Lecture 4 1 29 13 magnetism caused by electric currents in the liquid iron core of planet magnetic pole is historically 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 pump Atoms and Heat II Phases of Matter Most materials can exist in three phases solid liquid gas solid atoms are locked in position can t really move sometimes crystallized liquid they can move but they re always touching gas 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 a funny 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 degrees C 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 system increase in internal energy heat transferred in work done on basically the conservation of energy we cannot create or destroy energy 2nd 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 away 2 idling stop light is 17 driveline losses 6 so only 13 went to real use Heat 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 heat
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