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Transcript
HW #3 (Due 9/16) 1. If for some reason all the air molecules in the atmosphere of the Earth instantly stopped moving, what would happen to our atmosphere. Explain. Galileo showed that all objects fall at the same rate, regardless of the objects mass. This means that anvils fall at the same rate as a pebble. This is no different for air molecules. IF the air molecules were to stop moving, then they would immediately fall to the ground. The atmosphere would be a very thin layer on the surface of the Earth. The atmosphere is only extended because the energy from the Sun heats the surface of the earth, which in turn imparts kinetic energy to the air molecules. 2. Explain why temperature is defined as the average kinetic energy of a gas, instead of just the kinetic energy of a gas. Even in a small container of gas there are unimaginable numbers of particles. A liter jug filled with air could potentially contain 1021 particles. These particles are constantly moving and changing their velocity as they collide with other particles. It would be virtually impossible to know the kinetic energy of all the particles. Furthermore it would not be a good idea to characterize the kinetic energy of a gas by the kinetic energy of any one particle, because a single particle could have virtually any kinetic energy. Fortunately, when dealing with a large number of particles, the kinetic energy changes very little, regardless of the motion of an individual particle. 3. What is the difference between potential energy, kinetic energy, and radiant energy? You can do this by way of an example. Make up a situation where these three type change from one to another. In your example explain what the total energy is like throughout the situation. I will use an example that we will see again for star formation. When a star begins to form out of a large, extended gas cloud, the gravity of all the particles in the cloud begins to pull inward. At first the gas molecules are not moving much but have a large potential energy. Just like a book held in the air. As gravity pulls the cloud together, the gas molecules begin to pick up speed and fall inward. This is a change of potential energy into kinetic energy. When the particles are moving very fast they begin to collide, accelerating electrons and causing EM radiation. The forming star begins to shine. This is a change of kinetic energy into radiant energy. In the process, total energy is conserved. So the radiating electrons slow up, and this allows the gas to collapse even farther. 4. Explain how an electron makes light. An electron has around it an electric field which stretches from the electron to infinity. When an electron encounters an external force, such as another charged particle it accelerates. The electric field lines near the particle continue to point toward the charge. But very far from the particle, the electric field doesn’t “know” that the particle has been moved. The far field remains pointing at the previous position of the particle. Because the near field and far field are not aligned, there is a kink produced in the electric field where the two come together. This kink propagate outward from the electron at the speed of light. The kink is a change in the electric field. A change in the electric field produces a magnetic field which in turns produces an electric field. This constant changing of the electric and magnetic fields is what propagates the wave outward. This is electromagnetic radiation, or light.