16-22;U/Specific Heat/Phase/Evaporation/Heat transer Flashcards
internal energy
total energy of all particles within an object, sum of kinetic energy and potential energy of molecules
absolute temp
condition under which molecular motion ceases, 273K, 0 C
temp
measure of avg kinetic energy per particle
difference between internal energy and temp
two objects of the same temp doesn’t have the same amount of internal energy per gram, temp is not directly proportional to internal energy
thermal equil
2 objects at the same temp, no transfer of internal energy between them
heating
transfer of energy from a high-temp object to low temp object
temp scale
to define reproducible temp scale, should be at least 2 standard reference points to calibrate thermometer: freezing/boiling point
universal standard for temp scales is
760mmHg standard Patm
fp and bp of water at standard pressure
fp=0 degree Celsius
bp=100 degree Celsius
heat
internal energy in transit
1st law of TD
dU=Q+W
Q=transfer of heat, microscopic process
W=work done on the system, macroscopic process
conservation of energy
thermal expansion
creates destructive stress when bottom of glass vessel is heated so will crack due to large forces that break brittle glass
conditions for temp measurements
precise and reproducible manner
liquid expansion thermometers
expansion is proportional to temp = height of column in capillary can be calibrated in degrees C or F EG. mercury filled clinical thermometers
bi-metallic strip thermometers
2 metals A+B w/ different thermal expansion coefficients bonded together used to measure temp
EG. oven thermometers and thermostats, as they can tolerate wide range of temps
constant vol gas thermometer
when V constant = P of enclosed gas directly proportional to Kelvin T
easy to calibrate but large compared to liquid in glass thermometers
heat capacity
quantity of heat needed to raise its temp by 1 degree C
high heat capacity = warm up more slowly = absorb greater amount of heat, also cool more slowly = give off more heat
specific heat capacity
amount of heat in cal required to raise temp of 1g of substance by 1 degree C
dietary calorie
kilocalorie, used to specify energy value of food
used since using food is an oxidation process which is similar to burning in terms of chemical energy released
mechanical equivalent of heat
1 cal, 4.186 J
heat of combustion
amount of heat released during combustion of a specified amount
calorimeter
composed of oxidation chamber surrounded by measured vol of water = when food completely burned = heat of combustion is given to water = energy given off = energy gained by calorimeter = energy of combustion
problems with calorimeter
proteins are not completely oxidised by body, energy residues are excreted in urine
not all processes of converting food into energy is in form of heat since body also does mechanical work
change of phase means
giving internal energy to a solid body not at melting T
increasing Ek of molecules = weakens bonds = thermal expansion
if enough U added then bonding forces can breaks = frees molecules from rigid positions = melting
how to change phase liquid to gas
continuously adding Q to liquid after solid-liquid phase = gradually raises temp to BP = enough U will completely overcome attractive forces = change to gas occurs rapidly
540 cal/g required to convert water to steam at 100 degrees C
latent heat of fusion
energy required to melt 1g of solid once it has reached melting temp
refrigeration cycle
expansion valve at beginning of cooling coils force refrigerant liquid to evaporate using Bernoulli effect to lower P via boiling to gas = cooling coils will prevent vaporisation = cools interior = energy carried out to compressor = increases P = forces condensation of refrigerant = gives of latent heat of vaporisation = large amount of U = efficient method for cooling
how does body cool
via perspiration evaporating = extracts latent heat of vaporisation
when does evaporation occur
at T > 0 since higher Ek more molecules escape from liquid into air
what increases evaporation rate
increasing T of liquid = more molecules have energy necessary to escape
saturation vapour pressure
when vapour is saturated and P exerted on container walls
saturation vapor density
mass of water vapour per unit vol under saturation condition
relative humidity
% of saturation humidity at given temp
body membranes tend to be sensitive to relative humidity than absolute humidity
dew point
temp at which moisture content present in air will saturate air
heat transfer
via conduction, convection and radiation of heat
conduction
primary method of heat transfer, transfer of heat by direct interaction of molecules in hot area w/ molecules in cooler area via collisions between molecules
efficiency depends on no. of collisions, amount of energy transferred during each collision
why are metals generally better heat conductors than nonmetals
free e- in metals move at high speeds = transfer energy by collisions with other e- and with atoms in the metal lattice
Gases are poorer heat conductors = smaller no of molecular collisions in gaseous state
explain Fourier’s law
doubling thickness of material = 1/2 rate of heat loss
convection
heat transfer by movement of fluid
convection current
movement of fluid that carries heat
air rises in T = expands = less dense than surrounding air = buoyant force causes it to rise = more dense cool air moves down to replace it
EG. radiator
radiation
energy that comes from a source and travels through space, can penetrate some materials
black body
ideal emitter - absorber to absorb all incident light
