Download HW #3 (Due 9/16)

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.