Lightning begins with a process that's less mysterious: the water cycle. To fully understand how the water cycle works, we must first understand the principles of evaporation and condensation.
Evaporation is the process by which a liquid absorbs heat and changes to a vapor. A good example is a puddle of water after a rainfall. Why does the puddle dry up? The water in the puddle absorbs heat from the sun and the environment and escapes as a vapor. "Escape" is a good term to use when discussing evaporation. When the liquid is subjected to heat, its molecules move around faster. Some of the molecules may move quickly enough to break away from the surface of the liquid and carry heat away in the form of a vapor or gas. Once free from the constraints of the liquid, the vapor begins to rise into the atmosphere.
Condensation is the process by which a vapor or gas loses heat and turns into a liquid. Whenever heat is transferred, it moves from a higher temperature to a lower temperature. A refrigerator uses this concept to cool your drinks and food items. It provides a low-temperature environment that absorbs the heat from your beverages and foodstuffs and carries that heat away in what is known as the refrigeration cycle. In this respect, the atmosphere acts like a huge refrigerator to gas and vapors. As the vapors or gases rise, the temperatures in the surrounding air drop lower and lower. Soon, the vapor, which has carried heat away from its "mother" liquid, begins to lose heat to the atmosphere. As it rises to higher altitudes and lower temperatures, eventually enough heat is lost to cause the vapor to condense and return to a liquid state.
Let's now apply these two concepts to the water cycle. Water or moisture on the earth absorbs heat from the sun and the surroundings. When enough heat has been absorbed, some of the liquid's molecules may have enough energy to escape from the liquid and begin to rise into the atmosphere as a vapor. As the vapor rises higher and higher, the temperature of the surrounding air becomes lower and lower. Eventually, the vapor loses enough heat to the surrounding air to allow it to turn back into a liquid. Earth's gravitational pull then causes the liquid to "fall" back down to the earth, thereby completing the cycle. It should be noted that if the temperatures in the surrounding air are low enough, the vapor can condense and then freeze into snow or sleet. Once again, gravity will claim the frozen forms and they will return to the earth.
http://science.howstuffworks.com/lightning.htm
In an electrical storm, the storm clouds are charged like giant capacitors in the sky. The upper portion of the cloud is positive and the lower portion is negative. How the cloud acquires this charge is still not agreed upon within the scientific community, but the following description provides one plausible explanation. In the process of the water cycle, moisture can accumulate in the atmosphere. This accumulation is what we see as a cloud. Interestingly, clouds can contain millions upon millions of water droplets and ice suspended in the air. As the process of evaporation and condensation continues, these droplets encounter many collisions with other moisture that is in the process of condensing as it rises. Also, the rising moisture may collide with ice or sleet that is in the process of falling to the earth or located in the lower portion of the cloud. The importance of these collisions is that electrons are knocked off of the rising moisture, thus creating a charge separation.
The newly knocked-off electrons gather at the lower portion of the cloud, giving it a negative charge. The rising moisture that has just lost an electron carries a positive charge to the top of the cloud. Beyond the collisions, freezing plays an important role. As the rising moisture encounters colder temperatures in the upper cloud regions and begins to freeze, the frozen portion becomes negatively charged and the unfrozen droplets become positively charged. At this point, rising air currents have the ability to remove the positively charged droplets from the ice and carry them to the top of the cloud. The remaining frozen portion would likely fall to the lower portion of the cloud or continue on to the ground. Combining the collisions with the freezing, we can begin to understand how a cloud may acquire the extreme charge separation that is required for a lightning stroke.
When there is a charge separation in a cloud, there is also an electric field that is associated with the separation. Like the cloud, this field is negative in the lower region and positive in the upper region.
The strength or intensity of the electric field is directly related to the amount of charge build-up in the cloud. As the collisions and freezing continue to occur and the charges at the top and bottom of the cloud increase, the electric field becomes more and more intense -- so intense, in fact, that the electrons at the earth's surface are repelled deeper into the earth by the strong negative charge at the lower portion of the cloud. This repulsion of electrons causes the earth's surface to acquire a strong positive charge.
All that is needed now is a conductive path for the negative cloud bottom to contact the positive earth surface. The strong electric field, being somewhat self-sufficient, creates this path.
The strong electric field causes the air around the cloud to "break down," allowing current to flow in an attempt to neutralize the charge separation. Simply stated, the air breakdown creates a path that short-circuits the cloud/earth as if there were a long metal rod connecting the cloud to the earth. Here's how this breakdown works.
When the electric field becomes very strong (on the order of tens of thousands of volts per inch), conditions are ripe for the air to begin breaking down. The electric field causes the surrounding air to become separated into positive ions and electrons; the air is ionized. Keep in mind that the ionization does not mean that there is more negative charge (electrons) or more positive charge (positive atomic nuclei / positive ions) than before. This ionization only means that the electrons and positive ions are farther apart than they were in their original molecular or atomic structure. Essentially, the electrons have been stripped from the molecular structure of the non-ionized air.
The importance of this separation/stripping is that the electrons are now free to move much more easily than they could before the separation. So this ionized air (also known as plasma) is much more conductive than the previous non-ionized air. Incidentally, the ability or freedom of the electrons to move is what makes any material a good conductor of electricity. Often times, metals are referred to as positive atomic nuclei surrounded by a fluid-like cloud of electrons. That makes many metals good conductors of electricity.
These electrons have excellent mobility, allowing for electrical current to flow. The ionization of air or gas creates plasma with conductive properties similar to that of metals. Plasma is the tool nature wields to neutralize charge separation in an electric field. Those readers who are familiar with the chemical reaction of fire will recall that oxidation plays an important role. Oxidation is the process by which an atom or molecule losses an electron when combined with oxygen. Simply put, the atom or molecule is changed from a lower positive potential to a higher positive potential. Interestingly enough, the process of ionization, which creates plasma, also occurs through the loss of electrons. By this comparison, we can view the ionization process as "burning a path" through the air for the lightning to follow, much like digging a tunnel through a mountain for a train to follow.
Earthquakes
Shaking Ground
An earthquake is a vibration that travels through the earth's crust. Technically, a large truck that rumbles down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to think of earthquakes as events that affect a fairly large area, such as an entire city. All kinds of things can cause earthquakes:
volcanic eruptions
meteor impacts
underground explosions (an underground nuclear test, for example)
collapsing structures (such as a collapsing mine)
But the majority of naturally-occurring earthquakes are caused by movements of the earth's plates, as we'll see in the next section.
We only hear about earthquakes in the news every once in a while, but they are actually an everyday occurrence on our planet. According to the United States Geological Survey, more than three million earthquakes occur every year. That's about 8,000 a day, or one every 11 seconds!
The biggest scientific breakthrough in the history of seismology -- the study of earthquakes -- came in the middle of the 20th century, with the development of the theory of plate tectonics. Scientists proposed the idea of plate tectonics to explain a number of peculiar phenomenon on earth, such as the apparent movement of continents over time, the clustering of volcanic activity in certain areas and the presence of huge ridges at the bottom of the ocean.
The basic theory is that the surface layer of the earth -- the lithosphere -- is comprised of many plates that slide over the lubricating athenosphere layer. At the boundaries between these huge plates of soil and rock, three different things can happen:
Plates can move apart - If two plates are moving apart from each other, hot, molten rock flows up from the layers of mantle below the lithosphere. This magma comes out on the surface (mostly at the bottom of the ocean), where it is called lava. As the lava cools, it hardens to form new lithosphere material, filling in the gap. This is called a divergent plate boundary.
Plates can push together - If the two plates are moving toward each other, one plate typically pushes under the other one. This subducting plate sinks into the lower mantle layers, where it melts. At some boundaries where two plates meet, neither plate is in a position to subduct under the other, so they both push against each other to form mountains. The lines where plates push toward each other are called convergent plate boundaries.
Plates slide against each other - At other boundaries, plates simply slide by each other -- one moves north and one moves south, for example. While the plates don't drift directly into each other at these transform boundaries, they are pushed tightly together. A great deal of tension builds at the boundary.
2007-03-27 02:55:16
·
answer #6
·
answered by Curly 4
·
0⤊
0⤋