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

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SUMMARY

The discussion focuses on the physics behind why a finger can pass through an 800°C candle flame without being burned, provided the exposure is limited to approximately 0.25 seconds. Key calculations involve the convection heat transfer coefficient (h = 100 W/m²·K) and modeling the finger as a semi-infinite medium. The burn threshold for human skin is established at 65°C, and participants suggest calculating the energy transferred to the finger during the brief exposure to determine the maximum temperature increase. The conversation emphasizes the importance of understanding heat transfer equations in this context.

PREREQUISITES
  • Understanding of convection heat transfer principles
  • Familiarity with thermal properties of human flesh
  • Knowledge of heat transfer equations, particularly for semi-infinite media
  • Basic thermodynamics concepts related to temperature thresholds
NEXT STEPS
  • Study the derivation and application of the heat transfer equation for semi-infinite solids
  • Learn how to calculate energy transfer using the specific heat capacity of biological tissues
  • Explore the implications of convection heat transfer coefficients in different materials
  • Investigate the thermal response of human skin to brief high-temperature exposures
USEFUL FOR

Students in physics or engineering, educators teaching heat transfer concepts, and anyone interested in the thermal properties of biological tissues.

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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