When two different systems at different temperatures are in contact, they will eventually reach an average temperature. From this observation we can safely conclude that the system that had a higher temperature has given thermal energy to the one with lower temperature. The thermal energy given or received is called heat.
It’s wrong to think of heat as a substance. Itisn’t something that the objects possesses, but something that they give or receive.
In the SI (International System of Units), heat is measured in Joules, because heat is a form of energy. But three others heat measuring units are still used. These first measuring units were based on the thermal energy required to produce a change. These units are the calorie, the kilocalorie and the British ThermalUnit (Btu).
T1 > T2
1. The concept of heat is best defined by:
b) Chaotic molecular energy.
c) Internal energy increment of a system.
d) Energy that moves between two objects at different temperatures.
e) Kinetic and potential energy of an object’s atoms.
- Onecalorie (cal) is the amount of energy required to raise the temperature of one gram of water by one degree Celciuslñ
- The heat capacity (C) of an object, is the relation between the supplied heat and the temperature difference of the object. We can also define it as the amount of heat necessary to raise the temperature of an object by one degree.
From this definition we can observe thatwhen heat is given to an object, the temperature will rise, therefore:
- The specific heat (c) of a material is the amount of heat needed to raise the temperature of one gram of said material by one degree.
The specific heat of water is, by definition, 1 cal/gºC . The next table shows the specific heat for some materials:
|Material|Specific Heat (cal/g ºC) |
|Oil |0.47 |
|Water |1.00 |
|Alcohol |0.66 |
|Copper |0.093 |
|Ice |0.55 |
|Wood |0.42 |
|Glass |0.20 |
|Aluminium |0.22 |
Using the definition of specific heat and heat capacity, we can define the heat (Q) transferred between an object and itssurroundings as:
Notice that when heat is added the temperature rises and Q will be positive. When heat is substracted, the temperature decreases and Q is negative.
The unit for measuring heat in the SI is the Joule, but the most commonly used is the calorie, whose relation with the former is 1 cal = 4.18 Joule (Mechanical equivalent of heat).
Thermal Energy ConservationWhen two objects A and B, at different temperatures, are put inside a calorimeter, they will enter in thermal contact and eventually reach thermal equilibrium. Considering that the heat given by one object is received by the other we find that:
Calorimeter: Container in whose inside heat exchanges happen. The calorimeter is thermally isolated to...