Solving Heat Transfer Equations for Rtot: Understanding Units and Formulas

  • Thread starter Thread starter stunner5000pt
  • Start date Start date
  • Tags Tags
    Heat Heat transfer
AI Thread Summary
The discussion focuses on calculating the total heat resistance (R_tot) of a room's walls using the formula R_tot = (18 - (-5)) / (400 + 2(80)), resulting in a value of 0.041 K/W. However, participants note that the units do not align correctly, as the calculation mixes the rate of heat transfer (in watts) with the R-value of the walls. It is suggested that the relevant equation should incorporate the total wall area of 35 m² to ensure accurate results. Additionally, the correct unit for the heat resistance should be expressed in m²K/W. Clarification on the appropriate formula and units is necessary for accurate calculations.
stunner5000pt
Messages
1,443
Reaction score
4
Homework Statement
Consider a room in a house that has composite walls with some heat resistance R(tot). The total cross sectional area of the walls in 35 m^2. During winter, the temp outside is -5 deg C. Inside there is an electric heater emitting heat at a rate of 400 W so the room temperature is constant at 18 deg C. The room is occupied by 2 people each emitting 80 W each. What is the heat resistance of the walls in W?
Relevant Equations
[tex] \frac{dQ}{dt} = \frac{T_{out} - T_{in}}{R_{tot}} [/tex]
I believe that just using the equation above should yield the following:

R_{tot} = \frac{18 -(-5)}{400 + 2(80)} = 0.041

but the units don't seem to match up... The units of the above answer are in K / W.

Is there a different formula i should be using?

Your help is appreciated!
 
Physics news on Phys.org
stunner5000pt said:
Homework Statement:: Consider a room in a house that has composite walls with some heat resistance R(tot). The total cross sectional area of the walls in 35 m^2. During winter, the temp outside is -5 deg C. Inside there is an electric heater emitting heat at a rate of 400 W so the room temperature is constant at 18 deg C. The room is occupied by 2 people each emitting 80 W each. What is the heat resistance of the walls in W?
Your relevant equation is missing the total area of the walls of the room.
Also, the question seems to be mixing rate of heat transfer (watts) and R-value of the composite walls.

Please, see:
https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

:cool:
 
Last edited:
Lnewqban said:
the question seems to be mixing rate of heat transfer (watts) and R-value of the composite walls.
Yes, it should ask for ##Km^2/W##.
 
Thread 'Collision of a bullet on a rod-string system: query'
In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
Back
Top