Defining Thermal Energy
Thermal energy refers to the total kinetic energy of the atoms and molecules within a substance. It is an intrinsic property of a system, dependent on its temperature, mass, and the specific heat capacity of its material. The hotter an object, the more vigorously its particles are vibrating or moving, hence the greater its thermal energy.
Defining Heat
Heat, on the other hand, is defined as the transfer of thermal energy between systems or objects due to a temperature difference. It is energy in transit, moving from a region of higher temperature to a region of lower temperature. Heat is not a property that an object "possesses"; rather, it is a process of energy exchange.
A Practical Example
Imagine a cup of hot coffee and a bathtub full of warm water. The coffee has a higher temperature, meaning its particles have a higher average kinetic energy. However, the bathtub contains a vastly larger number of water molecules, so it possesses a greater *total thermal energy* despite its lower temperature. If you pour the hot coffee into the warm bath, heat will flow from the coffee (higher temperature) to the bath (lower temperature) until thermal equilibrium is reached.
Importance in Thermodynamics
Understanding this distinction is crucial in thermodynamics, which studies how energy is transformed and transferred. Thermal energy is a state function—a property that depends only on the current state of the system, not the path taken to reach that state. Heat, conversely, is a path function, meaning its value depends on the process by which energy is transferred. This difference is fundamental to concepts like work, internal energy, and the laws of thermodynamics.