Thermal Physics Flashcards
heat capacity definition
amount of energy required to raise temperature of sample by 1 Kelvin
energy required to raise temperature of sample formula
energy required to raise temperature of sample = heat capacity (K/J) x difference in temperature (K, °C)
Q = C x ∆T
energy required to raise temperature of specific mass of sample formula
energy required to raise temperature of mass of sample (J) = heat capacity (J/K) x mass (kg) x difference in temperature (K, °C)
Q = C x M x ∆T
power definition and formula
rate of energy change
power (W) = energy emitted (Q) / time (s)
P = Q / t
how heat flows
hotter object to cooler object
greater difference = faster rate of cooling
energy is transferred
melting
heat is added
particles given energy
particles are in fixed position and vibrate
adding sufficient heat causes particles to move faster to break bonds holding them (becomes liquid)
boiling
liquid heated
particles gain energy, move faster about their container and liquid expands
interactions between particles become weaker as distance between particles increases
addition of sufficient heat results in liquid expanding where particles interact negligibly (gas)
why temperature doesn’t increase during melting or boiling
additional energy is used to break/overcome intermolecular forces
instead of supplying particles with more kinetic energy
latent heat definition
amount of energy required to change the state of an object from one state to another
addition/subtraction of heat energy without observation of temperature change
latent heat of fusion definition
latent heat when converting from solid to liquid or vice versa
latent heat of vaporisation definition
latent heat when converting from liquid to gas or vice versa
amount of energy required to convert given mass from one state to another formula
amount of energy required to convert given mass of substance from one state to another (J) = mass of substance (kg) x l (kJkg^-1)
Q = m x l
factors affecting rate of condensation
temperature
Surface temperature
airflow
density
how temperature affects rate of condensation
as it decreases, rate of cond. increases
particles have lower average energy so
easier and more likely to slow down enough to form liquid
how surface temperature affects rate of condensation
as it decreases, rate of cond. increases
cold surface will transfer heat away from gas faster, cooling it quicker and forming condensate faster
how airflow affects rate of condensation
as it becomes slower, rate of cond. decreases
as conc. of gas will be higher so more cond. will occur on the surface
how density affects rate of condensation
as it increases, rate of cond. increases
forces between particles will be stronger so fewer particles will have enough energy to overcome these forces
more likely to form liquid
how condensation occurs
Gas changing to liquid
as gas cools, particles move slower and spend longer in close proximity to each other
intermolecular forces increase as temperature drops
once average temperature is below boiling point, gas particles condense into liquid droplets
how evaporation occurs
liquid into gas without boiling
occurs despite temperature of liquid below boiling point and average KE of particles being insufficient to do so
some particles with high KE (above average energy) and travelling toward surface of liquid
why evaporation leads to cooling
evaporation removes particles with highest energy from liquid
remaining liquid has lower average kinetic energy so lower temperature achieved
factors affecting rate of evaporation
temperature
airflow
density
surface area
how temperature affects rate of evap.
rate of evap. increases when temp. of liquid increases
higher temp. = average KE = more particles with enough energy to escape from liquid surface
how airflow affects rate of evap.
rate of evap. increases when airflow over surface of liquid increases
as conc of evap. substance is lower immediately surrounding liquid
air carries gas away and prevents it from condensing back into liquid so higher net evaporation
how density affects rate of evap.
rate of evap. increases when density lowers
lower density = particles further apart = forces between particles weaker = more particles with enough energy to overcome them and escape liquid
how surface area affects rate of evap.
rate of evap. increases as SA increases as more particles nearer the surface, easier to escape liquid
conduction of heat definition
process by which vibrating particles pass on their KE to neighbouring particles in a material
factors affecting rate of conduction
tighter neighbouring particles = the more they influence each other = more effective conduction
material (e.g. metals)
state (solid > liquid > gas at due to intermolecular distances)
SA (more room for particles to collide)
temp. difference (heat moves faster with greater temp. diff)
length (longer conductor = longer it takes for heat to pass from one side to another)
why metals good heat conductors
ions tightly bound with small distance (so more influence) between neighbouring ions
also delocalised electrons can move freely and quickly inside metal and collide with other electrons and ions to transfer energy quickly
electrons from hot regions move through structure and collide with ions and electrons at colder parts
insulators
no free electrons (can only transfer energy by vibration of neighbouring atoms)
liquids and gases poorer conductors as greater intermolecular distances than solids (so less influence)
fluid definition
no fixed shape but flows and spreads to fill the shape of the container it occupies
convection definition
method of transporting heat through a fluid due to particle movement
convection mechanism
when heated from the bottom, particles gain KE
move more quickly so collide more regularly and move further apart
take up more volume, decreases density
less dense regions of fluids rise and displace colder, denser regions colder denser fluids sink to bottom and be heated, cycle continues
convection current definition
currents of moving fluids
how you stay warm with jumper/blanket
body heats fixed amount of air around you
warm air trapped by blanket
without blanket, convection carries away warm air, replaced by colder air
heat radiation definition
emission of electromagnetic waves from warm bodies
doesn’t require a medium to travel through
heat radiation features
doesn’t require a medium to travel through
surfaces and absorbing and emitting infrared radiation
dark matt surfaces > bright glossy surfaces
best absorbers and emitters of infrared radiation as black absorbs all colours and matt surfaces reflect light less
why objects become hotter/colder
emit more/less radiation than it absorbs
proportional to how hot it is
reducing heat transfers in vacuum flask
vacuum between container and flask prevents convection and conduction since both need medium to occur
inside and outside of liquid container are shiny (not black) so heat radiation from outside of bottle reflected back out and radiation from liquid reflected back into liquid
bottle supported by insulating foam (conducted little heat to outside as possible)
stopped (made out of insulator) prevents heat transfer by convection by trapping air above liquid
biology of heat exchange
heat regulated to conserve energy and resources
when cold, hair in mammals skin pull upright to trap layer of insulting air around body (reduced her loss by convection)
also blood vessels constrict around extremities to reduce blood flow there
due to high SA:V ratio, more susceptible to heat loss via radiation
opposite occurs when hot + body sweats for heat loss via evaporation
animal adaptations in extreme conditions
cold:
fur traps insulating air
thick layer insulating blubber to reduce heat transfer away from body
smaller ears
hot:
thin layers of fur
large ears (higher SA so heat dispersed more effectively by passing blood through dilated blood vessels near surface)
density definition and formula
measure of mass of a substance per unit volume
density = mass / volume
p = M/V
how buoyancy depends on density
objects denser then the liquid they are in sink and vice versa
how to measure volume of irregular shaped object
place object into beaker of water
find volume of displaced water
density and states of matter
solid > liquid > gas
except for water
