ConceptThermodynamics
← All conceptsKinetic theory & temperature
matter as molecules, Brownian motion & diffusion as evidence, temperature as a measure of AVERAGE molecular KE (Kelvin scale, t=T-273.15), average translational KE E=(3/2)kT, average vs individual molecular speeds, molecule counting via NA (N=(m/M)NA), oil-film monolayer estimate
13 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Average KE proportional to Celsius tempUses Celsius rather than absolute (Kelvin) temperature when relating average molecular KE to temperature, so doubling Celsius is read as doubling KE.→
- Average speed equals individual speedTreats the average molecular speed as the speed of every individual molecule, ignoring the distribution of speeds.→
- Brownian motion blamed on currentsAttributes Brownian jiggling to air/convection currents or vibration rather than to unbalanced molecular collisions.→
- Brownian particle is a moleculeInterprets the visible Brownian particle itself as a molecule rather than as a larger particle revealing molecular bombardment.→
- E=(3/2)kT fed Celsius temperatureSubstitutes a Celsius temperature into E=(3/2)kT, or drops the 3/2 factor, when finding average translational KE.→
- Mass ignored for molecular speedAssumes molecules of different mass have the same speed at the same temperature, ignoring that average speed depends on molecular mass.→
- Molar mass units mismatchedMixes grams and kilograms (or molar mass vs molecular mass) in N=(m/M)NA, giving a count off by 1000.→
- Molecular speed unaffected by TAssumes molecular speeds do not change with temperature, or that every molecule moves at the same single speed.→
- N=(m/M)NA skippedReports the number of moles or the mass as the number of molecules, forgetting to multiply by Avogadro's number NA.→
- Oil film not treated as monolayerForgets that the oil-film estimate assumes a single-molecule-thick layer, so diameter = volume/area is not applied.→
- Oil-film thickness formula invertedComputes molecular diameter as area/volume instead of volume/area in the oil-film monolayer estimate.→
- Temperature as total KELinks temperature to the total molecular kinetic energy of the sample rather than to the average per molecule.→
- k Coulomb constant used for kBPlugs the Coulomb constant k (~9 × 109) into E=(3/2)kT instead of the Boltzmann constant kB (~1.38 × 10-23), confusing the two symbols.→
