A Scientific Way to Define Heat Energy

Illustration of an iron on a piece of clothing, with the definition of heat energy above it
ThoughtCo.

Most people use the word heat to describe something that feels warm, however in science, thermodynamic equations, in particular, heat is defined as the flow of energy between two systems by means of kinetic energy. This can take the form of transferring energy from a warm object to a cooler object. More simply put, heat energy, also called thermal energy or simply heat, is transferred from one location to another by particles bouncing into each other. All matter contains heat energy, and the more heat energy that is present, the hotter an item or area will be.

Heat vs. Temperature

The distinction between heat and temperature is subtle but very important. Heat refers to the transfer of energy between systems (or bodies), whereas temperature is determined by the energy contained within a singular system (or body). In other words, heat is energy, while temperature is a measure of energy. Adding heat will increase a body's temperature while removing heat will lower the temperature, thus changes in temperature are the result of the presence of heat, or conversely, the lack of heat.

You can measure the temperature of a room by placing a thermometer in the room and measuring the ambient air temperature. You can add heat to a room by turning on a space heater. As the heat is added to the room, the temperature rises.

Particles have more energy at higher temperatures, and as this energy is transferred from one system to another, the fast-moving particles will collide with slower moving particles. As they collide, the faster particle will transfer some of its energy to the slower particle, and the process will continue until all the particles are operating at the same rate. This is called thermal equilibrium.

Units of Heat

The SI unit for heat is a form of energy called the joule (J). Heat is frequently also measured in the calorie (cal), which is defined as "the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius." Heat is also sometimes measured in "British thermal units" or Btu.

Sign Conventions for Heat Energy Transfer

In physical equations, the amount of heat transferred is usually denoted by the symbol Q. Heat transfer may be indicated by either a positive or negative number. Heat that is released into the surroundings is written as a negative quantity (Q < 0). When heat is absorbed from the surroundings, it is written as a positive value (Q > 0).

Ways of Transferring Heat

There are three basic ways to transfer heat: convection, conduction, and radiation. Many homes are heated through the convection process, which transfers heat energy through gases or liquids. In the home, as the air is heated, the particles gain heat energy allowing them to move faster, warming the cooler particles. Since hot air is less dense than cold air, it will rise. As the cooler air falls, it can be drawn into our heating systems which will again allow the faster particles to heat up the air. This is considered a circular flow of air and is called a convection current. These currents circle and heat our homes.

The conduction process is the transfer of heat energy from one solid to another, basically, two things that are touching. We can see an example of this can be seen when we cook on the stove. When we place the cool pan down on the hot burner, heat energy is transferred from the burner to the pan, which in turn heats up.

Radiation is a process in which heat moves through places where there are no molecules, and is actually a form of electromagnetic energy. Any item whose heat can be felt without direct connection is radiating energy. You can see this in the heat of the sun, the feeling of heat coming off a bonfire that's several feet away, and even in the fact that rooms full of people will naturally being warmer than empty rooms because each person's body is radiating heat.