What Is A Nuclear Reaction?


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Photovoltaic Cells, Solar Power, and LEDs

Most of the world’s energy can go back to our sun. Every day we are heated by its electromagnetic rays, and plants use the sun’s energy to make sugars and ultimately proteins and other good things to eat. Fossil fuels were also once made from these plant and other organisms that relied on the sun’s energy millions of years ago. Today, humans can convert the sun’s energy directly into electricity, through solar thermal and solar photovoltaic systems.


Solar panels, also called solar thermal, convert sunlight to heat and then heat to electricity. Photovoltaic cells, or solar cells, convert sunlight directly into electric current by way of carefully-engineered semiconductor materials.

Though solar photovoltaics are more efficient converters of sunlight, they are also more expensive.

As of May 2011, the world’s largest solar power plant is a concentrating solar thermal power plant in the Mohave desert in California. Solar Energy Generating Systems has a capacity of 310 megawatts and uses parabola-shaped reflective troughs to concentrate electromagnetic radiation.

The world’s largest solar photovoltaic plant is probably the Sarnia Solar Project in Ontario, Canada. It has a capacity of roughly 80 megawatts.


Sunlight heats a design element (water, air, chemical fluids), and that thermal energy is transmitted for other applications, such as heating water, heating space, or generating electricity. In solar thermal power plants, sunlight heats a specialized fluid, which in turn heats water into steam, which can run turbines and produce electricity.

Solar thermal power plants use concentrators that bounce the sunlight off elliptical mirrors to a central tube, in which the specialized fluid lies.


Photovoltaic cells are made of specialized diodes. Electrons (natural components of atoms) in the photovoltaic cells absorb light, which excites them to a state where they can be conducted as electrical current. This difference in energy, between the valence band (the state of a normal electron staying around its home atom) to the conduction band (electron free to move between atoms) is called the band gap.

Solar photovoltaic farm in Indonesia. Photo by Chandra Marsono.

Well-engineered photovoltaics have a band gap that coincides with the energies of as broad a spectrum of light as possible, to convert the maximum amount of the sunlight into electricity.

As sunlight energy pops electrons into the conduction band and away from their home atoms, an electric field is produced. The negatively-charged electrons separate from the positively-charged “holes” they leave behind, so that when electrons are freed into the conduction band, they move as electric current in the electric field, electricity.


An ever-expanding variety of semiconductor materials can be used to make solar cells; universities and companies worldwide are researching these options, from special bio-plastics to semiconductor nanocrystals. Nonetheless, the photovoltaic cells available today require precise manufacturing conditions and are therefore far more expensive to produce than solar panels.

Silicon has to be processed under clean room conditions — carefully regulated atmospheres — to remove impurities and prevent introducing contaminants, both of which can change the band gap. Thin film-based photovoltaics require special production methods, like chemical vapor deposition. Semiconductor processing also uses strong acids and often dangerous chemicals for etching.

Today, commercially-sold cells are made from purified silicon or other crystalline semiconductors like cadmium telluride or copper indium gallium selenide.


Silicon is plentiful in the Earth’s crust. Cadmium is a readily available but highly toxic heavy metal, as is arsenic, another chemical used in some cells. As tellurium demand is only recently rising in response to solar demand, it’s unknown what the global supply is for this unusual element but it may be quite abundant. Photovoltaics are a lively area of research, and the future production and environmental costs of starter materials, production, and pollution are difficult to predict.

California, Massachusetts, Ohio, and Michigan produced the most photovoltaics in 2009. However, that year, 58 percent of photovoltaics were imports, primarily from Asian countries like China, Japan, and the Philippines.


Photovoltaic cells work in the opposite direction of light-emitting diodes, or LEDs. LEDs are used interchangeably with other lighting, like light bulbs. However, LED’s work in a completely different manner, far closer to the way photovoltaics work.

Click here see a bar chart comparing how much energy is used by various light sources.

LEDs absorb energy in the form of electricity, exciting electrons into the conduction band. When the electrons in the semiconductor material drop back into the valence band from the conduction band, they emit energy in the form of photons, or electromagnetic radiation.

It’s a highly efficient process because energy isn’t wasted on producing heat, which happens with standard tungsten filament bulbs. LEDs also last a much longer time as they do not have filaments to burn out, and because they are very small and several units are used to replace one large traditional lamp, they do not all burn out at once. That makes LEDs a good choice for stoplights or other safety critical applications.


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Thermodynamics and Thermal Energy

Thermodynamics is the study of how energy moves and changes form, usually by way of heat, as suggested by the components of its name: thermo-dynamics. Its laws and equations help to predict what could happen in various situations, based on the temperature, pressure, materials, and shape of a system.

Thermodynamics tells us how to calculate the ultimate temperature of a refrigerator or how much energy we can get out of a steam engine. Thermodynamics can also be applied to chemistry and the world on an atomic level, predicting which compounds are stable at specific temperatures and pressures. Thermodynamics explains why diamonds form naturally and spontaneously from carbon-based compounds deep inside the Earth, but they cannot form spontaneously here on the surface.

Thermodynamics relies on the idea that energy is conserved, even if it is transferred from or to a system to its surroundings through heat, changes in momentum, or other forms of energy.



Heat and thermal energy are directly related to temperature. We can’t see individual atoms vibrating in solids, liquids, and gases, but we can feel their kinetic energies as temperature. Atoms in solids, liquids, and gases do vibrate. If they didn’t, they would be at absolute zero, a theoretical state of zero thermal energy at ­-459.67 Fahrenheit.

When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat. Something doesn’t have heat. Instead, as an object or system gains or loses heat, it increases or decreases its thermal energy.

Adjacent objects that exhibit different temperatures will spontaneously transfer heat to try to reach the same temperature as each other, or equilibrium. However, how much energy it takes to change the temperature of an object is based on what its made of, a property called heat capacity or thermal capacity.

Water has a higher heat capacity than steel, for example. An empty pot on the stove takes almost no time to get to 212 degrees Fahrenheit, or the boiling temperature of water. A pot with some water in it will take far much longer to reach the same temperature, because water needs to absorb more energy — per weight, per degree — to gain the same number of degrees as metal. (Even though the vaporization temperature of metal is far, far higher than the water’s).



Thermal energy storage exploits the difference in temperature between a system and the environment. In the late 1800s, Americans used thermal energy storage by cutting blocks of lake ice during the winter and storing them underground packed in insulating wood shavings. When the summer rolled around, they retrieved that stored ice to make food cold, exploiting the difference in temperature to force thermal energy out of the food and into the ice.

Thermal energy storage can also happen in the other direction. Electricity or other forms of energy can be used to heat various materials, which are stored in insulated containers. Later, when the energy is needed, the hot materials can heat water into steam, and that steam can push turbines, which in turn produce electricity.

Solar panels use thermal energy storage. The panels absorb the heat of sunlight and store that energy so it can be transformed into electricity with turbines. There are several kinds of solar panels, but all rely on heat for energy, unlike photovoltaic cells.

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Forms of Energy: Motion, Heat, Light, Sound

What forms of energy is Raul using to move his LEGO car?

When he was a teenager in Romania, Raul Oaida became obsessed with building things: a jet-engine bike, a tiny spaceship, a LEGO car that runs on air. Why? Well, why not?

You can see more cool stories about energy at The Adaptors website.

Like video and audio? Check out The Adaptors Podcast.


Energy comes in two basic forms: potential and kinetic

Potential Energy is any type of stored energy. It can be chemical, nuclear, gravitational, or mechanical.

Kinetic Energy is found in movement. An airplane flying or a meteor plummeting each have kinetic energy. Even the tiniest things have kinetic energy, like atoms vibrating when they are hot or when they transmit sound waves. Electricity is the kinetic energy of flowing electrons between atoms.

energy_forms_pie-chartEnergy can shift between forms, but it is never destroyed or created.

A car transforms the potential energy trapped in gasoline into various types of energy that help the wheels turn and get the car to move. Most of the energy is converted to thermal energy, which is an unorganized form of energy that is difficult to convert into a useful form.

Power plants transform one form of energy into a very useful form, electricity. Coal and natural gas plants use the chemical potential energy trapped in fossil fuels. Nuclear power plants change the nuclear potential energy of uranium or plutonium into electricity too. Wind turbines change the kinetic energy of air molecules in wind into electricity. Hydroelectric power plants take advantage of the gravitational potential energy of water as it falls from the top of a dam to the bottom.

These transformations are hardly perfect. An efficient fossil fuel power plant loses more than half of the energy it creates to forms other than electricity, such as heat, light, and sound.

Forms of Potential Energy


Systems can increase gravitational energy as mass moves away from the center of Earth or other objects that are large enough to generate significant gravity (our sun, the planets and stars).

For example, the farther you lift an anvil away from the ground, the more potential energy it has. Lifting the anvil is called work, which is an interaction in which energy is transferred from one system (the person) to another (the anvil). The person has to do more work in order to carry the anvil higher, and the higher the anvil is carried, the more gravitational potential energy is stored in the anvil. If the anvil is dropped, that potential energy transforms to kinetic energy as the anvil moves faster and faster toward Earth.


Chemical energy is stored in the bonds between the atoms in compounds. This stored energy is transformed when bonds are broken or formed through chemical reactions. Like letters of the alphabet that can be rearranged to form new words with very different meanings, atoms move around during chemical reactions, and they form new compounds with vastly different personalities.

When we burn sugar (a compound made of the elements hydrogen, oxygen, and carbon) in our bodies, the elements are reorganized into water and carbon dioxide. These reactions both absorb and release energy, but the overall result is that we get energy from the sugar, and our bodies use that energy to do work.

Chemical reactions that produce net energy are exothermic. When wood is burned, the chemical reactions taking place are exothermic. Electromagnetic and thermal energy are released. Only some chemical reactions release energy. Endothermic reactions need energy to start and to continue, such as by adding heat or light.


Today’s nuclear power plants are fueled by fission. Uranium or plutonium atoms are broken apart, freeing lots of energy. Hydrogen atoms in the sun experience nuclear fusion, combining to form helium and subsequently releasing large amounts of energy in the form of electromagnetic radiation and thermal energy.

Nuclear energy is the stored potential of the nucleus of an atom. Most atoms are stable on Earth; they keep their identities as particular elements, like hydrogen, helium, iron, and carbon, as identified in the Periodic Table of Elements. The number of protons in the nucleus tells you which element it is. Nuclear reactions change the fundamental identity of elements by splitting up an atom’s nucleus or fusing together more than one nucleus. These changes are called fission and fusion, respectively.


Elastic energy can be stored mechanically in a compressed gas or liquid, a coiled spring, or a stretched elastic band. On an atomic scale, the stored energy is a temporary strain placed on the bonds between atoms, meaning there’s no permanent change to the material. These bonds absorb energy as they are stressed, and release that energy as they relax.

Forms of Kinetic Energy


A moving object has kinetic energy. A basketball passed between players shows translational energy. That kinetic energy is proportional to the ball’s mass and the square of its velocity. To throw the same ball twice as fast, a player does more work and transfers four times the energy.

rotationalIf a player shoots a basketball with backspin or topspin, the basketball will also have rotational energy as it spins. Rotational energy is proportional to how many times it spins per second, as well as the ball’s mass, and the size and shape of the ball.

In shooting a basketball, players often try to add rotational energy as backspin, because it results in the greatest slowdown in speed when the basketball hits the rim or the backboard, increasing the chance that the ball stays near the basket. The opposite direction of spin, a topspin, can be used in games like tennis, because it will help speed up a ball after impact and lowers the angle it travels after the bounce.


Thermal energy is directly related to temperature. We can’t see individual atoms vibrating, but we can feel their kinetic energies as temperature. When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat.

tea kettleA hot cup of tea loses some of its thermal energy as heat flows from the tea to the air in the room. Over time, the tea cools to the same temperature as the room air. At the same time, the thermal energy in the room air increases due to heat transfer from the tea. However, the thermal capacitance of the room air is much larger than the tea, so the temperature of the air in the room increases by very little – so little that a person in the room wouldn’t notice it.

Heat  flows spontaneously from high temperature objects to nearby low temperature objects, until all objects reach the same temperature, called thermal equilibrium. Some materials are easier to heat up or cool down than others. The thermal capacitance, or heat capacity, of a material tells us how much energy it takes to raise that material one degree in temperature. A pound of water has greater thermal capacitance than the same amount of stainless steel, for example. In moments, an empty one pound pot on the stove heats to 212 degrees Fahrenheit (the boiling temperature of water). If you pour a pound of water into the pot, it will take much longer than the empty pot to reach the same temperature, because water needs more energy to get as hot as steel.


Sound waves are made when stuff vibrates – like strings on an instrument or gas molecules in the air. Sound waves travel when the vibrating stuff causes stuff surrounding it to also vibrate. This happens in liquid, solid, or gaseous states. Sound cannot travel in a vacuum because a vacuum has no atoms to transmit the vibration.

Solids, liquids, and gases transmit sounds as waves, but the atoms that pass along the sound don’t travel the way photons do. The sound wave travels between atoms, like people passing along a “wave” in a sports stadium. Sounds have different frequencies and wavelengths (related to pitch) and different magnitudes (related to how loud).

Even though radio waves can transmit information about sound, they are a completely different kind of energy, called electromagnetic energy.


PlantElectromagnetic energy is the same as radiation or light. This type of energy can take the form of visible light, like the light from a candle or a light bulb, or invisible waves, like radio waves, microwaves, x-rays and gamma rays. Radiation — whether it’s coming from a candle or an x-ray tube — can travel in a vacuum. Sometimes physicists describe electromagnetic radiation as being composed of particles – tiny packets of energy called photons. Each photon has a characteristic frequency, wavelength, and energy, but all photons travel at the same speed, the speed of light, or nearly 1 billion feet per second.

Electromagnetic energy can be converted to the chemical energy stored in plants through photosynthesis, the process by which plants and algae use the sun’s radiation to turn carbon dioxide gas into sugar and carbohydrates.


Electric energy is to the kinetic energy of moving electrons, the negatively-charged particles in atoms. For more information about electricity, see Basics of Electricity.


-Anrica Deb


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Petroleum, Natural Gas, and Coal

The world depends on fossil fuels for its energy, and the United States is no exception. The vast majority of U.S. energy — more than 80 percent in 2009 — comes from burning fossil fuels. (more…)

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Historical Events in Nuclear Fission

As is the case with so many scientific fields, the history of nuclear physics and energy development has always been wrapped up with the history of modern warfare.

An unprecedented level of research went into the American bomb program, applying a rapidly evolving understanding of nuclear physics immediately to building a weapon. That investment spurred the rest of the world to pursue nuclear fission, often using the energy as an excuse for the weapons development. Rather than isolate nuclear energy from its less peaceable counterpart, the timeline incorporates all types of nuclear history.

Recent nuclear media coverage:

Germany begins shutting down old reactors and considers swearing off nuclear power entirely. Germany Dims Nuclear Plants, but Hopes to Keep Lights On.

New evidence that Japan’s troubled reactors were destined to malfunction, tsunami or not, in The Explosive Truth Behind Fukushima’s Meltdown.

Add more here. TK

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