What Can Heat Be Classified As?

Heat can be defined as energy that is transferred between objects or systems due to temperature difference. Heat is a form of energy that exists in different types, which allows it to be categorized and classified in various ways. The purpose of classifying heat is to better understand the underlying physics of how energy is transferred and converted. Having a framework for categorizing heat also enables scientists and engineers to apply the laws of thermodynamics more systematically.

Kinetic Energy

Heat can be classified as a form of kinetic energy. More specifically, heat is the kinetic energy associated with the random motions of atoms or molecules in matter. The hotter an object is, the faster its molecules vibrate and move around. This increased molecular motion corresponds to an increase in kinetic energy of the molecules.

Temperature is a measure of the average kinetic energy of molecules and atoms in a substance. As heat is added to matter, the molecules gain kinetic energy and start moving faster. This increase in molecular kinetic energy is detected as a rise in temperature. Essentially, temperature indicates the intensity of molecular motion – hotter substances have faster moving molecules with more kinetic energy. This is why heating an object increases its temperature. The added heat boosts the kinetic energy of the molecules.

In summary, heat is kinetic energy at the molecular level. Temperature measures the average kinetic energy of molecules, indicating how rapidly they vibrate and move. Heating increases molecular kinetic energy, speeding up molecular motion and raising the temperature. This relationship between heat, kinetic energy, and temperature is fundamental to thermodynamics.

Thermal Energy

heat is the kinetic energy associated with molecular motion
Thermal energy refers to the total kinetic energy of molecules within a substance. This kinetic energy is related to the temperature of the substance. The higher the temperature, the greater the thermal energy since the molecules are moving faster.

Heat is the transfer of thermal energy between substances or within a substance. There are three main mechanisms of heat transfer:

Conduction – The transfer of heat between substances in direct contact. Heat flows from high to low temperature regions as molecules collide and transfer kinetic energy. Metals are good conductors.

Convection – The transfer of heat by the movement of heated fluid. As the fluid (gas or liquid) warms, it becomes less dense and rises carrying heat upwards. Cooler denser fluid then sinks to take its place causing circulation.

Radiation – The transfer of heat via electromagnetic waves/photons. All objects emit infrared radiation related to their temperature. Net heat transfer occurs from high temperature to low temperature objects. Vacuum allows for radiation only.

Infrared Radiation

Heat can be classified as infrared radiation, which is a type of electromagnetic radiation located below the visible light spectrum. All objects emit some level of infrared radiation correlating to their temperatures. The hotter an object is, the more infrared radiation it emits.

Infrared radiation has some unique properties:

  • Infrared waves have longer wavelengths than visible light, ranging from about 700 nanometers to 1 millimeter.
  • Infrared radiation is invisible to human eyes but can be detected as heat.
  • Infrared waves exhibit transmission, absorption, and reflection properties when interacting with matter.
  • Infrared radiation transfers heat between objects through electromagnetic waves until equilibrium is reached.
  • Earth’s atmosphere is partly transparent to some infrared wavelengths, allowing heat to escape into space.
  • Common sources of infrared radiation include hot objects, fires, and living organisms.

Overall, infrared radiation is a key mechanism of heat transfer through electromagnetic waves that behaves according to the laws of thermodynamics.


Thermodynamics is the branch of physics that deals with heat and temperature and their relation to energy and work. The fundamental principles of thermodynamics help explain the movement of heat between different systems.

Heat itself can be understood as the transfer of thermal energy between systems due to a temperature difference. Thermal energy consists of the kinetic energy of atoms and molecules in a system. Heat always moves spontaneously from a hotter system (higher temperature) to a colder system (lower temperature). The transfer of heat is driven by the second law of thermodynamics, which states that the entropy of an isolated system always increases over time.

The laws of thermodynamics describe the relationships between thermal energy, heat, work, and the properties of matter. The first law of thermodynamics is essentially an expression of the conservation of energy – the total energy of a closed system remains constant. This means heat transfer into or out of the system must result in an equal but opposite change in the system’s internal energy.

The second law of thermodynamics, as mentioned above, describes the natural direction of heat transfer from hot to cold objects. It also defines the concept of entropy – a measure of molecular disorder within a thermodynamic system. Entropy in an isolated system always increases or remains constant over time. Entropy increases when heat is transferred or work is done on a system.

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero (the minimum possible temperature) is zero. This provides an absolute reference point for the measurement of entropy.

Together, these thermodynamic laws allow the precise calculation of heat transfer between objects and systems and place fundamental limits on how much useful work can be extracted from any given amount of thermal energy.

State Function

Heat is a state function, meaning it does not depend on the path or process used to transfer energy to the system. The amount of heat flow required to cause a certain change in the state or condition of a system depends only on the initial and final states, not the process. For example, whether heating water from 20°C to 80°C using a stove or a microwave, the net heat transfer is the same.

This is different from work, which is a process function that depends on the specific path taken. For example, lifting a book from the floor to a table requires different amounts of work if you take the stairs versus using an elevator. However, the change in thermal energy of the book is the same regardless of path.

Latent Heat

Latent heat is the heat absorbed or released by a substance during a change of phase, like during melting, boiling, vaporizing, or freezing. This heat does not change the temperature of the substance, but allows the molecule’s physical state to change by breaking or forming intermolecular bonds.

For example, when water is heated to its boiling point of 100°C, additional heat added will convert the water from liquid to gas through the process of vaporization. This heat is known as the latent heat of vaporization. Even though the temperature does not rise during this phase change, heat is being used to overcome the intermolecular forces between water molecules, allowing the molecules to escape the liquid phase and enter the gaseous phase.

Similarly, when water freezes into ice, its molecules enter a more structured crystalline phase, releasing energy in the form of latent heat of fusion. The amount of latent heat absorbed or released depends on the amount and type of substance undergoing a change of state.

Specific Heat

Specific heat is the amount of heat required to raise the temperature of one unit of mass by one unit of temperature. It is an intensive property, which means it is independent of the amount of material present. The specific heat capacity depends on the type of material.

For example, water has a very high specific heat capacity of 4.18 J/g°C. This means it takes 4.18 joules of energy to raise one gram of water by one degree Celsius. In comparison, iron’s specific heat capacity is only 0.449 J/g°C. Therefore, iron heats up and cools down much faster than water when the same amount of heat is added or removed.

Materials like water that can absorb a lot of heat before changing temperature have a high specific heat capacity. Materials like metals that don’t absorb much heat before changing temperature have a low specific heat capacity. The specific heat capacity is related to the strength of the molecular bonds in a substance. Stronger bonds require more energy to vibrate and move, resulting in a higher heat capacity.


Calorimetry is the measurement of heat transfer during physical and chemical processes. Scientists use calorimeters to experimentally determine the energy changes involved in reactions and phase transitions. There are two main types of calorimeters used:

Bomb calorimeters are used to measure the energy released during combustion reactions. A small sample is placed in an insulated chamber filled with oxygen gas. The sample is ignited, and the temperature change of the surrounding water is used to determine the amount of heat energy released. Bomb calorimeters allow for precise measurements under constant volume conditions.

Reaction calorimeters monitor heat changes during chemical reactions. The sample and reactants are added to an insulated vessel containing water. As heat is released or absorbed during the reaction, the temperature change of the water is measured. This allows the enthalpy change of the reaction to be quantified. Reaction calorimeters provide insights into thermodynamics and allow reaction rates to be studied.

Calorimetry experiments require precise temperature measurements and insulated calorimeter devices. The specific heat capacities of the system must be known to determine the heat transfer based on observed temperature changes. Calorimetry provides scientists with an important experimental tool for studying chemical and physical processes involving heat.


Heat can be classified in several key ways as we have explored in this article. First, heat is a form of energy called thermal energy that results from the kinetic motion of atoms and molecules. Thermal energy is transferred between objects through conduction, convection, and radiation. Infrared radiation is a type of electromagnetic radiation that transfers heat. In thermodynamics, heat is defined as energy transfer due to temperature difference only. Heat is also a state function, meaning it depends only on the current state of a system and not its past. Other important classifications of heat include latent heat, which is energy absorbed or released during a phase change, specific heat capacity, which is the amount of energy needed to change a substance’s temperature, and calorimetry, which is the measurement of heat transfer.

These classifications allow us to better understand the nature of heat and how it interacts with matter. We can use heat classifications to engineer more efficient heating and cooling systems, improve thermal management of electronics, optimize industrial processes, and gain insights into thermodynamic processes. The transfer and measurement of heat is critical in many scientific fields and technologies, so properly categorizing heat phenomena is key to advancement.

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