Change of internal energy: Heat

The world around us is made of tiny particles - molecules and atoms, which carry energy. Some of this energy comes from forces that hold individual particles together - intramolecular forces. And some energy comes from the forces that are between different particles, which attract each other - intermolecular forces. Imagine Michael at the disco. He might ask Lina to dance with him.

There is a chance they would dance together. But Lina is busy dancing with Maria right now, so the probability of Michael dancing with Lina is low. The bonds within or between the particles determine the probability - or potential - of the particles to move somewhere else. That’s why we call the energy related to molecular bonds POTENTIAL ENERGY. But that’s not all!

All particles move constantly, and they move in different ways. They spin, move freely in space, or vibrate. These different types of movement carry energy related to motion - KINETIC ENERGY. The sum of kinetic energies of all the particles in an object is called THERMAL ENERGY. There are so many moving particles in every object that we can’t measure the thermal energy directly.

But we can measure the average kinetic energy as temperature! The sum of kinetic and potential energy of all particles inside an object make up the object’s INTERNAL ENERGY. An object can exchange energy with anything around it, like other objects or even air. This causes a change in their internal energy. For example, if you take a glass of water and you put something hot inside it, like a very hot spoon, the water will soon become warmer.

This happens because the spoon gives away some of its energy to the water. This energy transferred from a warmer to a colder object is called HEAT. When energy moves into an object, we say that the object absorbs or receives heat. The object’s temperature rises, and its internal energy increases. If you now take an ice cube and put it in the glass, the water inside will soon get colder.

This happens because water gives away some of its energy to the ice. When energy moves out of an object, the object’s temperature drops, and its internal energy decreases. We say it loses heat. But what happens to the ice? Its temperature is still zero degrees Celsius.

It absorbs heat from the water but its temperature doesn’t change! This is because the ice is melting - it’s going through a process of PHASE TRANSITION. In this situation, the heat absorbed by the ice is not used to increase its kinetic energy and temperature - but to change the bonds between molecules. The bonds between the molecules of solid ice loosen up, so the potential energy increases. The internal energy rises.

Internal energy is the sum of potential and kinetic energy of all molecules inside an object. By adding or removing heat we cause a change in internal energy. This happens because the transfer of heat changes either the kinetic or the potential energy of the object. If the kinetic energy increases, the temperature rises. If the kinetic energy decreases, the temperature drops.

When the potential energy changes, the bonds between molecules loosen or become stronger. The molecules move closer together or further apart and the object changes its state - from solid to liquid or gas, or the other way round!