"Heat is thermal energy in transit".


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:

Sensitive Heat
It is when a system receives or yields heat, thus suffering a temperature variation and the matter of this system remains in the same physical state, i.e., if the system is a liquid it continues in this physical state.
Latent Heat
Unlike sensible heat, the latent heat does not change the temperature of the system, it causes a change in the physical state. For example, if a system is liquid and after receiving a small amount of heat it begins to evaporate, we say that this heat is latent.
Heat Capacity \((C)\)
The heat capacity \(C\) of a body is the quotient between the amount of heat \(\Delta Q\) it exchanges with the outside and the corresponding increase in temperature \(\Delta T\). \( C = \frac{\Delta Q}{\Delta T}. \)
Specific Heat \((c)\)
It is the thermal capacity per mass unit of a body and depends on the nature of the substance of which this body is constituted. In general, it is represented by the letter \(c\). Two bodies with the same mass and different sensible heat will have a different temperature variation for the same amount of heat received. The body with lower specific heat suffers greater temperature variation.

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 ÂșC) 0.530 2220
Mercury 0.033 140
Sea Water 0.93 3900
Water 1.00 4190
Water vapor 0.48 2011

Calorimetry Fundamental Formula

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:

Principle of Exchanges
"If someone won, it is because someone lost."

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.

Principle of Reverse Transformation.
"The price of the one-way ticket is the same as the return ticket."

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.

Theory of the Caloric

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.