thermodynamics (I + II) Flashcards
what is metabolism?
the processes involved by the body in energy intake, storage, and use
- also the sum of the chemical processes performed by all cells to keep the body alive
the body uses energy from food to
- operate organs
- maintain a constant temperature
- do external work
- build a stored energy supply (fat) for later uses
what is thermodynamics?
the study and application of the thermal energy of systems
what is heat (∆Q)?
the thermal energy that flows from one (hotter) body or system to another (colder) system in contact with it
what does the zeroth law of thermodynamics state?
if bodies A and B are each in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other
in what other way can conservation of energy in the body be written?
changed in stored energy in the body = heat lost from the body + work done
for a temperature change (∆T), a change (∆L) in any linear dimension (L) is given by
∆L = Lα∆T
the change ∆V in the volume of a solid or liquid is given by
∆V = Vβ∆T
what is specific heat capacity (c)?
the quantity of heat required to change the temperature of unit mass of a substance by one degree
what is the average specific heat of the body?
0.83 kcal/kg˚C
what is thermal conductivity (K)?
- it describes how temperature varies spatially due to the heat flow between different regions that are separated by a distance ∆x
- it also describes how much heat flows due to this spatial variation in temperature
what does the efficiency of a heat engine describe?
how efficiently it turns heat into work
η = work done / energy consumed
humans have an efficiency of
5.8%
what is mechanical work?
the force you apply to an object multiplied by the distance you push or pull it
what is latent heat of transformation (L)?
the amount of energy required per unit mass to change the state (but NOT the temperature) of a particular material
equation of latent heat
Q = Lm
L - latent heat
m - mass
what is the heat of vaporization (Lv)?
the amount of energy per unit mass that must be added to vaporize a liquid or removed to condense a gas
what is the heat of fusion (Lf)?
the amount of energy per unit mass that must be added to melt a solid or removed to freeze a liquid
what are the 4 modes of heat loss?
- radiation loss (54-60% heat loss)
- convection and conduction (=25% loss)
- evaporation of sweat (=7% heat loss)
- evaporation of water through breathing (=14% heat loss)
the magnitude and importance of modes of heat loss depend on
clothing, environment, surroundings
what is meant by radiation?
an object and its environment can exchange energy as heat via electromagnetic waves
ex: standing in front of a big fire and getting warmed by absorbing thermal radiation from the fire
energy transferred through radiation is known as?
thermal radiation
in planck’s law, a black body is essential. what is a black body?
- a body that absorbs all the thermal radiation falling onto it
- at thermal equilibrium, it emits as much energy as it absorbs
- a good absorber of radiation and a good emitter of radiation
radiation incident on an object can be
- reflected by it
- absorbed by it
- transmitted through it
what is emissivity?
the fraction of energy incident on the object that is absorbed
what surfaces have the highest emissivity?
dull, black surfaces
what does the Stefan-Boltzman Law state?
it gives the energy per second (power) radiated by the surface
P = e σ A T4
P - power radiated (watts)
e - emissivity (no units)
σ - stefan-boltzman constant
A - surface area (m^2)
T - temperature (K)
what is the value of the Stefan-Boltzman constant?
5.67 x 10^-8
the equation to calculate the difference between the energy radiated by the body and the energy absorbed from radiation from the surroundings
Hr = Kr Ar e (Ts - Tw)
Hr - rate of energy loss or gain due to radiation
Kr - parameter about 2.1 x 10^4
Ar - effective body surface area emitting the radiation
e - emissivity
Ts - skin temp (in ºC)
Tw - surrounding walls temp (in ºC)
what is meant by conductivity?
it occurs when thermal energy moves through a material as a result of collisions between free electrons, ions, atoms, and molecules of the material
what is the quantity ∆T/∆x?
- the temperature gradient
- the rate of change of temperature with distance (in a body)
the temperature difference of two faces of a body is given by
∆ T = T1 - T2
T1 is one face, T2 is the other
the quantity of heat ∆Q transmitted from face 1 to face 2 of the body in time T is given by
∆Q / ∆t = kT A ∆T / ∆x
kT - thermal conductivity (depends on the material of the body)
what is the coefficient of conduction heat transfer (h)?
h = kT / ∆x
so we can rearrange the equation into ∆Q / ∆t = h A ∆T
what is meant by convection?
- energy transfer that occurs when a fluid (ex: air) encounters an object (ex: match) whose temperature is higher than that of the fluid
- the temp of the part of the fluid in contact with the hot object increases, the fluid expands and becomes less
- because of this expanded fluid is now lighter than the surrounding cooler fluid, buoyant forces cause it to rise
the heat lost due to convection (Hc) is given by
Hc = Kc Ac (Ts - Ta)
Kc - parameter with value 3.1
Ac - effective body surface area
Ts - skin temp
Ta - air temp
what occurs during convection within the body?
it does not change the average body temperature, but the distribution of temperature within the body
when is heat production high?
at low temperatures
heat production decreases with increasing temperature until _____ becomes constant, and increases again with _____
Tc (lower critical temperature)
Th (upper critical temperature)
- between Tc and Th is?
- below Tc is?
- above Th is?
- thermoneutral zone (the zone of minimal metabolism)
- the heat loss increases due to radiation and convection
- the heat loss is dominated by evaporation
what is hypothermia?
when body temperature cannot be regulated and there is body cooling
when does vasoconstriction occur?
when the internal body temp gets too low, the blood vessels constrict to reduce the rate of heat loss
what is piloerection?
- when the fine body hairs stand on end in an attempt to reduce convective heat loss from the skin
- the raising of hairs is intended to create a stationary layer of air between the skin & surroundings
when does perspiration/sweat occur?
when the latent heat of evaporation of water is high
