Active Oldest Votes. Improve this answer. Ignacio Vergara Kausel Ignacio Vergara Kausel 2, 1 1 gold badge 17 17 silver badges 20 20 bronze badges. A lot of materials absorb visible radiation too e. Radiation is fun! Ryan Ryan 1 1 1 bronze badge. In a crystal this would be called a phonon emisison; I'm not sure what it is called in a big water bag.
Think about how a CO2 laser works. Featured on Meta. Now live: A fully responsive profile. Related 5. Hot Network Questions. A Joule is the amount of energy released by a g apple that falls a distance of 1 m. A kWh is the amount of electricity used by ten watt incandescent light bulbs for an hour. Another measure of heat is the calorie. We can think of heat by the kind of work it does when it is transferred from one object to another.
Sensible heat is that which causes the temperature of an object to increase. But does adding heat always cause the temperature to increase? Adding heat to an ice cube may cause some of it to melt, but the water changed from solid to liquid may still be at the same temperature of 0 o C.
When the temperature remains constant, but the added or removed heat causes a change in state, this is called latent heat. Recall that a change of state occurs when substances change forms between the conditions of being a solid, liquid, or gas. Generally it takes a lot more heat transfer to do a change of state than it does to simply raise the temperature of an object.
For example, it takes 1 calorie to raise the temperature of 1 gram of liquid water by 1 o C. Once at a temperature of 0 o C, For a gram of liquid water that is at o C, it takes another calories to change it into water vapor.
As you will see below, by using change of state rather than just change of temperature, much more energy can be stored or released from a system designed to do useful work. Heat always flows spontaneously from an object of higher temperature to an object of lower temperature. When the objects are touching, the heat is flowing by conduction.
If you place your hand on a hot cup of coffee, heat from the coffee will flow to your hand. If instead you place your hand near the hot cup of coffee, say 2 cm away, you will still feel your hand getting warmer. The infrared rays of heat energy are flowing away from the cup, and you are feeling them on the skin of your hand. When energy flows through space like this, it is called radiation. That is exactly how the energy travels from the sun to Earth, by solar radiation.
Heat can also move from one place to another by being carried in a moving fluid liquid or gas. This is called convection. Passive convection occurs when a warm object transfers heat to a fluid, and as a result the fluid becomes less dense, and floats up. The air above the hot cup of coffee is warmed by conduction and radiation. Being warmer, it becomes less dense and floats up, being replaced by cooler air that slides in to take its place. On a much larger scale this is what happens in the atmosphere and oceans, and it is how heat is transported around Earth.
Active convection occurs when a force is applied to move the fluid that is carrying the heat. We use electric-powered fans to circulate heated air in our houses. The rate of heat flow between objects is proportional to the difference in temperature between them. When there is a big difference, heat flows fast.
If the temperature difference is small, the flow of heat is reduced. Consider a hot pot taken off the stove and placed on a tile countertop. At first it cools quickly. As it cools, the difference in temperature between the pot and the room air becomes less, and so it takes a long time for the pot to lose enough heat to match room temperature. Two objects at the same temperature are said to be in equilibrium. At that point any heat gained by the pot from the air is equal to the same amount of heat lost by the pot to the air.
What is sunlight and how is it produced? The sun is our star. It is a massive ball of dense gases, mostly hydrogen The vast gravity of the sun packs together the gases in the core so tightly that it causes 4 hydrogen atoms 1 proton and 1 electron to fuse together to produce 1 helium atom 2 protons, 2 neutrons and 2 electrons and energy.
The energy released comes from a loss of a small amount of mass during the fusion process. This nuclear fusion results in the release of a large amount of energy.
The released energy includes heat infrared radiation , visible light, ultraviolet light, and various high energy rays and particles. See an animation of fusion in the sun. Converting Sunlight to Heat. Energy travels in waves. The distance between one wave top to another is called the wavelength. The wavelength determines the kind of energy.
Heat infrared has a longer wavelength than visible light. What is the wavelength in nanometers nm for infrared? Einstein showed that while light travels in waves, it also is made of particles, called photons.
The energy associated with a photon is determined by its wavelength. Shorter wavelength waves of photons have more energy per photon than longer wavelength photons. When sunlight hits an object, it can be reflected or absorbed. If it is reflected it bounces off at the same wavelength. But if it is absorbed, the short wavelength energy is changed to long wavelength heat.
One reason that Earth can support life is that it is very warm, considering its distance from the sun. The average temperature at the surface for the entire earth is 15 o C. The moon is the same average distance from the sun as is Earth.
Yet the moon has a much lower average surface temperature of o C. Recall that water freezes at 0 o C, room temperature is 22 o C and water boils at o C. So why is Earth so much warmer than the moon? The mass of the moon is only 1. Since gravitational force is proportional to mass, the moon has much less gravity than Earth and cannot hold on to a gaseous atmosphere. It is Earth's atmosphere that keeps it warm. The infrared energy coming from the sun is not enough to keep Earth as warm as it is.
Energy from visible light and ultraviolet light has to play its part, too. Much of what is reflected back to space is visible light, which is why Earth appears as a glowing blue and white globe when photographed from some distance away.
When visible light is absorbed by an object, the object converts the short wavelength light into long wavelength heat. This causes the object to get warmer. But this is only part of the story as to why Earth is warm. Something has to keep that heat from quickly radiating back out to space. What keeps the heat from radiating away so quickly? It is greenhouse gases. The most important of these are carbon dioxide, methane CH 4 , water vapor and nitrous oxide N 2 O.
These are called greenhouse gases because they let the short wavelength visible light pass through the atmosphere, but block much of the long wavelength heat infrared energy from escaping. These gases provide the same function as does the glass in the roof and walls of a greenhouse used to grow plants in cold climates. Sunlight passes through the glass and it is absorbed by the plants and other objects in the greenhouse. When absorbed, the short wavelength sunlight is converted to long wavelength heat.
The glass walls and roof keep much of the heat from escaping. Anybody opening a closed automobile that has been sitting in sunshine on a clear cold day has experienced the greenhouse effect firsthand. The spaces between N2 and O2 molecules in the atmosphere are large enough to let both long wavelength and short wavelength radiation pass through.
Although CO 2 accounts for only a tiny fraction 0. The longer wavelengths essentially cannot fit through the distance between CO 2 molecules. The other greenhouse gases have the same effect. As mentioned at the beginning of this unit, burning of fossil fuels since the start of the Industrial Revolution has significantly increased the level of CO 2 in the atmosphere.
In , near the start of the Industrial Revolution, the CO 2 concentration in the atmosphere was about parts per million ppm. In May of , CO 2 concentrations exceeded ppm, the highest level in the past 3 million years. Accordingly, Earth's average temperature increased 1 o C over the last century.
This is the reason for the ongoing global climate change. In , Swiss-French scientist Horace Benedict de Saussure built the first solar collector to make use of the greenhouse effect. He noted that the closed carriages of the day that had glass windows would get warm even on cold days — just like your experience with automobiles. He built boxes covered with layers of glass that had black cork in them to absorb the light. He recorded temperatures above o C. URL for image of image of de Saussure's hot box.
Interestingly, de Saussure put his "hot box" to work as a scientific instrument. He was interested in discovering why it is generally colder at higher latitudes.
He brought the box to the top of a mountain to measure the maximum temperature produced, and repeated the procedure the next day on the low altitude plain. The box reached the same temperature at both locations, despite the outside air of the plain being 43 o F warmer than on top of the mountain.
From this he concluded that the thicker blanket of air overlying the plain provided more insulation than the thinner atmosphere of the mountaintop. The hot box concept was put to practical use by astronomer Sir John Herschel who was on an expedition in South Africa in the s. He built a solar hot box to cook meals. Solar cookers There are two different approaches to using solar energy to cook food. Solar concentrators use either mirrors or lenses to collect sunlight from a larger area and focus it on a smaller area where the cooking takes place.
Greenhouse-style solar ovens use the system invented by de Saussure discussed above. These have glass doors that allow sunlight in, but seal tightly to minimize the escape of heat. The interior of the cookers is black to maximize the absorption of light. Some of the greenhouse-type cookers also have attached reflective mirrors to help concentrate the light. Solar cookers require no fuel. There are two advantages to this.
Fuel is often scarce in poor countries. Kerosene is expensive and firewood, charcoal, dried manure, etc. Second, cooking fuels often burn in a very dirty way, causing much soot and smoke. This creates real health problems, particularly for women and children in countries with traditions of cooking in houses that have poor ventilation.
They state, "The World Health Organization WHO estimates that exposure to smoke from the simple act of cooking is the fifth worst risk factor for disease in developing countries, and causes almost two million premature deaths per year — exceeding deaths attributable to malaria or tuberculosis. In addition, tens of millions more fall sick with illnesses that could readily be prevented with increased adoption of clean and efficient cooking solutions.
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A kitchen can be blackened by soot from an indoor stove that has no chimney. These reactions generate copious energy that slowly migrates out toward the photosphere and then into space. Astronomers believe that the Sun formed approximately five billion years ago, at which time it initiated these reactions. They have continued ever since. As the Sun ages, it slowly grows hotter due to the accumulation of residual energy emitted by these core reactions. Consequently, the solar constant, the energy Earth receives from the Sun, would have been correspondingly lower.
Throughout the subsequent billions of years, the Sun's luminosity increased gradually and will continue to increase in the future. This increase might seem slight, but it will render Earth inhospitable to life in about 1.
The planet will be too hot to support life. When stellar astronomers first understood the Sun's energy generation mechanism, they believed that Earth's life would survive until the Sun expanded into the red giant stage. Today they know that our time is much shorter, albeit still more than one billion years.
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