Week of September 10th

Week of September 10th - Class notes for for week of...

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Class notes for for week of September 10 th The four fundamental forces We discussed the four fundamental forces of classical physics: The strong force, responsible for holding the quarks in subatomic particles like the proton together. The weak force, responsible for holding the atomic nucleus together The electromagnetic force, responsible for holding atoms together, as well as bonding atoms together into molecules, and molecules together into solids. This is the force that mediates the attraction between positive and negative charge, and is the basis for all of electrical engineering, from radio to computers. Gravity, the very weak force that nevertheless holds the stars and planets together, and keeps us from floating off into space. These forces differ in their strength (they are listed in stronger to weaker order above) and effective range. The strong and weak forces only act within the atomic nucleus and are of little relevance to our task at hand. Both the electromagnetic and gravitational forces act over large distances, and they also share some common characteristics: The forces due to gravity, F g , and electromagnetism, F e , are described by similar looking equations. Both depend on the inversely on the distance between the two bodies under consideration, d , squared. Both contain a physical constant, G , for gravity, and (4πε 0 ) -1 for the electromagnetic force. Both also depend on the product of some physical property of the two bodies, the mass, m (measured in kilograms), for gravitation and the charge, q (measured in coulombs) , for the electrical force. A key difference is that mass is only positive, while the charge can be either positive or negative. This has important implications: the gravitational force is always attractive, while the electromagnetic force can be either attractive or repulsive. Since the universe is equally populated with positive and negative charges, static electromagnetic forces tend to average out to zero over long distances. This is why gravity dominates over very large distances even though it is much weaker than the electromagnetic force. Energy and power We all know that physical effort is require to move heavy objects up hills. This physical effort is a form of work or energy . Energy comes in a large number of forms, and can be converted between these forms (with varying degrees of efficiency, depending on the technology). In the case of moving a rock up a hill, we have converted the chemical energy of our food into the potential energy of a rock at the top of a hill. If the rock rolls down the other side of the hill, its potential energy is converted to kinetic energy
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associated with its motion. When the rock finally comes to a halt at the bottom of the hill, it is because its kinetic energy has been converted to thermal energy (heat) due to friction with the ground. Energy is conserved and can not be created or destroyed. A gasoline engine is about 30%
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Week of September 10th - Class notes for for week of...

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