## Definitions

Thermodynamic System – Most of the time when studying the bahaviour of an ideal gas in a particular situation, we focus on the macroscopic behaviour of the gas as a whole. The gas can gain or lose thermal energy, and either do work or have work done on it. Because of this, the gas can be considered a thermodynamic system.

The Surroundings – No system is completely isolated. To some extent, every system interacts with its' surroundings, exchanging heat, doing work or having work done by the surroundings.

Heat– Refers to the exchange of heat energy between a system and its' surroundings as a result of a temperature difference – heat flows from hotter to colder bodies until the temperatures are the same.

Work– Refers to the macroscopic transfer of energy. For example, Work is done by a forceif it forces a piston back a distanceSuch a force could be exerted by a gas – when the gas expands it does work on its' surroundings. Conversely when the gas is compressed, the surroundings do work on the gas.

Internal Energy – Labelledwith a change in internal energy being labelledThe internal energy is the energy of the particles of a system because of their random movement. It is in fact kinetic energy arising from the random motion of the particles of a system.

It is different from the energy of a system, because a system may have in general many kinds of energy – gravitational or elastic potential energy, chemical energy...

The internal energy of a system is in fact the heat energy, because heat is due to the random motion of the particles of a system. An increase in internal energy therefore means an increase in temperature, as does a change in phase from liquid to gas for example.

Ideal gas - An ideal gas is a gas that obeys the equation of state for an ideal gas

$pV=nRT$
.

Mol  - The mol is the basic SI unit for 'amount of substance'. One mol of a substance is the amount of that substance that contains the same number of particles as 12g of Carbon-12, ie
$6.02 \times 10^{23}$
particles.

Avogadro's Constant - The number of particles in 1 mol, equal to
$6.02 \times 10^{23}$
.

Molar Mass - The mass of 1 mol of a substance.