Question

Please explain me

Answer 1

in Example 16.4
Question 1: for the wall R-sum = 11+0.17+0.65+0.55 = 12.37 , they calculate R = 12.4
for the roof, R-sum = 30 + 0.17+0.65+0.55 = 31.37 , they calculate R = 31.4
Why? they round the number or else?
Question 2: H = -kA(dT/dx)
and k = dx/R
--> H = -(1/R)A (dT)
they calculate all of them but - sign. Why?

Answer 2

Does anyone else feel it could be because of significant figures? Personally it's the only reason I can think of, but that doesn't seem right because then the numbers would be rounded to 13 and 31, not 12.4 and 31.4.
@LastDayWork
@broken_symmetry
can you guys help here?

Answer 3

oops, I meant 12 and 31 not 12.4 and 31.4, my rounding was off.

Answer 4

For addition and subtraction, the result should have as many decimal places as the measured number with the smallest number of decimal places (for example, 100.0 + 1.111 = 101.1).

If the first non-significant figure is a 5 followed by other non-zero digits, round up the last significant figure (away from zero). For example, 1.2459 as the result of a calculation or measurement that only allows for 3 significant figures should be written 1.25.

http://en.wikipedia.org/wiki/Significant_digits

It is unlikely that the 11 and 30 had an uncertainty greater than +/ 0.1. This is then would be the numbers with the smallest number of decimal places. You would not expect to be MORE precise than these numbers.

The negative just means that for any positive change in temperature versus distance then H will decrease.

Answer 5

I got what you mean
One more question please
we have Q = mc (delta T)
and in this example, they use Q stands for the heat loss
Question: Is there any relationship between them?

Answer 6

heat wants to be in "equilibrium" so the 70 degrees will want to go to the 20 degrees outside. It's a matter of what you determine to be initial temperature and final temperature in the delta T of the equation.
the equation can represent heat loss or heat gain depending.

Answer 7

@Loser66
Why don't you take only magnitude from calculations and use physics to get the sign ??

Answer 8

@roadjester the first Q relates to m (mass)
the example Q relates to Area
how can they link?
@LastDayWork in previous example, they use - sign to show the direction of the heat (which go from outside to inside) , in this example, they totally ignore it. That's why I made that question

Answer 9

m is moles and c is specific heat? This looks like enthalpy, the amount of heat that is required to change the temperature of a substance delta T degees

Answer 10

I feel that @LastDayWork explanation is more eloquent but yes.
Basically, determining which is initial and which is final is the physics aspect.

Answer 11

@ybarrap m stands for mass
c stands for specific heat

Answer 12

@Loser66 yes, that is correct

Answer 13

I see that now -- see page 2 here: http://ocw.nctu.edu.tw/upload/classbfs120912375412714.pdf

Answer 14

@ybarrap Thank you very much, it explains well why H = Q

Answer 15

:) This is the first lesson in my course. I don't want to miss anything. I am sorry for making many questions.

Answer 16

according to this document,

delta H = Q for constant pressure
delta U = Q for constant volume

Answer 17

R-factor = r ( R with the curve on the top left)
and r = RA ,
for example R-30 --> r =30 , how can I calculate r with r = RA , what is R?

Answer 18

@ybarrap
are you talking about isobaric, isothermal, adiabatic, etc processes?

Answer 19

yes

Answer 20

@Loser66 what is "R with the curve on the top left"?

Answer 21

In general; H = PV
Thus; ∆H = P∆V + V∆P

Answer 22

That makes sense - the chain rule for energy!

Answer 23

:)

Answer 24

@LastDayWork you should probably write that in differential form

Answer 25

at the bottom of the page

Answer 26

the right hand side is R(with the curve at the top left)
middle one is R . What is it?
and in R-factor, the "factor" is R(with the curve)
how about the R?

Answer 27

@roadjester
I need to go to sleep..
If you are trying to relate how ∆H = Q at constant pressure
See First Law

The interesting point is that - at constant pressure Q starts behaving as a state function :)
Gud nite..

Answer 28

@LastDayWork Thanks for your help. Good night. :)

Answer 29

lol ok good night although it's 11 am where i am

Answer 30

It's 2:10pm here. :)

Answer 31

Texas, right?

Answer 32

hey, why don't you work?

Answer 33

ooohh, nvm today is Saturday.

Answer 34

Actually a correction -
H = PV + U (or E)

Answer 35

Now, gud nite XD

Answer 36

hahaha.... enthusiastic friend. Sleep well, friend

Answer 37

:D

Answer 38

That's what delta U = Q for constant volume looks like