Physics Lesson 13.3.2 - Joule or Calorie?

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Welcome to our Physics lesson on Joule or Calorie?, this is the second lesson of our suite of physics lessons covering the topic of Absorption of Heat, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Joule or Calorie?

Since heat is a form of energy, its unit is normally Joule, J. However, it was not always so in the past. Initially, another unit known as calorie, cal was used as a unit of energy. It represents the amount of heat supplied to 1 g of water to increase its temperature by 1°C. Nowadays, calorie is used as a unit of energy in food industry. Since calorie is very small, a multiple of it, known as kilocalorie, kcal (or Cal with uppercase C)is used as a unit of food energy. Thus,

1 Cal = 1
kcal = 1000 cal

Example 2

The unit calorie (cal) is based on the heat absorbed by water. Thus, 1 cal is the amount of heat energy needed to increase the temperature of 1 g water by 1 degree (Celsius or Kelvin). Based on this info, find the conversion factors between calorie and Joule given that the specific heat capacity of water is 4186 J/kgK.

Solution 2

The value of specific heat capacity of water shows the amount of heat (in Joules) needed to increase the temperature of 1 kg water by 1 degree (Celsius or Kelvin). Therefore, given that 1 kg = 1000 g, the amount of heat energy (in Joules) needed to increase the temperature of 1 g water by 1 degree (Celsius or Kelvin) is

c(1g) = 4186 J/1000
= 4.186 J

From the info provided in the clues, this value corresponds to the heat in calories. Therefore, we can write

1 cal = 4.186 J

This number represents the conversion factor between calorie and Joule. For example, if we see on a chips package that the nutritional values per 100 g are 0.22 kCal (22°Cal), it means that if you eat 100 g of such chips, your body gets

(220 × 1000) cal × 4.186 J/cal
= 920 J

of food energy, which is partially converted into heat, in order to maintain a constant body temperature (37°C). (The rest is converted into other forms of energy needed to the body for operating regularly).

You have reached the end of Physics lesson 13.3.2 Joule or Calorie?. There are 5 lessons in this physics tutorial covering Absorption of Heat, you can access all the lessons from this tutorial below.

More Absorption of Heat Lessons and Learning Resources

Thermodynamics Learning Material
Tutorial ID	Physics Tutorial Title	Revision Notes	Revision Questions
13.3	Absorption of Heat
Lesson ID	Physics Lesson Title	Lesson	Video Lesson
13.3.1	Absorption of Heat. Good and Bad Absorbers of Heat
13.3.2	Joule or Calorie?
13.3.3	States of Matter
13.3.4	Change of State
13.3.5	Latent Heat. Specific Latent Heat of Fusion and Vaporization

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