Heat and related entities

Heat is a form of energy that can be intuitively associated to molecular motion. Transferring heat to a system is tantamount to increasing the motion of its constituent molecules. Heat has been traditionally measured in calories (cal), one calorie being the amount of heat requited to increase the temperature of 1 g of liquid water by 1 °C at 1 atm. However, in order to stress the interconvertibility of all forms of energy, it is better to express heat in joules (J), the international unit for energy, 1 J being approximately equivalent to 0.24 cal. 

Note that, in spite of the obvious relation between heat and temperature, they are very different entities. Heat is an extensive entity (i.e. it varies with the amount of matter) while temperature is an intensive entity. 

As mentioned above, thermotropic phase transitions may be brought about at constant pressure by adding or removing heat. (In biology, atmospheric pressure is considered constant for all practical purposes.) This is the case when ice is heated up to give liquid water, or when a lipid bilayer in the Lp phase is heated up to give an La phase. In thermodynamics, the amount of heat gained or lost by a system at constant pressure is called enthalpy (H). We can write  

A related usefiil parameter is Cp, or heat capacity at constant pressure. Heat capacity refers to the amount of heat required by a system in order to have its temperature increased by a given amount, at constant pressure. Its units are energy temperature.  

The transition temperature should he measured at the onset of the transition (Ton, also called Tc). However, for hroad transitions, the temperature is measured at the midpoint of the transition, i.e. in the middle of the peak (T ). 

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