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

[chronicals]

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

 

[Mapes]

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

 

[chronicals]

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?

 

[Mapes]

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?

 

[chronicals]

i think finger may be modeled as semi-infinite medium, initially at a uniform
temperature 37°C, anybody help me overcome this problem

 

[Mapes]

Good choice. What equations have you found for modeling convective heat transfer to a semi-infinite medium?

 

[chronicals]

[PLAIN]http://img253.imageshack.us/img253/4029/eqn464.gif

Am i use this equation?

 

[Mapes]

That's what I'd use.

 

[chronicals]

Can you show your calculations or explanations how you solve this problem?

 

[donutz610]

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?