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Concept · Thermodynamics
Internal energy & heat
internal energy U as molecular KE+PE (state variable), heat Q as transferred energy (process quantity), phase change at constant T (latent heat raises molecular PE, KE-sum unchanged), heat flows hot→cold
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Named traps
12 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- Absolute zero misplacedPlaces absolute zero at 0 Celsius, or claims molecules move fastest there instead of having minimum motion.→
- Boiling/evaporation confusedTreats vaporization as occurring only at the boiling point, ignoring surface evaporation below boiling and its cooling effect.→
- Cold flows into objectsDescribes 'coldness' as a substance flowing into an object, rather than heat flowing out of it.→
- Heat flows cold to hotBelieves heat spontaneously flows from a colder body to a hotter one without any external work.→
- Heat vs temperature confusedUses heat (energy transferred) and temperature interchangeably, e.g. says a hotter object necessarily contains more heat.→
- Internal energy flat during phase changeConcludes internal energy is unchanged during a constant-temperature phase change because temperature is constant, ignoring the PE rise.→
- Internal energy ignores amountEquates internal energy with temperature alone, ignoring that more particles at the same temperature hold more internal energy.→
- Internal energy is heatTreats internal energy as 'the heat stored in a body' rather than as a state variable, with heat being energy only in transit.→
- Internal energy is only KEDefines internal energy as molecular kinetic energy alone, omitting the molecular potential-energy contribution.→
- Kelvin-Celsius offset droppedConverts a single temperature between Kelvin and Celsius without the 273 offset, or wrongly applies the offset to a temperature difference.→
- Latent heat raises molecular KEClaims the latent heat absorbed during melting/boiling raises average molecular KE, when at constant T it raises molecular PE and the KE-sum is unchanged.→
- Temperature rises during phase changeAssumes temperature keeps increasing while a substance melts or boils, ignoring the constant-temperature plateau.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
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