It is a process of transferring thermal energy between systems that are at different temperatures. Since heat is a process, it is not stored in the system, that is, we cannot say that a given system has heat. Heat is the process of transferring thermal energy. What the system acquires in function of this process is energy.
The heat always flows spontaneously from bodies of higher temperature to those of lower temperature. The heat flow ceases when both reach thermal equilibrium, i.e., the same temperature.
Heat is a form of energy, whose unit in the \(IS\) is the Joule (J), but the caloric unit (\(cal\)) is also used, where \(1 cal = 4.186 J \). $$ 1 Kcal = 10^3 cal = Cal. $$
To study the flow of thermal energy we need to know the following definitions:
Below are specific heat values for some materials.
Specific Heat | ||
---|---|---|
Substance | \(\frac{Cal}{g \cdot K}\) | \(\frac{J}{kg \cdot K}\) |
Copper | 0.923 | 386 |
Aluminium | 0.251 | 900 |
Granite | 0.19 | 790 |
Glass | 0.20 | 840 |
Ice (-10 ºC) | 0.530 | 2220 |
Mercury | 0.033 | 140 |
Sea Water | 0.93 | 3900 |
Water | 1.00 | 4190 |
Water vapor | 0.48 | 2011 |
For a mass \(m\) of a specific heat substance \(c\), which is not at the melting or boiling temperature, to undergo a change in temperature \(\Delta T\), it must receive a quantity of heat which is: $$Q = m \cdot c \cdot \Delta T.$$
It is important to highlight some principles, which are:
The algebraic sum of the heat waves in a thermally isolated system is null. That is:$$ \pm Q_1 \pm Q_2 \pm Q_3 \pm \cdots \pm Q_n = 0,$$ where each sign should be chosen as the system receives or yields heat, respectively.
If a system receives (yields) a certain amount of heat when undergoing a transformation, then it will yield (receive) the same amount of heat upon undergoing the reverse transformation.
The concept of heat is rather subtle. In the past people thought (mistakenly) that heat was a kind of fluid contained in objects, called caloric fluid. They thought that hotter objects flowed calorically into the colder ones in order to warm them. However, this supposed substance would have to have rather strange properties compared to typical fluids. Lastly, this theory was discredited and abandoned.