The first law of thermodynamics states that, in a closed system, energy can be neither created nor destroyed: it can merely change its form. However, the implications are somewhat different for open systems. Energy still cannot be created or destroyed, but it can exchange energy - such as heat or light - with surrounding systems.
The laws of thermodynamics are a set of fundamental principles describing the nature of energy, heat, and work. The first law of thermodynamics, discussed here, states that energy can neither be created nor destroyed, and is also known as the law of conservation of energy. The second law of thermodynamics states that in a closed system, the level of entropy (disorder) always increases over time, until it reaches a point where no further work (transformation of energy) is possible. Finally, the third law of thermodynamics states that as an object's temperature is cooled, it approaches a point where all motion and potential for work ceases (known as absolute zero) and entropy becomes minimal. However, near-absolute zero temperatures can only be achieved through heat transfer to the surroundings, and not in a closed system.
In a closed system, one which has no contact with its external surroundings, the first law of thermodynamics indicates that the total amount of energy present will always remain the same. Only the form of this energy can change, through work. In practice, however, closed systems generally do not exist in nature except at the level of the universe as a whole; to at least some minimal extent, energy transfer occurs between most seemingly closed systems.
What this means is that, in practice, energy can both appear to be created (more energy is present in the system than before) as well as destroyed (less energy is present in the system than before). In practice, however, the energy in question has been neither created nor destroyed: in contrast, it has been gained or lost through exchange with the surroundings. From an engineering perspective, and in terms of fluid dynamics, some open systems are steady flow systems - that is, at least for the purposes that system is being observed for, the mass or energy which enters a system is roughly equivalent to that which leaves the system. In one sense, for example, a room heater is a steady system: the same mass of air which enters the heater then leaves it, combined with (most of) the energy used to heat the air to a higher temperature. Alternatively, open systems can be seen as unsteady flow systems - that is, those where the mass or energy which enters the system is clearly not equal to that which leaves, either because more is retained within the system or because more is released to the surroundings.