Heat, Work and Energy

In physics, the word heat is used to describe a quantity of heat that is being transferred. Heat is energy that is in the process of being transferred from one place or object to another. Similarly, work is energy that is being transferred when a force is acting on an object.

Kinetic Particle Model

The following is a list of assumptions that form the kinetic particle model:

  • All matter is made up of many very small particles (atoms or molecules).
  • The particles are in constant motion – They have kinetic energy.
  • No kinetic energy is lost or gained overall during collisions between particles.
  • There are forces of attraction and repulsion between the particles in a material.
  • The distances between particles in a gas are large compared with the size of the particles.

States of Matter

There are four states of matter: solid, liquid, gas and plasma. Plasma is a state of matter that is similar to gases that is so hot that the electrons are not held by the nuclei of atoms. Plasma is not considered in this kinetic theory of matter.

  • Solids have a fixed shape and volume. Solids are incompressible. They are held together by strong attractive forces. Repulsive forces also exist between solid particles, otherwise the solid would collapse in on itself. The attractive forces hold the particles in almost fixed positions. The particles are not completely still and vibrate around an average position. 
  • Liquids take the shape of the container and have a fixed volume. Liquids are mostly incompressible. Particles in liquids have more freedom to move and this allows them to have no fixed shape, except that of the container. Particles in liquids move freely, colliding with each other but constantly attracted to each other.
  • Gases have no fixed shape or volume and are compressible. The particles in gases are in constant motion and will fill a container. The particles often collide and are attracted to each other but this attraction is not enough to keep gas particles close together.

The Energy Model

The kinetic particle model is used to explain the idea of heat transfer. Heat is measured in the SI unit of Joules (J). When a substance is heated, the particles in the substance will gain either kinetic energy (move faster) or potential energy (shift from their equilibrium position). The term internal energy refers to the total kinetic and potential energy of the particles within a substance. Heating (the transfer of thermal energy) a substance increases the kinetic and/or potential energy of the particles within a substance. During a phase change, the particles do not gain kinetic energy. Rather, the energy is used to break the intermolecular bonds between particles and this allows the particles to move further apart.


Temperature is the average kinetic energy of the particles contained in a substance. Temperature is not a measure of total thermal energy of an object. The hotter an object is, the faster the motion of the molecules inside it. Thus, the heat of an object is the total energy of all the molecular motion inside that object. Temperature, on the other hand, is a measure of the average heat or thermal energy of the molecules in a substance. 

Kelvin Vs Celsius

Kelvin and Celsius temperature measurement use the same scale. The Kelvin scale determines 0ºK as absolute zero – the point at which all particle motion becomes zero (0ºK =-273.15ºC). On the Celsius scale, 0ºC is the freezing point of water at standard atmospheric pressure.