IB Physics B4 Notes

Laws of thermodynamics

In Topic B.1 – B.3, you learned about the basics of thermal energy transfers and gas laws. For HL students, you are expected to know more detail, starting with the four laws of thermodynamics.

0. Zeroth law – if two systems are in thermodynamic equilibrium with a third system, then they are in thermodynamic equilibrium with one another. This is a roundabout way of saying that these two systems have the same temperature.
1. First law – as per the law of conservation of energy, a closed system exhibiting a change in energy converts this into other forms to maintain total energy. In other words, energy cannot be created nor destroyed, but only changes forms.
2. Second law – in a closed system, spontaneous processes are not spontaneously irreversible, and consequently increase the disorder (entropy) of the universe. 
3. Third law – when an isolated system reaches a temperature of absolute zero, its entropy reaches a constant value.

Within these laws, two terms need to be defined:

1. Closed system – a closed system does not allow mass to transfer in or out but allows energy to transfer in and out in the forms of heat or work.
2. Isolated system – an isolated system does not allow mass or energy to transfer in or out.

The Zeroth law was effectively covered in B.1, so does not need further evaluation. However, you are expected to know the first and second laws in depth, so let’s cover these one-by-one.