Temperature, Heat, Internal Energy

Temperature

Thermodynamics, "motion of heat", is a branch of physics that focuses on heat, work, temperature, energy, and entropy. In thermodynamics, temperature is a physical quantity expressing the hotness of an object or radiation. On microscopic level, hotness is observed as how volatile atoms are moving. Its SI unit is kelvin (K) for scientific purpose.

Celsius scale $^\circ C$ is used for daily life in most part of the world. $0\ ^\circ C$ is the freezing point of water, and $100\ ^\circ C$ is the boiling point of the water at atmospheric pressure at sea level.

Fahrenheit scale $^\circ F$ is used for daily life in the United States. $32\ ^\circ F$ is the freezing point of water, and $212\ ^\circ F$ is the boiling point of the water at atmospheric pressure at sea level.

Conversion for those 3 units:

$$x\ ^\circ F = \frac{5}{9}(x-32)\ ^\circ C$$ $$x\ ^\circ C = (x+273.15)\ K$$

Heat

In thermodynamics, heat is the energy transferred from or to an object through radiation, conduction, and convection. It SI unit is joule (J).

Radiation is the emission or transmission of energy in the form of waves or particles through space, such as electromagnetic radiation (microwaves, visible light), particle radiation (alpha radiation, beta radiation), acoustic radiation (sound, seismic waves).

Conduction is the transfer of internal energy as heat from the hotter end to the colder end of an object. Convection is the transfer of internal energy as heat from one place to another due to the motion of fluid.

Internal energy

Internal energy of a thermodynamic system is the total energy within the system. It includes microscopic motion of particles (translation, rotation, vibration), and potential energy associated with microscopic forces, such as chemical bonds and phases of matter.

Similar to potential, it is used to keep account of the gains and losses of energy due to changes in its internal state, so a reference state can be defined and changes from that state are of interest.