3.1 Introduction to the First Law of Thermodynamics
Simply stated, the First Law of Thermodynamics is a conservation of energy principle. In a
thermodynamic sense, the energy gained by a system is exactly equal to the energy lost by the
surroundings (look at figure below) ...
surroundings, T=290 K
Energy can cross the boundary in a closed system in the form of heat and/or work. These energy forms
are different, making it important to be able to distinguish between them.
3.2 Heat Transfer
Heat is defined as the form of energy that is transferred between two systems (or a system and
its surroundings) by virtue of a temperature difference. That is, an energy interaction is heat if and only if
it occurs because of a temperature difference. Heat is energy in transition. It is only recognized as it
crosses the boundary of a system.
A process during which there is no heat transfer is called an adiabatic process . There are two
ways a process can be adiabatic: either the system is well-insulated so that no heat flows across the
boundary, or the system and surroundings are both at the same temperature (zero temperature
Heat transfer per unit mass, q, of a system is defined as:
The total heat transferred to/from a pure substance between states 1 and 2 can be expressed in
terms of the heat transfer rate as:
where is the heat transfer rate per unit time. For constant
, the equation above becomes ...
Q = Q
Very Important Sign Convention:
Since heat is a directional quantity (it can be lost or gained), it is very important to establish a
sign convention! Heat transfer to a system is positive and heat transfer from a system is negative!
Heat can be transferred in three different ways: conduction, convection, and radiation. Below is
a brief description to familiarize you with the basic mechanisms of heat transfer.