1. The 1st Law of Thermodynamics
Heat Flow
The 1st Law of Thermodynamics

2. Thermal (Internal) Energy
You saw in the previous lesson how temperature is really an indicator of the amount of kinetic energy possessed by the molecules that make up a gas.
Because these molecules are microscopic and thus too small to see, we refer to the TOTAL amount of kinetic energy possessed by ALL of the molecules as the thermal energy (sometimes referred to as internal energy) of the gas.

3. The Direction of Heat Flow
When something cold touches something hot, the two objects exchange thermal energy by a process that we refer to as heat flow.
More specifically,
the object that starts out at a higher temperature LOSES thermal energy
the object that starts at a lower temperature GAINS energy

4. The Result of Heat Flow
When energy (in the form of heat flow) leaves the warmer object
The molecules slow down.
The temperature drops
When energy (in the form of heat flow) enters the cooler object
The molecules speed up
The temperature increases

5. Equilibrium
As long as two objects have different temperatures, they will continue to exchange thermal energy and their temperatures will get closer to being equal.
When the temperature of the two objects becomes equal, they cease to exchange thermal energy. We call this condition equilibrium, and the temperature at which it occurs is called the equilibrium temperature.

6. Modes of Heat Flow
The precise process by which thermal energy is exchanged during heat flow depends mostly on the states of matter involved.
When solids exchange energy with other solids, they do so by a process called CONDUCTION.

7. Modes of Heat Flow
When heat flows through liquids and gases, it does so largely by a process called CONVECTION.
Finally, if thermal energy is being transferred through empty space (such as the 93 million miles between the Earth and the Sun), it happens by a process called RADIATION.