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

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Discussion Overview

The discussion revolves around the physics of why a finger can pass through a candle flame without being burned, focusing on heat transfer principles and modeling the finger's response to the flame. Participants explore theoretical and mathematical approaches to understand the conditions under which skin does not reach the burn threshold.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant asks for help with equations to solve the problem, noting the burn threshold of human skin and properties of human flesh.
  • Another participant questions the significance of the convection heat transfer coefficient (h) and suggests modeling the finger as a finite object or semi-infinite medium.
  • It is noted that h represents the convection heat transfer coefficient, and a cylindrical model for the finger is proposed.
  • A suggestion is made to calculate the energy transferred to the finger in 0.25 seconds and how much flesh that energy could heat up to the burn threshold.
  • One participant proposes modeling the finger as a semi-infinite medium initially at a uniform temperature of 37°C.
  • Another participant asks for equations relevant to modeling convective heat transfer to a semi-infinite medium.
  • A link to an equation is shared, with a participant confirming its use for the problem.
  • A request is made for calculations or explanations on how to solve the problem mathematically.
  • One participant expresses uncertainty about the mathematical explanation but suggests that the time it takes for the finger to reach a burnable temperature is a key factor.

Areas of Agreement / Disagreement

Participants generally agree on the importance of heat transfer modeling and the relevance of the burn threshold, but there is no consensus on the specific approach or calculations to be used. Multiple competing views on modeling the finger and the equations to apply remain present.

Contextual Notes

Participants have not resolved the specific assumptions regarding the modeling of the finger or the exact equations to apply, leaving some steps in the mathematical reasoning unresolved.

chronicals
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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 threshold 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?
 
Last edited by a moderator:
That's what I'd use.
 
Can you show your calculations or explanations how you solve this problem?
 
  • #10
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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