Work is the mechanical transfer of energy to a system or from a system by an external force on it.
Heat is the non-mechanical transfer of energy from the environment to the system or from the system to the environment because of a temperature difference between the two.
When a system is isolated, it means that it is separated from its environment in such a way that no energy can flow in or out of the system.
- The total energy of the isolated system will remain constant as there is no extra energy contributed to the system by work done or heat.
A non-isolated system will interact with its environment and exchange energy with its environment. Thus, the total energy of the system may not be constant and ΔE may not be equal to zero all the time.