Sunday, 22 July 2012

How much heat is given off by a human...

(An updated version of this post is available here).  Application to hypo- and hyper- thermia are also discussed there.

Heat is generated in biochemical reactions in cells. Each gram of carbohydrate generates 4 Calories, while fat and protein generate 9 and 4.5 Calories respectively.
Humans are hot-blooded and their body temperature is maintained constant at about 37C, the heat produced in the body has to be given off to the outside. This is done through breathing, sweating and radiating away heat energy to cooler surroundings, say at 20C.

Your calorie consumption goes up if you are physically active, like running or climbing stairs than when you are sitting calmly. In one hour, a 70 kg man would burn, approximately, 80 Calories sitting; 110 standing; 320 walking and 500 jogging.

Suppose, that every day a human consumes 2000 Calories (Some eat much more and get obese!).
We can calculate, using physics, the amount of energy produced in a day.
The Calorie used by nutritionists is actually equal to 1000 times the calorie used by physicist to measure energy. Note C and c in the two definitions of energy.

1 Calorie = 1000 calories or 1 kcal

In physics the usual unit to express energy is a Joule (J), and 1 calorie = 4.18 J

So we consume 2000 x 1000 calories/day
= 2,000,000 calories/day
= 2,000,000 x 4.18 J/day
= 8,360,000 J/day

To make sense of Joule per day, it is better to talk about power that is J per second or Watt (W)

A day has 24 x 3600 second = 86,400 seconds

We produce 8360000/86400 J per second (W) = 96.76 W (nearly 100 W) and this is given off to the surroundings as heat.

A human gives off heat equal to that of a 100 W incandescent light bulb (old style light bulbs) which actually converts most of the electricity to heat.
The new fluorescent light bulbs and particularly LED lights are more efficient in converting electric energy to light and that is why they do not get too hot when in use.
They are also cheaper to use.

Now you know why the room feels very warm when a lot of people are present and talking animatedly.
Or why you sweat and breath faster when jogging - you are burning extra Calories and producing heat faster.  The body has to get rid of the extra heat by sweating and respiration.

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