The Mystery of Human Luminosity: A Comparison to Daily Energy Intake

  • Thread starter Thread starter Xalkias
  • Start date Start date
  • Tags Tags
    Human Luminosity
Click For Summary
SUMMARY

The discussion centers on calculating human luminosity based on the assumption that humans radiate like black bodies. Using the formula L=AσT4, the calculated luminosity is 733 watts for a human with a surface area of 1.3 m2 at a temperature of 310K. In contrast, the average daily energy intake from food is approximately 3000 calories, equivalent to 12500 joules, leading to the conclusion that humans radiate more energy than they consume. This discrepancy raises questions about energy balance and the definitions of "calorie" in scientific versus layman contexts.

PREREQUISITES
  • Understanding of black body radiation principles
  • Familiarity with the Stefan-Boltzmann Law
  • Basic knowledge of thermodynamics and energy units
  • Concept of caloric energy and its measurement
NEXT STEPS
  • Research the Stefan-Boltzmann Law and its applications in thermodynamics
  • Explore the differences between scientific and dietary definitions of calories
  • Investigate energy balance in biological systems
  • Learn about black body radiation and its implications in physics
USEFUL FOR

Students in physics, nutritionists, and anyone interested in the relationship between energy intake and thermodynamic principles in living organisms.

Xalkias
Messages
7
Reaction score
0

Homework Statement


If we assume that a human radiates like a black body, calculate the luminosity in watts, then compare it to the daily energy a human gets from eating. (The area of a human A=1.3 m2)

The Attempt at a Solution


The temperature of a human is 310K so L=AσT4 = 1.4*5.6*10-8*(310)4=733 watts and the average energy of a human gets from eating is about 3000 calories=12500 joules.
So in one day a human seems to radiate too much energy compared to the daily energy he gets. What is going on?
 
Physics news on Phys.org
Xalkias said:
What is going on?
Let's say you're not looking at a human, but at a rock. It gets zero Joules from eating. How come it stays at a comfy 30-ish degrees centigrade on a warm day?
 
  • Like
Likes   Reactions: Bystander
Xalkias said:
...3000 calories=12500 joules.
... What is going on?
Along with paying attention to Bandersnatch's comment, you might also want to look into the sciency vs laymanish use of the word "calorie".

hint: They have two different values.

ps. I'm going to ignore your "area" typo, as it's not that big a deal.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 1 ·
Replies
1
Views
19K
  • · Replies 15 ·
Replies
15
Views
6K
Replies
18
Views
7K
  • · Replies 7 ·
Replies
7
Views
4K
  • · Replies 7 ·
Replies
7
Views
4K