What is one calorie in J/g*K?
1 cal = 4.184 J/g*K
Difference between STATE and FLOW functions?
STATE functions: Pathway independent, e.g. P, ρ, T, V, G, U, G, S
FLOW functions: Pathway DEPENDENT; e.g. Q and W
Heat: def, units
Def: a FLOW function; the transfer of energy into or out of a system
Temperature: def, units
Def: A STATE function; directly proportional to the internal energy of a system
(T is proportional to U)
Energy of a system from its atoms and molecules
Work, Heat, and Internal Energy (relationship and def)
Heat and Work are ways of changing the Internal Energy of a system:
∆Eint = Q + Won system
Heat Capacity: def, eq, units
Def: energy needed to raise temperature of a sample by 1 degree (this is the "mc" component of the equation)
Eq: Q = mc∆T
Specific heat: def, eq, units
Def: Heat capacity per unit mass
(this is the "c" component of the equation)
Q = mc∆T
Latent Heat: def, eq
Def: heat absorbed that does NOT change the temperature
Eq: Q = mL
Walk through the HEATING CURVE of water; which components are flat? Which parts have slope? Why is Qd so large?
Flat component: PHASE CHANGE (e.g. liquid --> gas)
Sloped lines: heating during a single phase (e.g. heating of liquid phase)
- Qd is large because it is difficult to break H-bonds and move particles far apart --> which is why it takes so much energy
How would heating curve change if ambient pressure was 0.95 atm (e.g. Denver); aka less than 1 atm?
By decreasing the amount of air pressure, you decrease the amount of water pressure needed to overcome it (and convert liquid to gas)
Therefore, water will boil at a lower temperature and food requires a longer time to cook
(The temperature at which water boils is related to the vapor pressure required for boiling, which is equal to the atmospheric pressure)
How many feet of increased elevation results in 1 C decrease in BP?
For every 1000 ft. in altitude, the boiling point of water decreases by about 1 °C.
How does a pressure-cooker take advantage of pressure-boiling point relationship?
A pressure cooker is a tightly sealed pot which uses the steam from water to increase the internal pressure of the pot. As the pressure inside the pot increases, the boiling point rises, and a higher temperature can be achieved for cooking. The internal temperature can be determined by controlling the vapor pressure inside the pot. Since the internal temperature can be raised substantially above 100°C, food cooks faster.
Conduction: def, eq
def: Exchange of kinetic energy between microscopic particles
Eq: Q/t = [k A (T2 - T1)]/ d
Thermal conductivity: def, units
def: Ability to conduct heat
objects w/ large κ = metals
small κ = gases, non-metals
Convection: def, ex
Energy transferred by movement of a fuid
E.g. air conditioning
Radiation: def, ex
Conversion of thermal energy into electromagnetic waves
E.g. solar radiation
Thought question: which will be cooler? A clear night, or a cloudy night?
A clear night would be cooler because it is losing the electromagnetic waves stored in the earth throughout the day
A cloudy night, on the other hand, would help insulate the earth and keep it warmer
Compression and Expansion as it relates to work
When Wby system > 0 →expansion
When Wby system < 0 → compression
Work is path (independent/dependent)
Adiabatic thermodynamic process: what is constant? What changes in compression? What changes in expansion?
Adiabatic = no net change in heat
Q = 0
Compression: ∆T increases (increase in temp when heat is constant)
Expansion: ∆T decreases (decrease in temp)
Isothermal process: what is constant? What changes in compression/expansion?
Isothermal = No ∆T
∆T = 0
Compression: Q < 0 (heat is being removed in compression to ensure no increase in temperature)
Expansion: Q > 0 (heat is being added to the system in expansion to ensure no decrease in temperature)
Isobaric thermodynamic process: what is constant? What changes in compression? Expansion?
Isobaric = No ∆P
∆P = 0
Compression: ∆T decreases; it gets colder
Expansion: ∆T increases; it gets warmer
(for this, you use PV = nrT*)
*So if V is decreasing in compression, then T decreases
Isochoric process: what is constant, what changes in compression/expansion?
Isochoric = no change in Volume → Work = 0
Work is 0 (constant)
Compression: ∆U = Q*
Expansion: ∆U = Q
*So to have compression w/out changing the volume → change heat
Equation for internal energy of system
∆U = ∆Eint (synonymous)
∆Eint = Q + Won sys
Walk through diagram of Thermodynamic Processes
A: isochoric cooling
B: adiabatic expansion
C: Isothermal expansion
D: isobaric expansion
What are the lines T1, T2, etc. in the thermodynamic processes diagram?
The lines are isothermal process lines;
Adiabatic processes would jump between lines (due to change in both volume and pressure -- like B)
What's the effect on the system: rapidly pumping up a bicycle tire?
System: air in pump
Q = 0
W = -
∆Eint = +
Internal energy is increasing,
What's the effect on system: pan of room-temp water sitting on hot stove?
System: water in pan
Q = +
W = ~0
∆Eint = +
What is the effect on the system: air quickly leaking out of a balloon?
System: air originally in balloon
Q = 0
W = +
∆Eint = -
If you change the atmospheric pressure from 1 atm → 0.95 atm, what is the effect on cooking food?
- Lower BP bc it takes less to reach vapor pressure
- Quicker to reach boiling
- But takes longer to cook bc you aren't transferring as much energy into the food, because the boiling water doesn't have as much energy as it does at 1 atm boiling