thermal energy
Summary
Thermal energy is the energy of random particle motion in a substance, set by its temperature and by how much of it there is. Heat is thermal energy transferred because of a temperature difference. Testable point: temperature tracks average kinetic energy per particle, not total thermal energy.
Detail
Meaning: Thermal energy is the internal energy from the random translational, rotational and vibrational motion of particles. Higher temperature means greater average kinetic energy per particle, and a larger sample at the same temperature holds more thermal energy. In an ideal gas it is all kinetic. In real liquids and solids, intermolecular potential energy also counts, so heat added during a phase change does not raise the temperature.
Equation: U = (3/2)nRT for a monatomic ideal gas, where U is thermal energy in J, n is amount of gas in mol, R is 8.31 J/(mol·K), and T is absolute temperature in K.
On the MCAT: Friction or an inelastic collision stops a sliding block, and the lost mechanical energy appears as thermal energy. Calorimetry uses Q = mcΔT to track heat transfer between samples. Temperatures must be in kelvin when used in gas relations.
Common confusion: Thermal energy, heat and temperature are different. A bathtub of warm water has more thermal energy than a cup of hot coffee, yet a lower temperature. Heat is thermal energy in transit, so an object does not contain heat. It flows from the higher-temperature object to the lower-temperature one.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Knight, Jones, Field)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.