Heat energy is the form of energy that can be transferred from a higher temperature body to a lower temperature or one object with the higher temperature loses heat energy and others gain it. For example, heat flow increases the temperature of the colder body and decreases the temperature of the hotter body. All the other forms of energy are completely converted into work but they can not be wholly converted into work. Any attempt to convert the whole quantity would affect a permanent change in the system. For example, a substance absorbed heat without increasing temperature changing the physical state or phase (solid, liquid, gas, or vapor) of the system by melting, sublimation, and boiling. In this category, we discuss different topics and formulas related to heat energy.