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Energy at the microscopic scale

These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as a combination of energy associated with the motion of particles and energy associated with the configuration (relative position of the particles). In some cases the relative position energy can be thought of as stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. Created by Khan Academy.

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Video transcript

- [Instructor] Welcome. Today, we're going to take a look at forms of energy, such as kinetic, electrical, thermal, gravitational, potential energy. It turns out when you start thinking about energy on smaller scales, or the microscopic level, all of these forms of energy are basically two things: one, kinetic energy, particles moving around, and two, potential energy, energy stored by a field, such as electric, magnetic or gravitational. Let's start with a deceivingly simple example and explore properties of energy as we zoom in. I'm drawing a glass of water. This water has a temperature. I can warm up the water by adding energy from an electric stove, or cool down the water by putting it in the fridge, thus removing energy in the liquid water. From this example, we can see that temperature is related to energy. But how is it related to kinetic or potential energy? Hmm. This big picture, the macroscopic scale, allows us to look at energy in terms of temperature. But let's go to the microscopic or small-scale to get a better picture of the physics at play. As we look inside this liquid, we see lots of water molecules moving around. Zoom, zoom, zoom. Well, they don't actually make a sound. The average speed at which these molecules move is related to their kinetic energy and its temperature. This is thermal motion. If we zoom in again, we can look into the strong chemical bonds within molecules. Here, the individual atoms, in this case for water, is hydrogen and oxygen. They can vibrate back and forth and rotate, so they also have kinetic energy. Let's look at another example between macro and microscale energy interactions. When you burn something, think of a fire. A chemical reaction takes place, and it releases a lot of energy. How does it do that? Let's look at the microscopic scale to find out. In this example, I'm gonna burn methane gas, and the chemical reaction that takes place is methane, CH4, and oxygen, O2. They rearrange to create water, H2O, and carbon dioxide, CO2, plus energy. Before the reaction, there is a greater chemical potential energy than afterwards. But don't worry. Energy is still conserved because that potential energy is converted to kinetic energy and radiation. What is the source of chemical potential energy? At this level, we can think of individual bonds between atoms storing energy so that energy can be absorbed or released as bonds are broken and reformed. But where does the energy and chemical bonds come from? We need to zoom in again. In a single atom, there's a nucleus that contains protons and neutrons and overall has a positive charge. This creates an electromagnetic field. The interaction of other charged particles, like negatively charged electrons relatively far away from the nucleus, with this field provide potential energy. You can think of this electric potential energy as the same kind of concept as a potential energy of a mass and a gravitational field. Zooming back out, each molecule has its own particular configuration of charged particles within the electromagnetic fields, right? This means it has an associated potential energy. As we've seen, this chemical potential energy is the result of energy stored in fields. Okay, so we've covered electrical, chemical, thermal types of energy, but there's other forms out there, right? For instance, what about sound waves? Here's a speaker and an ear. The energy in sound waves is transferred through the vibrations, a back and forth motion of molecules in the air. Another example could be elastic potential energy or the energy stored in a spring. At the microscopic level, as you stretch the spring, see the hand stretching the spring? The atoms are being pulled out of their equilibrium position within a solid and thus gain potential energy from the electromagnetic force that holds the solid together. How neat is this? We can describe all these energies as just kinetic or potential. So there's one more microscopic form of energy that we need to talk about, and it might seem a little complicated at first. Let's go back to the combustion example we talked about earlier. As I said before, this process releases radiant energy. We can see burning objects. They glow brightly. We can also put our hand near the burning object. Don't touch it! And feel the radiant heat. This radiation that's emitted carries energy with it. So how do we explain the radiant energy? Does it fit it into one of these two categories, either kinetic or potential energy that we've been discussing? Well, it turns out it sort of fits into both groups. Whoa, electromagnetic radiation, such as light, can be modeled in a couple of different ways, which we'll go into more detail in another video. But one way to model the light is as a wave of electric and magnetic fields. Another way to think about light is being made up of particles called photons. In this instance, the particles are carrying the energy. So with both of these models, radiant energy can be explained by the same microscopic interactions that cause the other forms of energy. In conclusion, we can see energy at the macroscopic scale, like temperature or light being emitted. However, we must look at the microscale to observe the different forms of energy that we experience are really just the result of kinetic and potential energy of particles. How cool!