The other process by which the internal energy can change is through
so called heat flow. This concept is more subtle and covers a range of
physical processes which we shall give examples of in the following.
In summary heat flow is the exchange of internal energy between
thermodynamic systems. We denote heat flow into our system
in consideration by dQ. Thus in a process involving only heat flow and no
mechanical work we have
Again we have chosen the sign convention for dQ so that it is mainly a
positive quantity for thermodynamic processes by which heat is converted
to work.
The three most important physical processes by which internal energy is exchanged between thermodynamic systems are radiation, conduction, and convection.