Misconceptions: 10 Entries

#	Misconception	Truth
1	"Adiabatic means constant temperature."	Adiabatic means Q = 0 (no heat exchange), not $\Delta T$ = 0. Temperature changes significantly — expansion cools, compression heats.
2	"In adiabatic expansion, $\Delta U$ = 0."	$\Delta U$ = −W in adiabatic process. Expansion (W > 0) means $\Delta U$ < 0, so internal energy and temperature decrease.
3	"Carnot efficiency can be calculated in Celsius."	Carnot formula η = 1 − $T_{2}$ / $T_{1}$ requires absolute (kelvin) temperatures. Using Celsius gives completely wrong answers.
4	"A 100% efficient heat engine violates the First Law."	A 100% efficient engine would violate the Second Law (Kelvin-Planck), not the First. Energy is still conserved if W = $Q_{1}$ and $Q_{2}$ = 0.
5	"v_mp is the maximum possible molecular speed."	v_mp is the most probable speed (peak of Maxwell distribution), not the maximum. Molecules can have any speed up to infinity (theoretically).
6	"v_rms = v_avg because both are averages."	v_rms ≠ v_avg. v_rms = √(3RT/M), v_avg = √(8RT/πM). v_rms > v_avg > v_mp always.
7	"COP of a refrigerator is always less than 1."	COP of a refrigerator = $T_{2}$ /( $T_{1}$ − $T_{2}$ ) and can be much greater than 1. A fridge cooling from 300 K to 270 K has COP = 270/30 = 9.
8	"The work done in a thermodynamic cycle is always zero."	Work per cycle = area enclosed by the cycle on the PV diagram. For a clockwise cycle (engine), W > 0. Zero work would mean a cycle with zero area (degenerate).
9	"Entropy decreases in a refrigerator because it makes things colder."	The total entropy of the system + surroundings always increases. The refrigerator decreases entropy of the cold reservoir by increasing it more in the hot reservoir.
10	"Internal energy of a gas depends on pressure and volume."	For an ideal gas, U = (f/2)nRT depends only on temperature. At the same temperature, two states with different P and V have the same U.