Thermal Physics Introduction

From mechanics to thermal physics Many concepts in thermal physics are based on mechanical concepts. For example, temperature is a measure of average kinetic energy. When mechanics is applied on millions and millions of atoms or molecules moving and colliding, it is often not possible to study every one in detail. When they are represented by a collective, we can more easily do measurements.

Thermal Energy Thermal energy refers to the non- mechanical transfer of energy between a system and its surroundings.

Direction of thermal energy flow Thermal energy flows from an object with a higher temperature to an object with a lower temperature. If there is no flow of thermal energy between two objects, the objects are said to be in thermal equilibrium.

Zeroth Law of Thermodynamics The "zeroth law" states that if two systems are at the same time in thermal equilibrium with a third system, they are in thermal equilibrium with each other. thermal equilibrium

O th Law (in other words) If two objects have the same temperature, then there is no flow of thermal energy between them and, vice versa, if there is no thermal energy flow between two objects, then the objects must be at the same temperature.

Measuring temperature Temperature is a measure of “hot or cold”. Temperature is proportional to the average kinetic energy of the atom or molecules of a material. Temperature is usually measured in Celsius where the freezing point is 0 °C and the boiling point is 100 °C. Although we have negative values on the Celsius scale, temperature is a scalar quantity (the negative sign does not indicate a direction).

The Kelvin Scale Temperature is also measured in Kelvin (K). In the Kelvin scale, the size of a degree is the same as in the Celsius scale but the scale has been shifted to avoid negative numbers. The lowest possible temperature in the universe ( °C) is given the value 0 K

Kelvin Scale continued To adjust a temperature from Celsius to Kelvin, just add 273. Ex. 25 °C would equal ( ) K or 298 K. To adjust a temperature from Kelvin to Celsius, subtract 273. Ex K would equal (400 – 273) °C or 127 °C.

Internal energy of a substance The internal energy of a substance is the total potential energy and random kinetic energy of the molecules of the substance. Kinetic energy arises from the molecules (atoms) vibrational, translational and rotational motion. The potential energy of the molecules arise from the forces between the molecules (atoms).