# 1.6 Mechanisms of heat transfer  (Page 14/27)

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A man consumes 3000 kcal of food in one day, converting most of it to thermal energy to maintain body temperature. If he loses half this energy by evaporating water (through breathing and sweating), how many kilograms of water evaporate?

2.59 kg

A firewalker runs across a bed of hot coals without sustaining burns. Calculate the heat transferred by conduction into the sole of one foot of a firewalker given that the bottom of the foot is a 3.00-mm-thick callus with a conductivity at the low end of the range for wood and its density is $300\phantom{\rule{0.2em}{0ex}}{\text{kg/m}}^{3}$ . The area of contact is $25.0\phantom{\rule{0.2em}{0ex}}{\text{cm}}^{2},$ the temperature of the coals is $700\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}$ , and the time in contact is 1.00 s. Ignore the evaporative cooling of sweat.

(a) What is the rate of heat conduction through the 3.00-cm-thick fur of a large animal having a $1.40{\text{-m}}^{2}$ surface area? Assume that the animal’s skin temperature is $32.0\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}$ , that the air temperature is $-5.00\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}$ , and that fur has the same thermal conductivity as air. (b) What food intake will the animal need in one day to replace this heat transfer?

a. 39.7 W; b. 820 kcal

A walrus transfers energy by conduction through its blubber at the rate of 150 W when immersed in $-1.00\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}$ water. The walrus’s internal core temperature is $37.0\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}$ , and it has a surface area of $2.00\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}$ . What is the average thickness of its blubber, which has the conductivity of fatty tissues without blood?

Compare the rate of heat conduction through a 13.0-cm-thick wall that has an area of $10.0\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}$ and a thermal conductivity twice that of glass wool with the rate of heat conduction through a 0.750-cm-thick window that has an area of $2.00\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}$ , assuming the same temperature difference across each.

$\frac{Q}{t}=\frac{kA\left({T}_{2}-{T}_{1}\right)}{d}$ , so that
$\frac{{\left(Q\text{/}t\right)}_{\text{wall}}}{{\left(Q\text{/}t\right)}_{\text{window}}}=\frac{{k}_{\text{wall}}{A}_{\text{wall}}{d}_{\text{window}}}{{k}_{\text{window}}{A}_{\text{window}}{d}_{\text{wall}}}=\frac{\left(2\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}0.042\phantom{\rule{0.2em}{0ex}}\text{J/s}·\text{m}·\text{°}\text{C}\right)\left(10.0\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}\right)\left(0.750\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-2}\text{m}\right)}{\left(0.84\phantom{\rule{0.2em}{0ex}}\text{J/s}·\text{m}·\text{°}\text{C}\right)\left(2.00\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}\right)\left(13.0\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-2}\text{m}\right)}$
This gives 0.0288 wall: window, or 35:1 window: wall

Suppose a person is covered head to foot by wool clothing with average thickness of 2.00 cm and is transferring energy by conduction through the clothing at the rate of 50.0 W. What is the temperature difference across the clothing, given the surface area is $1.40\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}$ ?

Some stove tops are smooth ceramic for easy cleaning. If the ceramic is 0.600 cm thick and heat conduction occurs through the same area and at the same rate as computed in [link] , what is the temperature difference across it? Ceramic has the same thermal conductivity as glass and brick.

$\frac{Q}{t}=\frac{kA\left({T}_{2}-{T}_{1}\right)}{d}=\frac{kA\text{Δ}T}{d}⇒$
$\text{Δ}T=\frac{d\left(Q\text{/}t\right)}{kA}=\frac{\left(6.00\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-3}\phantom{\rule{0.2em}{0ex}}\text{m}\right)\left(2256\phantom{\rule{0.2em}{0ex}}\text{W}\right)}{\left(0.84\phantom{\rule{0.2em}{0ex}}\text{J/s}·\text{m}·\text{°}\text{C}\right)\left(1.54\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-2}\phantom{\rule{0.2em}{0ex}}{\text{m}}^{2}\right)}=1046\phantom{\rule{0.2em}{0ex}}\text{°}\text{C}=1.05\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{3}\text{K}$

One easy way to reduce heating (and cooling) costs is to add extra insulation in the attic of a house. Suppose a single-story cubical house already had 15 cm of fiberglass insulation in the attic and in all the exterior surfaces. If you added an extra 8.0 cm of fiberglass to the attic, by what percentage would the heating cost of the house drop? Take the house to have dimensions 10 m by 15 m by 3.0 m. Ignore air infiltration and heat loss through windows and doors, and assume that the interior is uniformly at one temperature and the exterior is uniformly at another.

What is differential form of Gauss's law?
help me out on this question the permittivity of diamond is 1.46*10^-10.( a)what is the dielectric of diamond (b) what its susceptibility
a body is projected vertically upward of 30kmp/h how long will it take to reach a point 0.5km bellow e point of projection
i have to say. who cares. lol. why know that t all
Jeff
is this just a chat app about the openstax book?
kya ye b.sc ka hai agar haa to konsa part
what is charge quantization
it means that the total charge of a body will always be the integral multiples of basic unit charge ( e ) q = ne n : no of electrons or protons e : basic unit charge 1e = 1.602×10^-19
Riya
is the time quantized ? how ?
Mehmet
What do you meanby the statement,"Is the time quantized"
Mayowa
Can you give an explanation.
Mayowa
there are some comment on the time -quantized..
Mehmet
time is integer of the planck time, discrete..
Mehmet
planck time is travel in planck lenght of light..
Mehmet
it's says that charges does not occur in continuous form rather they are integral multiple of the elementary charge of an electron.
Tamoghna
it is just like bohr's theory. Which was angular momentum of electron is intral multiple of h/2π
determine absolute zero
The properties of a system during a reversible constant pressure non-flow process at P= 1.6bar, changes from constant volume of 0.3m³/kg at 20°C to a volume of 0.55m³/kg at 260°C. its constant pressure process is 3.205KJ/kg°C Determine: 1. Heat added, Work done, Change in Internal Energy and Change in Enthalpy
U can easily calculate work done by 2.303log(v2/v1)
Abhishek
Amount of heat added through q=ncv^delta t
Abhishek
Change in internal energy through q=Q-w
Abhishek
please how do dey get 5/9 in the conversion of Celsius and Fahrenheit
what is copper loss
this is the energy dissipated(usually in the form of heat energy) in conductors such as wires and coils due to the flow of current against the resistance of the material used in winding the coil.
Henry
it is the work done in moving a charge to a point from infinity against electric field
what is the weight of the earth in space
As w=mg where m is mass and g is gravitational force... Now if we consider the earth is in gravitational pull of sun we have to use the value of "g" of sun, so we can find the weight of eaeth in sun with reference to sun...
Prince
g is not gravitacional forcé, is acceleration of gravity of earth and is assumed constante. the "sun g" can not be constant and you should use Newton gravity forcé. by the way its not the "weight" the physical quantity that matters, is the mass
Jorge
Yeah got it... Earth and moon have specific value of g... But in case of sun ☀ it is just a huge sphere of gas...
Prince
Thats why it can't have a constant value of g ....
Prince
not true. you must know Newton gravity Law . even a cloud of gas it has mass thats al matters. and the distsnce from the center of mass of the cloud and the center of the mass of the earth
Jorge
please why is the first law of thermodynamics greater than the second
every law is important, but first law is conservation of energy, this state is the basic in physics, in this case first law is more important than other laws..
Mehmet
First Law describes o energy is changed from one form to another but not destroyed, but that second Law talk about entropy of a system increasing gradually
Mayowa
first law describes not destroyer energy to changed the form, but second law describes the fluid drection that is entropy. in this case first law is more basic accorging to me...
Mehmet
define electric image.obtain expression for electric intensity at any point on earthed conducting infinite plane due to a point charge Q placed at a distance D from it.
explain the lack of symmetry in the field of the parallel capacitor
pls. explain the lack of symmetry in the field of the parallel capacitor
Phoebe