ConceptThermodynamics
← All conceptsLaws of thermodynamics
First Law ΔU=Q+W (sign convention: Q>0 absorb, W>0 work done ON gas/compression; equivalently ΔU=Q-Wbygas), work by gas W=P*ΔV, Second Law (Clausius & Kelvin statements, irreversibility, no perpetual motion of the second kind)
13 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Adiabatic heat nonzeroIncludes a heat term Q for an adiabatic process where Q=0, instead of setting ΔU equal to the work alone.→
- All heat raises temperatureAssumes all heat supplied to a gas raises its temperature, ignoring that during expansion some goes to work done by the gas.→
- Expansion work taken as +WUses positive W for an expanding gas in ΔU=Q+W, when expansion means work is done BY the gas so W on the gas is negative.→
- First-law sign convention wrongMis-signs Q or W in ΔU=Q+W, e.g. treats heat released or work done by the gas as positive against the stated convention.→
- Isothermal ΔU nonzeroAssigns a nonzero internal-energy change to an isothermal process for an ideal gas, where ΔU=0.→
- P-V work area misreadReads work as the area to the axis at a single state rather than the area under the P-V curve between the two states.→
- Real process treated as reversibleAssumes a real spontaneous process (mixing, free expansion, heat flow) can run backwards on its own, ignoring irreversibility.→
- Single-source engine 100% efficientBelieves a heat engine drawing from one reservoir can convert all absorbed heat into work, violating the Kelvin statement.→
- Spontaneous cold-to-hot allowedClaims heat can flow from cold to hot with no other change (no work input), violating the Clausius statement.→
- W=P*ΔV uses absolute VComputes work by the gas as P times the absolute volume rather than P times the volume change ΔV.→
- Work by/on gas swappedConfuses work done BY the gas (expansion) with work done ON the gas (compression), flipping the sign of W in the first law.→
- Work nonzero at constant volumeComputes nonzero work for a constant-volume process, ignoring that no volume change means W=P*ΔV=0.→
- ΔU depends on pathTreats the internal-energy change as path-dependent rather than a state function fixed by the endpoint states.→
