The SI unit of energy: the joule

  

What is a joule? A joule (pronounced as in cool; symbol, J) is a unit of energy. It is approximately one quarter of a calorie – so one calorie equals four joules.

What is a watt? A watt (symbol, W) is a joule per second (in symbols, J/s). It is the unit of power – the rate of energy transfer.

And, “watt's a joule”? A joule is also a watt-second (symbol, Ws); it is the amount of energy transferred during one second in a process operating at one watt.

   

60 W

A typical power rating for a traditional incandescent light bulb is sixty watts.

It has an energy transfer rate of sixty joules per second (60 J/s).

In one second,

an electrical energy input of 60 joules converts to an energy output of 60 joules of heat and light.

A typical energy transfer rate for a woman at rest is similar:

her metabolic power (metabolic rate) is about 60 W.

In one hour,

an energy transfer of 216000 joules or 216 kilojoules (216 kJ).

In one day, 

an energy transfer of 5 184 000 joules or 5.184 megajoules (5.184 MJ).

   

The joule is the unit of energy in a system of units known as the International System of Units or “SI” after the French title: Systéme Internationale d'Unités.

In regard to energy, the SI makes significantly simpler the explanation of the basic concepts that are involved in the management of body mass: 

Energy input in food is in joules; energy output in exercise and metabolism is in joules; and energy stored in fat is in joules.

   

The calorie

Currently, the energy unit most widely known to the general public is the “calorie”. Unfortunately, a confusion is possible as there are two versions: the “calorie” and the “Calorie”, not distinguishable when spoken, and very often, not differentiated when written. 

The Calorie is one thousand times the calorie and should be more exactly referred to as the kilocalorie (kcal), which is more likely to be used by professionals in nutrition rather than the public. 

However, it is generally assumed by the technically interested that when the word is used, it is a shortened form of “kilocalorie”. 

Confusing: what about calling a “kilogram” a “gram” or a “kilometre” a “metre”? 

As Humpty Dumpty said: “When I use a word it means just what I choose it to mean – neither more nor less.”

 

    

For more on:

 

• Food Packaging

   

• EU – Guideline Daily Amounts

   

• United States – Percent Daily Value

   

• SDA: A Standard Daily Amount of Energy – ten megajoules (10 MJ)

   

• Labelling : Calories to SDA %

• Diet: Daily Structure  

 

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