# 11.2.2 Temperature, Heat, and Internal Energy

Let’s start by imagining a one-dollar coin. It is stationary so it has zero KE. It is on the floor so it is already at zero GPE (assuming it can’t fall lower). So does it mean that the coin has zero energy?

Nope. As a 7.62 g steel disc, the coin’s centre of mass is stationary. But the $8.4\times {{10}^{{22}}}$  atoms[1] that make up the coin are each jiggling like crazy about their own equilibrium positions. The energy of the individual atoms that make up the coin is the coin’s thermal energy. The average amount of jiggling manifests as the temperature of the coin. To be precise, you will learn later that temperature is a measure of the average translational KE of the particles.

If the coin is dropped into hot water, thermal energy is transferred from the water to the coin until the atoms of the coin are jiggling with the same average translational KE as the molecules of the water. This transfer of thermal energy is called heat. And the direction of heat transfer is always from high to low temperature. When the coin and water are done with the exchange of heat because they have reached the same temperature, they are said to have reached thermal equilibrium.

Finally, we introduce the concept of internal energy U. Basically, the internal energy of a system is the summation of the KE and PE of the individual particles (that make up the system).

$U=K{{E}_{{microscopic}}}+P{{E}_{{microscopic}}}$

Concept Test

1601

1641

[1] Just a rough estimation, based on molar mass of 55 g/mol for steel.