Heat and Thermal Energy
Heat is energy in transit that flows from the hotter body to the colder body. In Physics, “heat” is a flow quantity and not a property of an object. Heat flows into or out of an object. This heat comes from/goes into the heat energy or thermal energy of the object. Thermal energy is one of the components of internal energy.
Heat is just a form of energy and can be converted from mechanical energy. Although the idea that heat energy and mechanical energy are connected was first suggested by Benjamin Thomson (1753-1814), Joule established the equivalence of these two forms of energies experimentally.
Thermal Contact and Thermal Equilibrium
Thermal Contact: If two objects are in thermal contact with each other, heat can flow between them. Heat can flow by conduction, convection and radiation, so two objects do not have to come into physical contact to be in thermal contact.
Thermal equilibrium: If the two objects are in thermal contact but yet not net heat flow between them, the two objects are said to be in thermal equilibrium.
Note: Steady State = the state is not changing.
Is the Earth in thermal contact with the Sun?
Ans: YES!!! Heat flows from the Sun to Earth!
Then, is the Earth in thermal equilibrium with the Sun?
Ans: NO!! There is a net heat flow from the Sun to the Earth!