[SOUND] [MUSIC] >> In this lesson, we're going to focus on energy resources that come from the Earth's system. So, to start with, I want to understand, or I want to discuss it, what energy is and how we use it? Then, we're going to get in to the concept of what a fossil fuel is, because as we'll see, fossil fuels are the main source of energy that we use at the moment. Then, in some of the later lectures, we'll get in to alternatives to fossil fuel, and we'll see that several of them are also utilizing materials that come from the Earth's system. So what is energy? Energy is something that gives an object or a system the capacity to do work. Energy can cause heat or light, energy can cause a system to change state. For example, it can turn liquid water into steam. And energy can cause an object to move. Now, fundamentally, there are several sources of energy in the Earth's system. The one that may first come to mind is energy that comes from the Sun. You can think of the Sun as being up in space. It radiates out energy that comes to Earth in the form of waves. Those waves can heat things directly, or in some cases, the waves get absorbed inside the Earth and then re-radiate back in the form of heat. But in any case, it's energy from the Sun that's striking the Earth and providing the capacity to do work. The sunlight also does something that you may not think of off the bat. And that is that energy from the sun drives photosynthesis and we'll see that that energy that drives photosynthesis can be stored either short-term or long-term in the Earth's system and provides the capacity to do work later on. Now, in addition to energy from the sun, we also have energy that is due to gravity. For example, if you think of a hill slope and you imagine that there's a boulder perched on top of that hill slope, that boulder has gravitational potential energy because it's up higher than the land surface below. Now if that boulder starts to move. And so, it tumbles down and eventually reaches the bottom. It transforms that potential energy into another kind of energy, kinetic energy, which again, is able to do work. It's maybe going to loosen up rocks that are underneath it, or it may push something that it strikes when it gets to the bottom. In other words gravity is able to provide a source of energy in the Earth's system. Now in terms of energy that we use in society, gravity is particularly important because it, as we will see is what drives rivers. Basically what is a river? It's a channel that carries water from a higher elevation to a lower elevation. And what's driving that motion is gravity. And it's the motion, in turn, that is able to drive water wheels. Is able to push boats along, to do many things of use. In addition to gravity being able to simply cause an object to move from one position to another, or to cause two objects to pull together. Gravity plays an important role, because together with the heat provided by the sun it is able to drive convection. Convection is a flow that happens because you have a mixture of denser material and less dense material. And the less dense material will be able to rise or float to get above the denser material. Now in the Earth's system the variations in density are maybe driven or are commonly driven by variations in temperature. So for example, if you have hotter material lower down, it will try your hotter air lower down, it will try to rise to get above colder air. And colder air, at a higher elevation, will try to sink to get below warmer air. That creates a circulation, which we know of every day as the wind. The wind of course, can drive windmills. The wind can drive other sources of energy. The wind is also really important because it drives the surface currents in the oceans, and that transfers a huge amount of energy around the earth. As you can see in this animation, the oceans are in constant motion, and the water in them circulates both in large currents, and in local eddies. What we're seeing is the surface flow, but there's also flow under the surface in the oceans. That flow is called Thermohaline Circulation. Because it is a consequence of density variations and therefore is a kind of convection. Thermohaline circulation is driven by the fact that colder water is denser than hotter water or warmer water and sinks, and it's also a consequence of the fact that saltier water is denser than less salty water and then also sinks. As we can see, there's an overturn of the oceans that takes place for example, up in the polar regions and is driven by convection, which in turn is driven by thermohaline density variations. Another source of energy in the Earth's system is the Earth's internal heat. You may recall that the Earth gets warmer and warmer as you go deeper and deeper. And that difference or that change is called the Geothermal Gradient. Now the geothermal gradient exists because the Earth is hot inside. Where did that heat come from? Well it turns out that a lot of that heat or some of that heat is left over from the formation of the Earth. At the time of Earth formation a large amount of mass came together, squeezed together. The process of squeezing that mass together caused it to warm up. Friction of particles as they burned against each other during that process caused it to warm up as materials sunk to the interior it transformed it's potential energy into kinetic and that caused the earth to warm up. The result of that is the Earth became very hot inside in fact early in its history it was warm enough to cause the surface of the earth to melt for an interval of time. But object the size of the Earth over the billions years of Earth history, should have cooled significantly by now. But it turns out that in addition to elements like carbon and iron and so forth that are inside the Earth, there are also a number of radioactive elements. What is a radioactive element? Well, it is an element in which the atoms are unstable. And every now and then decay to form data products. That decay process generates heat. That heat generation warms up the Earth. Now it turns out radioactive elements are not distributed randomly throughout the Earth. They're concentrated to some extent in some of the upper layers. So in effect the Earth has a thermal blanket around it. It's like having an electric blanket surrounding the planet. Keeping the inside warm. One of the key consequences of having a warm inside and a soft inside is that the mantle of the Earth is able to undergo convection. In effect, warmer, less dense material at the base of the mantle is able to rise very, very slowly on the other, of centimeters a year, up to shallower levels in the mantle. Also, the cooler material up at the shallower levels of the mantle, including cold lithosphere plates, is able to sink back to depth because it's denser. Near the surface, the heat inside the Earth is manifested by volcanic eruptions, in which large quantities of molten rock spew out on the surface or erupt into clouds of debris. And it can also heat ground water, to create hot water springs. In those hot water springs are one source of energy that we can also use in the earth system. We'll see how that's done a little later on. Now I've mentioned radioactive decay. Radioactive decay in itself is an energy source, because if we are able to collect, enough of the radioactive atoms, bring them together in a small area, that mass of material can generate so much heat that it can serve to drive nuclear reactors and unfortunately as well, nuclear bombs. Okay, so we've seen now that there are four principle components of energy in the Earth system. Energy from the Sun, which is generated ultimately by the fusion reactions that are taking place in the sun. Gravity, which is a body force that's part of nature. Any two masses exert a force on each other that pulls them together. The Earth pulls objects that are high and causes them to fall to lower elevation. We've also seen that there's energy that's caused by radioactive materials in the Earth and that to some extent that radioactivity, combined with heat left over from the planet, results in Earth's internal heat which is also a source of energy. [MUSIC]
