lecture5-09-thermal - 1 H A&S 222d Introduction to Energy...

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1 H A&S 222d Introduction to Energy and Environment: Life Under the Pale Sun P.B.Rhines, M Ewert Spring 2009 Lecture 5 THERMAL ENERGY SCIENCE CORE: PHYSICS OF ENERGY, continued We are beginning to fill in the list of ‘forms of energy’ and to give examples of the magnitudes. ..the amounts. ..of energy present in familiar objects. The kinetic energy in a 1 kg. rock moving at 1 meter/sec is ½ Joule. As shown earlier, Ch. 2 sec. 2.11, a flowing river can contain a large kinetic energy by virtue of its size. ..its mass (although there it makes sense to consider the power flowing past an observation point on the river, as well as the kinetic energy per kg. of water). We will see again that thermal energy is ‘rich’, or concentrated and chemical energy is even more so. THERMAL ENERGY AND THE HEAT ENGINE Thermal energy…’heat’…can be transformed into mechanical energy or vice-versa. Heat can move by radiating light or invisible, infrared radiation. But some aspects of thermal energy are quite simple. Earlier we introduced the 1 st law of thermodyamics, and mentioned the idea of an engine that produces mechanical energy by converting some thermal or chemical energy. If you squeeze a gas it becomes warmer…instantly. This happens because ‘squeezing’ means exerting a force and changing the volume of gas. Recall that exerting a force on a moving object requires work in the amount: work = force x distance traveled….Joules and this corresponds to: power = force x velocity of the moved object … Watts. This is equal to the change in energy of the moving object. In our case the object is the gas, and compressing it requires work, and the work goes into internal heat energy of the gas: it warms up. Thinking of a gas like air as billiard-ball like molecules flying around, bouncing off each other and bouncing off the walls of their container, you can imagine that squeezing the gas could make the molecules move faster. We can illustrate this by squeezing the air in a spherical glass vessel, measuring the increase in pressure, and seeing the temperature rise (for example, by the change in color of a liquid crystal thermometer). {elaboration: When you hit a tennis ball with a racquet, the speed of the ball is greatly increased. Now think of a billiard ball bouncing off a wall. If no energy is lost (the collision is called ‘elastic’), then the speed of the ball perpendicular to the wall after rebounding, U 2 , is the same as before rebounding, U 1 , yet in the opposite direction: U 2 =U 1 . If, instead, the wall is moving steadily toward the ball before the collision, then the ball’s rebound speed will be greater, U 2 = U 1 + 2 U wall . We can see why by imagining that we are riding with the moving wall. In this frame of reference, the ball bounces simply, conserving its velocity which is U 1 +U wall both before and after the collision. An observer not riding on the moving wall thus sees speeds U 1 and U 1 + 2U wall before and after the collision. } In the 1 st Law of Thermodynamics, that is the thermal energy equation, int '' EQ W ∆= +
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This note was uploaded on 03/08/2012 for the course H A&S 222b taught by Professor P.b.rhines during the Spring '09 term at University of Washington.

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lecture5-09-thermal - 1 H A&S 222d Introduction to Energy...

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