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Difference between 'KJ' & 'Kcal'

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Thin man
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2006/02/05 15:39:56 (permalink)

Difference between 'KJ' & 'Kcal'

Hi guys, when you look up the energy in the nutritional information you can see both KJ and/or Kcal. I'm guessing the Kcal is the 'calories' ?
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    ink
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 15:54:19 (permalink)
    Joules or Kilojoules and calories or Kilocalories are both different units of energy. When someone says they had 500 calories they really mean they had 500,000 calories or 500Kcal.
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    sillynarbie
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 15:54:49 (permalink)
    kcals is the calories yes, just units of energy mate, nothing to worry about. The reason kJ is put on is because its in terms of the SI Uni Joules, just a technicality.
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    Thin man
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 16:15:09 (permalink)
    Cheers guys, it's was a bit confusing...
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    Big Les
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 16:32:40 (permalink)
    they are both units of energy
    if i remember rightly - a you times joules by 4.2 to get cals and oviously divided both joules and calories by 1000 to get kilo joules and kilo calories.
    Now isnt a calorie the amount of energy needed to raise the temperature of 1ltr of water 1 degree

    I should really know this better - lol

    big les

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    sillynarbie
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 16:43:10 (permalink)
    One joule is defined as the amount of work or energy exerted when a force of one newton is applied over a displacement of one metre, you're thinking of specific heat capacity Les
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    Big Les
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/05 18:39:24 (permalink)
    I was thinking of calorimitery and calorimiters (okay I cant spell) - which is to do with heat given off by the burning of different foods - in very simple terms -
    Which has a lot to do with heat and specific heat capacity - which is 4.2 for water isnt it

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    euphoria
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/06 05:23:32 (permalink)
    I think seungmena means multiply calories by 4.2 to get joules :P
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    Big Les
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/06 08:08:00 (permalink)
    okay - maths is not a strong point of mine but here you go:

    if you want to derive calories from kJoules you divide by 4.2

    Now the energy value of foods is determined using a bomb calorimiter and measuring the temperature change in the water.

    Thus you use the following formula

    heat= Mass x specific heat capacity x temperature change

    So you have the mass of water in g, the specific heat capacity of water which is 4.2 and the temp change in degrees C or kelvin as they are the same

    So for eg
    mass of water 100g
    Temp change of 15 degrees

    heat = 100x4.2x15 which equals 6300 J - or Joules
    so thats 6.3 kjoules or 6.3/4.2 to give 1.5calories

    This is the basic mathematics behind the calorie values on your food labels.

    hope that clears it all up now.

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    Tony Barnes
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/06 09:40:44 (permalink)
    Yep, I'm sure that's laid plenty of minds at ease Les

    Easy way to do it for calories in water is energy = mass (in kg) x temperature rise (in K) - so cals = 0.1 x 15 = 1.5cals

    Which is why drinking 4litres of ice cold water & warming it up in you, burns an incredible - 4 x 37 = 148cals - woop woop. That said, at 0 C, waters specific heat capacity actually drops, so that's not right anyway...

    The whole situation with Kj/Kcal on the back of packets is silly - short of people with an interest in science, Kj is completely meaningless, and Kcal confusing, is should just say "Calories" to keep it simple (even if I do hate it when stuff is dumbed down, there's no point in confusing people on an important point)
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    euphoria
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/07 11:38:08 (permalink)
    haha lol very imformative and specific
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    sillynarbie
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/07 13:39:50 (permalink)

    ORIGINAL: seungmena

    okay - maths is not a strong point of mine but here you go:

    if you want to derive calories from kJoules you divide by 4.2

    Now the energy value of foods is determined using a bomb calorimiter and measuring the temperature change in the water.

    Thus you use the following formula

    heat= Mass x specific heat capacity x temperature change

    So you have the mass of water in g, the specific heat capacity of water which is 4.2 and the temp change in degrees C or kelvin as they are the same

    So for eg
    mass of water 100g
    Temp change of 15 degrees

    heat = 100x4.2x15 which equals 6300 J - or Joules
    so thats 6.3 kjoules or 6.3/4.2 to give 1.5calories

    This is the basic mathematics behind the calorie values on your food labels.

    hope that clears it all up now.


    Don't mean to be a pedant but the mass is measured in kg not g, and kelvin and celsius aren't the same, but the correct quantity for the SHC equation is CHANGE in temperature, in which case the change in temp will be the same whether in kelvin or celsius.


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    Tony Barnes
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    RE: Difference between 'KJ' & 'Kcal' 2006/02/07 13:43:35 (permalink)
    Double?? lol

    Thought I'd said that in my post (though possibly didn't shout about it..)
    #13
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