Often in physics problems energy values are given for different processes in calories. However, in the international system of measurements (SI), the accepted unit for this physical quantity is the joule. In the article, we will consider in detail the issue of converting kilocalories to joules.
What does the value mean?
You should deal with the very concept of "calorie". And then go on to consider how to convert kilocalories to joules.
It was introduced into physics in 1824 by the French scientist Nicolas Clement. At the end of the 19th century and in the first half of the 20th century, calories were often used to quantify energy processes. It was used as an energy measurement in the Technical System of Units.
The value is called a small calorie. It represents energy (heat), which must be transferred to 1 gram of water in order to heat it by 1 degree Celsius.
There is also a large calorie. Its value is equal to the energy required for heating by 1 degree already 1 kilogram of water. Given the prefixes for units of measure, a large calorie is recorded as 1 kcal = 1000 cal.
What is 1 kilocalorie in joules?
This question will seem simple to many. Indeed, for this you need to take 1 kg of water, heat it at 1 o C and measure how much heat was transferred during this process. The problem is that the heat capacity of water depends on its temperature.
If you look at the tabular data, you can see how in the temperature range from 0 o C to 100 o C the heat capacity of H 2 O varies from 4174 kJ / kg to 4220 kJ / kg. Moreover, it first decreases, reaching a minimum at 30-40 o C, and then again increases up to the boiling point.
The way out of this difficult situation was found. Scientists have linked an extra-systemic unit of energy to a specific water temperature. Two temperatures are most commonly used: 15 o C and 20 o C.
In the first case, it is a standard calorie, it is 4.1868 J. In the second case, it is a thermochemical calorie, it is slightly smaller than the standard one and equal to 4.184 J. These values differ from each other by only 0.07%.
When solving problems in physics, it is recommended to use the larger of the given values, i.e. 1 cal = 4.1868 J, since this value is standard, recognized in many developed countries of the world.
Accordingly, 1 kilocalorie in joules will be equal to 4186.8 J or 4.1868 kJ.
Where is this knowledge important?
As mentioned above, calories are not used in science. There it is recommended to use the joule for measuring energy.
Nevertheless, in the production of food products, the energy value of their contents in kilocalories is applied to labels. As a rule, these values are duplicated by the corresponding figures in kilojoules. However, many modern diets are based on a calorie system.
Why is it important to know the energy value of foods that a person eats? Because the difference between calorie consumption and their intake in the form of food depends on whether the weight will increase or not.
Everyone who has come across this issue in their life knows that fats contain 2.25 times more energy per unit mass than proteins and hydrocarbons. In addition, there is such a thing as "empty calories", which indicates the harmfulness of products (they supply energy to the body, but at the same time do not contain nutrients and vitamins). A vivid example of products with a huge content of empty calories are alcoholic beverages.
Problem solving example
Here is a solution to the simple task of converting kilocalories to joules. Suppose a person bought 2 kg of meat in a store. He knows that this product contains 1250 kcal / kg. You must find the appropriate value in joules.
Since the mass of meat is 2 kg, then its calorie content is 2 [kg] * 1250 [kcal / kg] = 2500 kcal. Knowing that 1 kcal = 4186.8 J, we use the proportion. We get: 2500 * 4186.8 = 10467000 J or 10.467 MJ.