness of the threat, of Global warming. Most countries, have now come to an informal understanding, to limit emission of green house gases, by 2020, which will limit global warming, to 2 degrees centigrade.
Scientists and Engineers, in the future, not only have to be, inventive and innovative, they also need to be, ecologically sensitive. Future development, for our own good, has to be eco friendly.