Why Can We Pass Our Finger Through a Candle Flame Without Being Burned?

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The discussion focuses on understanding why a finger can pass through a candle flame without being burned when exposure is limited to about 0.25 seconds. Participants highlight the importance of the convection heat transfer coefficient (h) and suggest modeling the finger as a semi-infinite medium. The key calculations involve determining the energy transferred to the finger in that brief exposure time and how much flesh could be heated to the burn threshold of 65°C. The conversation emphasizes the need for appropriate equations to model convective heat transfer and the thermal response of the finger. Ultimately, the problem requires a mathematical explanation of heat transfer dynamics to understand the phenomenon.
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Homework Statement


I must solve this problem, please help me, which equations i should use?

Most of us have passed our finger through a 800C candle flame and know that if we limit exposure to about 0.25 s, we will not be burned. Why not? Show all the calculations that support your answer. The burn treshold of human skin is about 65C. Assume human flesh has properties similar to beef and h=100 W/m^2.K.



Homework Equations





The Attempt at a Solution

 
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What does h signify in heat transfer? How will you model your finger (e.g., finite object? semi-infinite medium)?
 
Mapes said:
What does h signify in heat transfer? How will you model your finger (e.g., finite object? semi-infinite medium)?

h is convection heat transfer coefficient. finger can be modeled as a cylinder, but what calculations i should do?
 
chronicals said:
h is convection heat transfer coefficient. finger can be modeled as a cylinder, but what calculations i should do?

Agreed. Now there are many approaches you can take; I don't know what techniques you've covered in class. One starting point is: how much energy is transferred to your finger in 0.25 seconds? How much flesh could that energy heat up to 65 degrees?
 
i think finger may be modeled as semi-infinite medium, initially at a uniform
temperature 37°C, anybody help me overcome this problem
 
Good choice. What equations have you found for modeling convective heat transfer to a semi-infinite medium?
 
[PLAIN]http://img253.imageshack.us/img253/4029/eqn464.gif

Am i use this equation?
 
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That's what I'd use.
 
Can you show your calculations or explanations how you solve this problem?
 
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Im not sure of how to explain it mathematically but isn't that just the time it takes for your finger to reach a temperature at which it can burn?
 
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