THERMODYNAMICS: Heat

Based on our own perceptions, it's hard to perceive what it is exactly, For a long time, Scientists described heat as a kind of fluid because it's flow from one system to another but these days we actually known that Heat is not actually a fluid.

Heat is energy that is transferred between a system and its environment because of a temperature difference between them. In Equations we represents heat as "Q". It can be measured in joules (J), calories (cal), kilocalories (Cal or kcal), or British thermal units (Btu).

This video can help you to visualize the concept of heat:

The Absorption of Heat by Solids and Liquids

Heat capacity

The heat capacity C of an object is the proportionality constant between the heat Q that the object absorbs or loses and the resulting temperature change of the object that is:

The word “capacity” in this context is really misleading in that it suggests analogy with the capacity of a bucket to hold water. That analogy is false, and you should not think of the object as “containing” heat or being limited in its ability to absorb heat. Heat transfer can proceed without limit as long as the necessary temperature difference is maintained.The object may, of course, melt or vaporize during the process.

Specific heat

Specific heat c that refers not to an object but to a unit mass of the material of which the object is made. can replace the Heat capacity in the previous equation making it:

Heats of Transformation

When energy is absorbed as heat by a solid or liquid, the temperature of the sample does not necessarily rise. Instead, the sample may change from one phase, or state,to another.

The amount of energy per unit mass that must be transferred as heat when a sample completely undergoes a phase change is called the heat of transformation L.Thus, when a sample of mass m completely undergoes a phase change, the total energy transferred is:

When the phase change is from liquid to gas (then the sample must absorb heat) or from gas to liquid (then the sample must release heat), the heat of transformation is called the heat of vaporization L_v.

When the phase change is from solid to liquid (then the sample must absorb heat) or from liquid to solid (then the sample must release heat), the heat of transformation is called the heat of fusion L_f.

Examples:

1. An aluminium plate with a mass of 1.5kg. Calculate the thermal energy stored in the plate when the temperature rises from 20°C to 200°C. The specific heat capacity of aluminium is 913 J/kg° C.