Tuesday, 16 January 2018

Heat given off by a human body is about 100 Watt

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Humans are warm-blooded mammals and their body temperature is maintained constant at about 37C. Comfortable ambient temperature is much cooler at about 20C.  We are constantly losing heat energy from the body at higher temperature to the  surroundings at a lower temperature.  The heat energy is lost by radiation, sweating and respiration.  That is why when a lot of people are present in a room, it feels warmer.

The body temperature is maintained at 37C by compensating the energy lost to the colder surroundings by converting the food we eat into energy. Heat is generated in biochemical reactions in cells (we say that food is burnt to produce energy). Each gram of carbohydrate generates 4 Calories, while fat and protein generate 9 and 4.5 Calories respectively.  In the end, all energy generated by  digesting food is converted into heat which needs to be lost to maintain a constant body temperature.

Therefore, energy given off by a human body in a day is equal to the energy produced by the food we eat per day.  We shall calculate this in the following:

Suppose, that every day one 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 (used in nutrition) = 1000 calories or 1 kcal (used by physicists)

In physics the usual unit to express energy is a Joule (J), and 1 calorie = 4.18 J 
(One Joule is not a lot of energy - if we lift a 100 gram tomato from ground to 1 meter height then we have spent 1 Joule of energy.  
If we took one second to do so then the rate of energy used is 1 J/s or 1 Watt; take 2 seconds, the energy spent is still one Joule but rate of energy used is 0.5 Watt)

So we consume 2000 Calories or 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 more sense of Joule per day, it is better to talk about power that is J per second or Watt (W). We divide J/day by the number of seconds in a day to obtain J/s or Watt.

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 eventually given off to the surroundings as heat.

10 humans will give off 1000W - that is like one bar of electric fire switched on!

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 style LED lights are much more efficient in converting electric energy to light and that is why they do not get hot when in use.  This also makes LED much cheaper to use for the same light output.

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 68 kg man would burn, approximately, 70 Calories sitting; 85 Calories standing; 340 Calories walking at 6 km/hr and 700 Calories running at 10 km/hr.

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.

Final Word:  In this blog, we have calculated the heat produced by the food (2000 Calories per day) we consume  - essentially our metabolic rate (BMR) - and to a good approximation it is 100 W.  Under ideal conditions of comfortable environmental temperature, this energy is lost to the environment. The question I would like to address in my next blog is what happens when the outside temperatures are either too cold or too high for our thermal regulation mechanisms to work efficiently or not very well at all.  With climate change, heat strokes have become more wide spread and a good understanding of the science of heat strokes will be very useful - this is the subject of my next blog. 
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