Some processes are reversible, and some processes are irreversible. If a process is reversible, this means that we can make an Infinitesimal change in the conditions of the process and reverse it, all the while the system being in or close to thermal equilibrium.
All other thermodynamic processes are irreversible. :)
\begin{equation} e = \frac{W}{Q_H} = 1 + \frac{Q_C}{Q_H} = 1 - \left| \frac{Q_C}{Q_H} \right| \qquad \textnormal{Efficiency of engine} \end{equation}
\begin{equation} e = 1 - \frac{1}{r^{\delta - 1}} \qquad \textnormal{(Efficiency, r = compression ratio, δ = heat cap. of working substance)} \end{equation}
\begin{equation} K = \left| \frac{Q_C}{W} \right| = \frac{\vert Q_C \vert}{\vert Q_H \vert - \vert Q_C \vert} \qquad \textnormal{(Coefficient of performance)} \end{equation}
The second law has to do with the direction of natural thermodynamic processes.
We can describe it in different ways,
and one way to describe it is that the number of microstates of a system is always increasing.
The equation is
\begin{equation}
S = k_B \, \textnormal{ln} \, \Omega \qquad \textnormal{(Entropy)}
\end{equation}
Ω are the microstates, and kB is the Boltzmann constant.
The units of the second law are Joules / kelvin. So if
S is always increasing, a simple example would be
1 Joule / kelvin, 2 joules / kelvin, 3 joules / kelvin, etc..
We can also give an equivalent engine description, which says that no cyclic process can convert heat into work completely.
The refrigerator description says that no cyclic process can transfer heat from a colder place to a hotter place without mechanical work.
We can evaluate the change in entropy for a reversible process, given by \begin{equation} \Delta S = \int_{T_1}^{T_2} \frac{dQ}{T} \qquad \textnormal{(Reversible process)} \end{equation}
\begin{equation} e = 1 - \frac{T_C}{T_H} = \frac{T_H - T_C}{T_H} \qquad \textnormal{(Efficiency of carnot engine)} \end{equation} \begin{equation} e = 1 - \frac{T_C}{T_H} = \frac{T_H - T_C}{T_H} \qquad \textnormal{(Coefficient of performance)} \end{equation}
Continue on to gen phys 2, where you'll learn about charge, circuits, electromagnetic waves, and much more here!