Energy can be expressed as both heat and work, $\Delta U = Q+W$, where $\Delta U$ is the change in total internal energy of a system, $Q$ is heat1, and $W$ is work. Both heat and work are expressed in joules.
Though heat and work have the same unit, they are actually different from each other. it’s necessary to understand this differentiation between them in a physical context before we further explore thermodynamics.
In a nutshell, heat is the transfer of energy purely by a temperature difference and work is the transfer of energy by means other than the temperature difference. In other words, heat is the thermodynamic process that adds or removes energy from a system in a surrounding due to the temperature gradient between them. And work is the process that transfers energy in ways that are not dependent on this gradient.
An interesting thing to note is that, no matter how you transfer energy, i.e, either as heat or work, the net change in energy will be independent of the thermodynamic path.
A common misconception is that ‘heat’ means the amount of thermal energy in a system. In a thermodynamic context, heat simply means the movement of energy. But nevertheless, we still use the term ‘Heat transfer’ in a redundant fashion, mostly in engineering than in physics. ↩