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# Systems and energy

## Systems in thermal physics

A system is a group of particles which are considered, for the purposes of study, to be a single entity.

A thermodynamic system is similar to a mechanical system. But in thermodynamics, systems are studied with respect to energies rather than forces.

In a closed system in mechanics there are no external forces acting on the system. A closed system in thermodynamics is one in which energy but not mass can be exchanged between the system and its surroundings.

A gas cloud, a box of excited particles and a fluid section in a pipe are examples of systems.

## Thermal equilibrium

Two bodies are in thermal equilibrium with each other if there is no thermal energy (heat) transfer between them. This happens when the two bodies are at the same temperature.

If the two bodies are not at thermal equilibrium, they will exchange thermal energy until thermal equilibrium is established.

Thermal energy that is transferred from one body to another is called heat. Heat always flows from a hotter body to a colder one.

Placing a cold spoon in a hot cup of coffee causes the spoon to warm up until it reaches the same temperature as the coffee.

Heat is transferred from the fire to the air, causing the room to become warmer.

## Internal energy

The internal energy $(U)$ of a system is the sum of the total energy of all the particles in the system.

Each particle in a system has an amount of kinetic energy due to its motion.

Each particle also has some potential energy due to its chemical bonds and intermolecular forces.

When an object is heated the particles in the system move faster. The random kinetic energy of the particles is increased. The potential energy of the particles stays constant.

This leads to an overall increase in the internal energy.

The internal energy of a body therefore depends on the temperature and the number of particles.

Gas molecules. The red arrows show movement (random kinetic energy) and the blue lines show chemical bonds (potential energy).

## Thermal energy

Thermal energy is the part of the internal energy of a system that is transferred between systems at different temperatures. Unlike heat, thermal energy is a property of a system or body.

Thermal energy consists of a part of the potential and a part of the kinetic energy of the system.

When the temperature of a system increases, the system gains thermal energy and accordingly, internal energy.

However, the temperature does not fully determine the amount of thermal energy contained within a body.

Temperature is a measure of the average kinetic energy possessed by the particles in a system. It therefore does not take into account the potential energy component of thermal energy.

Thermal energy, like internal energy, is a sum of energies and is dependent on the mass and composition of the body in question.

A large mass has a greater thermal energy than a small mass even though they might be at the same temperature.