Ch 5: 4.3.19 Weekly Quiz material Flashcards
the amount of moisture in the air compared to how much the air can hold at that temperature
relative humidity
going directly from solid to gas
sublimation
the study of heat and its conversion to work
thermodynamics
1st law of thermodynamics
heat into a system will either raise the internal energy of the system or do work.
2nd law of thermodynamics
no system can have 100% thermal efficiency.
3rd law of thermodynamics
absolute zero cannot be reached.
the measure of disorder in a system
entropy
“heat death” theory
in the very distant future there won’t be enough useful energy for stars to form or matter to exist
a device that forces heat in the “wrong” direction
heat pump
heat transfer by individual particle motion
conduction
heat transfer by bulk particle motion
convection
heat transfer by energy only
radiation
how are boiling and evaporation alike?
both are a phase change from liquid to gas.
how are boiling and evaporation different?
evaporation is slow and natural compared to boiling.
why does your skin feel cool when you take your hands out of hot water, or when you get out of a shower?
the water evaporating from your skin takes the *latent heat of vaporization* with it to make the jump, and that heat comes from your skin.
describe the “fight” that goes on at the surface of a liquid.
the liquid particles are trying to break free into the gas phase, but air pressure holds them down. the liquid particles slip by (which is *evaporation*), and the hotter the liquid, the faster they do so. when the liquid is hot enough, the energy of the particles overcomes the air pressure holding them down, and they “erupt” (which is *boiling*).
even though pure water is said to boil at 100℃, where is that only technically true?
at sea level, where the full atmosphere can hold the water molecules down. water at any level higher than that will have an actual boiling point of less than 100℃.
why must foods be cooked longer at higher elevations?
water boils at a lower temperature at higher elevations, so the water doesn’t have the same amount of heat in it; foods must be cooked longer in order to get the correct amount of heat into them.
how does a pressurized cooker work?
when it’s under pressure, water will still be liquid above 100℃, so it has more heat than normal. this allows foods to be cooked faster.
what is one feature of water that is unusual?
water is *less* dense in its solid phase than its liquid phase - e.g., ice floats.
at what point is water at its most dense? why?
at about 4℃. the molecules of water at a colder temperature will start trying to “lock” into a certain pattern that makes the molecules move away from each other. this makes the water less dense.
why does a glass of ice water “sweat” on a hot day?
the air holds a certain amount of moisture based on its temperature; as air comes up against the cold glass its temperature drops, so it can’t hold all of its moisture. this extra moisture comes out as condensation on the glass.
how does dew form?
the air holds a certain amount of moisture based on its temperature. as the air cools overnight, it can’t hold all of its moisture, so the extra moisture comes out on cooler surfaces (like grass, or a car) as “dew”.
how does frost form?
the air holds a certain amount of moisture based on its temperature. if the air gets cold enough while cooling overnight, the extra moisture will come out frozen as frost.
what are two examples of materials which undergo sublimation?
moth balls, dry ice (which is frozen CO2)
what is an example of a heat engine?
a car engine.
what is an example of a heat pump as defined in this chapter?
a refrigerator, or an air conditioner
what is the range of “impossible efficiencies” for a heat engine?
from 100% down to the *ideal* efficiency.
what is the natural flow of heat?
from hot to cold.
how is radiation different from the others (w/r/t heat transfer?)
radiation doesn’t require a carrier.
equation: 1st law of thermodynamics
heat = change in internal energy + work
equation**: thermal efficiency
thermal efficiency = (work ÷ heat in) * 100
equation: ideal efficiency
ideal efficiency = [(Temp High - Temp Low) ÷ Temp High] * 100
important: in this equation the temperatures must be in Kelvin; always convert from Celsius (TK = TC + 273.15)
