Ch 5: 4.3.19 Weekly Quiz material

Question 1

the amount of moisture in the air compared to how much the air can hold at that temperature

Answer 1

relative humidity

Question 2

going directly from solid to gas

Answer 2

sublimation

Question 3

the study of heat and its conversion to work

Answer 3

thermodynamics

Question 4

1st law of thermodynamics

Answer 4

heat into a system will either raise the internal energy of the system or do work.

Question 5

2nd law of thermodynamics

Answer 5

no system can have 100% thermal efficiency.

Question 6

3rd law of thermodynamics

Answer 6

absolute zero cannot be reached.

Question 7

the measure of disorder in a system

Answer 7

entropy

Question 8

“heat death” theory

Answer 8

in the very distant future there won’t be enough useful energy for stars to form or matter to exist

Question 9

a device that forces heat in the “wrong” direction

Answer 9

heat pump

Question 10

heat transfer by individual particle motion

Answer 10

conduction

Question 11

heat transfer by bulk particle motion

Answer 11

convection

Question 12

heat transfer by energy only

Answer 12

radiation

Question 13

how are boiling and evaporation alike?

Answer 13

both are a phase change from liquid to gas.

Question 14

how are boiling and evaporation different?

Answer 14

evaporation is slow and natural compared to boiling.

Question 15

why does your skin feel cool when you take your hands out of hot water, or when you get out of a shower?

Answer 15

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.

Question 16

describe the “fight” that goes on at the surface of a liquid.

Answer 16

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*).

Question 17

even though pure water is said to boil at 100℃, where is that only technically true?

Answer 17

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℃.

Question 18

why must foods be cooked longer at higher elevations?

Answer 18

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.

Question 19

how does a pressurized cooker work?

Answer 19

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.

Question 20

what is one feature of water that is unusual?

Answer 20

water is *less* dense in its solid phase than its liquid phase - e.g., ice floats.

Question 21

at what point is water at its most dense? why?

Answer 21

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.

Question 22

why does a glass of ice water “sweat” on a hot day?

Answer 22

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.

Question 23

how does dew form?

Answer 23

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”.

Question 24

how does frost form?

Answer 24

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.

Question 25

what are two examples of materials which undergo sublimation?

Answer 25

moth balls, dry ice (which is frozen CO2)

Question 26

what is an example of a heat engine?

Answer 26

a car engine.

Question 27

what is an example of a heat pump as defined in this chapter?

Answer 27

a refrigerator, or an air conditioner

Question 28

what is the range of “impossible efficiencies” for a heat engine?

Answer 28

from 100% down to the *ideal* efficiency.

Question 29

what is the natural flow of heat?

Answer 29

from hot to cold.

Question 30

how is radiation different from the others (w/r/t heat transfer?)

Answer 30

radiation doesn’t require a carrier.

Question 31

equation: 1st law of thermodynamics

Answer 31

heat = change in internal energy + work

Question 32

equation**: thermal efficiency

Answer 32

thermal efficiency = (work ÷ heat in) * 100

Question 33

equation: ideal efficiency

Answer 33

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)