Dimensional analysis, conversion of units in an equation

In summary, the professor said we should use the conversion factor 1ºC <> 1.8ºF when we have ΔT in an equation, but when we have a specific temperature we should use (ºF)=1.8(ºC)+32.
  • #1
tkfn
2
0
We are asked to transform this equation:

h=1147(1+0.05625T)[itex]\frac{V^{0.6}}{D^{0.4}}[/itex]

where the units of each variable h, T, V and D are:

h->[itex]\frac{kcal}{h·m^{2}·ºC}[/itex]
T->ºC
V->[itex]\frac{m}{s}[/itex]
D->cm

into another equation where the units of each variable h, T, V and D now are:

h->[itex]\frac{Btu}{h·ft^{2}·ºF}[/itex]
T->ºF
V->[itex]\frac{ft}{s}[/itex]
D->inches

Data:
1 Btu <> 0.252 kcal
1 ft <> 0.3048m
1 ft <> 12 inches
1 ºC <> 1.8 ºF
(ºF) = 1.8(ºC)+32

The professor said the solution is

h=2.479T[itex]\frac{V^{0.6}}{D^{0.4}}[/itex]

however I've tried the very same solution method which had worked for me in the past to solve other conversion problems but I can't seem to get the answer right, I'm confused as to which temperature conversion "factor" should we use, 1 ºC<>1.8 ºF or (ºF)=1.8(ºC)+32? I have really given up on this. :confused:
 
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  • #2
Show your work.

In your unit conversions, you have something per degree C. You are not trying to find a particular temperature, so F = 1.8C + 32 is not the correct conversion.
 
  • #3
Well what i did first is find out the units of the constant 1147 in the original equation so the principle of dimensional homogeneity at both sides of the = sign is satisfied:

[itex]\frac{kcal}{h·m^{2}·ºC}[/itex]=[itex]\frac{kcal·cm^{0.4}·s^{0.6}}{h·m^{2.6}·ºC^{2}}[/itex]·ºC·[itex]\frac{m^{0.6}}{cm^{0.4}·s^{0.6}}[/itex]

Then I repeated the same process for the units of the new desired equation:

[itex]\frac{Btu}{h·ft^{2}·ºF}[/itex]=[itex]\frac{Btu·inches^{0.4}·s^{0.6}}{h·ft^{2.6}·ºF^{2}}[/itex]·ºF·[itex]\frac{ft^{0.6}}{inch^{0.4}·s^{0.6}}[/itex]

Then I made an attempt to convert the constant with its units from the original equation into a constant with the units from the desired equation:

1147[itex]\frac{kcal·cm^{0.4}·s^{0.6}}{h·m^{2.6}·ºC^{2}}[/itex]·[itex]\frac{Btu}{0.252kcal}[/itex]·[itex]\frac{0.394^{0.4}inches^{0.4}}{cm^{0.4}}[/itex]·[itex]\frac{0.3048^{2.6}m^{2.6}}{ft^{2.6}}[/itex]· This is where I don't know how to proceed in order to convert the temperature units from ºC[itex]^{2}[/itex] to ºF[itex]^{2}[/itex], the professor said we should use the factor 1ºC <> 1.8ºF when we have ΔT in an equation BUT when we have a specified T we should use (ºF)=1.8(ºC)+32, but SteamKing disagrees so I don't know how to proceed at all. I have a hunch my professor made a mistake but I'm not sure.
 

FAQ: Dimensional analysis, conversion of units in an equation

1. What is dimensional analysis?

Dimensional analysis is a method used in science to convert units in an equation and ensure that the units on both sides of the equation are consistent. It involves identifying the physical dimensions (length, mass, time, etc.) of each quantity in an equation and using conversion factors to cancel out units and obtain the correct units in the final answer.

2. Why is dimensional analysis important in science?

Dimensional analysis is important because it allows scientists to check the validity of an equation and ensure that the units of measurement are consistent. It also helps to convert units from one system to another, making it easier to compare data from different sources.

3. How do you perform dimensional analysis?

To perform dimensional analysis, first identify the physical dimensions (length, mass, time, etc.) of each quantity in the equation. Then, use conversion factors to cancel out units and obtain the desired units in the final answer. It is important to keep track of the units throughout the calculation and make sure they cancel out correctly.

4. What are conversion factors?

Conversion factors are ratios that are used to convert units from one system to another. They are typically derived from known relationships between different units of measurement, such as 1 inch = 2.54 centimeters. These conversion factors are used in dimensional analysis to cancel out units and obtain the desired units in the final answer.

5. Can dimensional analysis be used for all types of units?

Yes, dimensional analysis can be used for all types of units as long as the units are consistent and can be converted using conversion factors. However, it is important to note that some equations may require more complex conversions, such as those involving exponential or trigonometric functions.

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