Cyclic process: $\Delta U_{cycle} = 0 \Rightarrow Q_{net} = W_{net}$
Work done in CA (isobaric) = 600 J (given)
Cyclic net work = area enclosed in P-V diagram
Given data yields $W_{net} = \mathbf{600}$ J
Answer: 600 J
Zeroth law: if A is in thermal equilibrium with C, and B is with C, then A and B are in equilibrium (defines temperature). First law: $\Delta U = Q - W$ (conservation of energy). Second law: heat flows spontaneously from hot to cold; entropy of isolated system increases. Third law: entropy of a perfect crystal at 0 K = 0; absolute zero is unattainable.
Isothermal ($T$ = const, ideal gas): $PV = $ const (Boyle\'s law). $W = nRT\ln(V_2/V_1) = P_1V_1\ln(V_2/V_1)$. Adiabatic: $PV^\gamma =$ const, $TV^{\gamma-1} =$ const. $W = (P_1V_1 - P_2V_2)/(\gamma-1)$. Isochoric ($V$ = const): $W = 0$, $Q = \Delta U = nC_v\Delta T$. Isobaric ($P$ = const): $W = P\Delta V = nR\Delta T$, $Q = nC_p\Delta T$.
Entropy $S$: measure of disorder. $dS = dQ_{rev}/T$. Entropy increases in irreversible processes. Carnot cycle: most efficient reversible cycle operating between $T_H$ and $T_C$. $\eta_{Carnot} = 1 - T_C/T_H$. Coefficient of performance (COP): refrigerator COP = $Q_C/W = T_C/(T_H - T_C)$. Heat pump COP = $Q_H/W = T_H/(T_H-T_C)$.
Ideal gas: $PV = nRT = NkT$ where $N$ = number of molecules, $k_B = R/N_A$. RMS speed: $v_{rms} = \sqrt{3RT/M} = \sqrt{3k_BT/m}$. Average speed: $v_{avg} = \sqrt{8RT/\pi M}$. Most probable speed: $v_p = \sqrt{2RT/M}$. Ratio: $v_p : v_{avg} : v_{rms} = 1 : 1.13 : 1.22$. Mean free path: $\lambda = 1/(\sqrt{2}\pi d^2 n)$ where $n$ = number density.
The first law is $\Delta U = Q - W$, where Q is heat added to the system and W is work done by the system. Getting the signs right decides the answer: heat absorbed is positive, heat released negative; expansion work is positive, compression work negative. Some textbooks write $\Delta U = Q + W$ with W as work done on the system — the physics is identical, but mixing the two conventions within one problem guarantees a wrong sign.
The Kelvin-Planck statement says no engine can convert heat entirely into work with no other effect, so 100% efficiency is impossible. The Clausius statement says heat cannot flow spontaneously from a colder to a hotter body. The two are logically equivalent — violating either allows you to violate the other. Both are really statements about entropy: the total entropy of an isolated system never decreases.
A refrigerator is a heat engine run backwards, using work to move heat from cold to hot. Its performance is measured not by efficiency but by the coefficient of performance, $\beta = \frac{Q_2}{W} = \frac{T_2}{T_1 - T_2}$. Unlike efficiency, this can exceed 1 — a good refrigerator moves several joules of heat per joule of work. Note that $\beta$ falls as the temperature difference grows, which is why a freezer costs more to run than a fridge.
Mixing sign conventions. Decide at the start whether W means work done by or on the system, and keep it for the whole question.
Using Celsius in efficiency formulas. Every thermodynamic temperature ratio needs kelvin. Using °C gives nonsense, sometimes even a negative efficiency.