Lecture 4 1/29/13magnetism: caused by electric currents in the liquid iron core of planet, magnetic pole ishistorically known to wander (evidence is rocks made at different times indicated differ-ent magnetic directions in their composition), no way to predict the “flips”- to be more efficient, the temp difference (between the ignited hot fuel and cool part of the engine) ...should be as large as possible- the melting temperature of an object is usually: equal to its freezing point- to heat a room using the least energy, do it by: using natural gas to run a heat pumpAtoms and Heat IIPhases of Matter: Most materials can exist in three phases: solid, liquid, gassolid: atoms are locked in position, can’t really move, sometimes crystallizedliquid: they can move, but they’re always touchinggas: all move fast at speed of sound, not touching- to go from phase to phase requires a lot of energy- specific heat: tells how much thermal energy we need to add to something to change its temp by 1 degree C (joules per kg, calories per gram)- thermal energy, random motion of “stuff”, temperature is the avg. kinetic energy, how fast the atoms/molecules are moving (has to do with mass and velocity)- can raise temp of aluminum with much less energy than water, why? water has afunny structure, the hydrogen and oxygen atoms are only 105 degrees apart, form a zigzag chain, each molecule like a triangle, positives end up by negative neigbor of next molecules, so water molecules are “sticky” due to opposing charges (this is seen in “beads” of water, they are so sticky they overcome the force of gravity, bug in PPT is suspended on water because molecules are so tight)Phase changes:melting ice- mix of water and ice is 32 degrees F- eventually it will absorb energy from the room and all melt to liquid, going above 32 degrees, must add 330,000 J of heat to melt 1 kg of ice at 0 degrees C into liquid water at 0 degrees C, because water is sticky, once the molecules are locked as ice, you have to add enormous amount of energy to melt it- must add 2,300,000 Joules to boil 1 kg of water at 100 degrees C at 100 degreesC, to change sticky water from liquid to gas requires 8x as much energy as it takes to go from solid to liquid- latent heat: the heat required to change a phase at constant temperature is called latent heat, powers hurricanes, storm sucks warm energy out of water (as va-por) and into atmosphere, it cools and condenses, 10 percent of energy is released as winds, rest goes out to space (so hurricanes powered by latent heat of condensation), everything depends on that 105 degree angle- sublimation: solid to gas, when you put an ice cube by itself in the freezer- if water didn’t need so much energy to change phases, we wouldn’t be alive, when we start to sweat we release an enormous amount of energy, we need to re-lease it as heat, sweat evaporates changing from liquid to gas (using energy from body’s hot tissue) so the temp of your tissue drops and keeps you alive; if dehydrated, you can’t cool yourself off- the Zeroth Law of Thermodynamics: Thermodynamic Equilibrium: When two ob-jects are separately in thermodynamic equilibrium with a third object, they are in equi-librium with each other (if all have the same temperature, no heat will flow between them)- Corollary to the Zeroth law: if two objects are not in thermal equilibrium, heat will flow from the hot object to the cold object until their temperatures are equal (from hot to cold) (touch two bowls, one metal, one plastic, the same temp as your skin, they feel like the same temperature, if they are both 60 degrees, they won’t feel the same)1st Law of Thermodynamics: work done by system = - work done on systemincrease in internal energy = heat transferred in + work done on- basically the conservation of energy, we cannot create or destroy energy2nd Law of Thermodynamics: The entropy (disorder or randomness in a system) of a closed system shall never decrease, and shall increase whenever possible, so entropy in a thermally isolated system never decreases (entropy in the room’s air, means all molecules randomly spread out, will not suddenly concentrate in one area, or the way dye spreads out in water, air freshener spreads out in a room)- when we cool a hot region, we decrease its entropy, but not by much- when we heat a cool region, we increase its entropy by a lot, so this works okay- heat engines work best with large temp differences- efficiency of your car is low, engine loses 62 % of energy, accessories burn away2%, idling/stop light is 17%, driveline losses 6%, so only 13 % went to real useHeat Pump: to “cheat”, move heat energy to already hot region (increase entropy)- when we cool an already cool region, we decrease its entropy by a lot, so we must do external work (doesn’t occur naturally)- to compress something you have to do work on it, because increase in internal energy = heat transferred in + work done on - the work you do increases the internal energy, and usually the temp rises, so much compression that it causes burning- cooling by expansion: when a system expands, it does work on its surroundings. Since it’s losing energy in the process, it gets cooler- to heat a room, used a fireplace loses a lot of energy up the chimney, the cheap-est, most efficient method is using a heat pump because the thermal energy is free (comes from outside air), you pay only for the electricity needed to move it inside - you only have to pay for 1 kW of electricity and it will produce 3 kW of
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