Hva er termodynamikk?

What is thermodynamics?

Thermodynamics is the science of how the heat and work of a system is related to the system state. Energy processes that convert heat energy into mechanical work are among the major concerns of this science. One example is the motor of a car, were fuel is used as heat energy source. But not only concrete examples like this are described by thermodynamics. In fact, the thermodynamics is everywhere, because our surroundings consist of systems where heat is flowing and work is done. The fridge and boiling of water are systems that can be described by thermodynamics. The same is valid for the air around us: A thin layer of air close to our bodies a cold day has got a higher temperature than the air further away from us. This is because heat from the body is flowing into the surroundings, by making the molecules in the air move faster than before.

A system always has a physical boundary, which can be permeable for mass and heat. If a system is isolated, the boundary is closed for both heat and mass transfer. A closed system can exchange heat, but not mass, with the surroundings.

Heat is a form of energy that is transferred from one system to another by crossing a boundary. Heat transfer can occur when there is a temperature difference, and the flow is always going from a hotter system to a colder one. This will continue until the temperature of the systems are the same, and the systems hence are in thermal equilibrium with each other. An equilibrated system is not changing its state.

Work is done on a system when for example the boundary of the system is moving in the same direction as a force acting on it. If the direction of the movement is reversed, then the work is done by the system on its surroundings.

Heat and work have the same unit (joule, J) as energy. They are not state functions. This means that by knowing only the amount of heat or work supplied to a system, it is not possible to decide the state of the system. The way the heat or work is added that is also interesting. For example, work can be done both reversible and irreversible. 

However, the change in internal energy, which can be written as the sum of heat and work added to a system ( U = Q - W), is a state function. Internal energy is the kinetic and potential energy of the molecules in a system, and it is a function of temperature. This explains why the temperature increases when heat is added: by making the molecules move faster, the kinetic energy is increasing, and the internal energy and hence the temperature is increasing.