I think that anyone would agree that heat is one of the most recognized characteristics of a fire. In general fires produce a lot of heat. But what exactly is heat? NFPA 921 "Guide for Fire and Explosion Investigations, section 3.2" describes heat as the energy that is needed to maintain or change the temperature of an object. When heat energy is transferred to an object, the temperature increases. When heat energy is transferred away, the temperature decreases.
Generally, heat can be viewed as a form of energy associated with an object or a body. At this point we need to differentiate between heat and temperature.
All matter, whether solid, liquid or gas is made up of molecules. These molecules have in them an inherent degree of internal energy. That internal energy causes the molecules to vibrate or move in-place, somewhat like you jogging in place. The greater the internal energy, the faster the molecules vibrate. The more heat that is transferred to that object, the greater its internal energy. So heat makes the molecules vibrate. Temperature is a relative measurement of how fast those molecules are vibrating or moving.
A simple example is that of water. Water freezes and is a solid at 32 degrees Fahrenheit. Even though it is cold at that temperature, the water still has internal energy associated with it. Negative 50 degrees F is even colder and the molecules are vibrating even slower, right? The colder the object is, the less internal energy there is in the object. Theoretically, the coldest temperature possible is about negative 460 degrees F, which, on the absolute temperature scale, is referred to as 0 degrees Rankine. This is called absolute zero. At this temperature an object has no internal energy!
Now let's take it the other way. What happens if I put that frozen water into a pot and place it on the stove? As I transmit heat into the water it causes the molecules to move faster, raising the temperature. As the temperature rises above 32 degrees F the water turns from a solid (ice) to a liquid. As I continue to add heat to the water it raises the temperature to 212 F where it begins to boil and turn into a gas.
Heat Transfer
How is heat transferred from one object to another?
The importance of understanding how heat is transferred is reflected in NFPA 921 Guide for Fire and Explosion Investigations, section 3.2 which states: The transfer of heat is a major factor in fires and has an effect on ignition, growth, spread, decay (reduction in energy output), and extinction. Heat transfer is also responsible for much of the physical evidence used by investigators who attempt to establish a fire's origin and cause.
Before we start, we want to consider a couple of basic scientific principals.
First heat always moves from a warmer body to a colder body. That should be easy to remember if you think about your heating bill during the winter. In cold weather if you warm your house to 75 degrees F and then cut your heat off, even if you leave your windows and doors shut, your house will relatively quickly grow cold. The reason is that heat is being transferred through the warmed walls, ceilings, and floors of your home to the colder outdoors all the time. If you don't replenish that heat by leaving your heat on, your house will cool down.
The second principal to keep in mind is that the greater the temperature difference between two objects the faster the heat is transferred.
In the science and engineering world there are three ways that heat is transferred. They are easiest to understand through real world examples.
The first means of heat transfer is called conduction. Conduction heat transfer is simply the transfer of heat through direct solid to solid contact. A metal pot sitting on the eye of a stove is direct solid-to-solid contact. If we turn the stove on, the initially cold pot is warmed by hot eye through conduction heat transfer. If you touch a hot pot or a hot iron heat will be conducted into your hand very quickly. Remember the greater the temperature difference between two bodies the faster the heat is transferred!
The second means of heat transfer is called convection. Convection heat transfer occurs by the motion or flow of gases or fluids over an object. An example would be blow drying your hair. The hot gases from the dryer flow past your hair warming and drying it.
Finally, the third means of heat transfer is by radiation. This is the transfer of heat from one body to another by electromagnetic waves. The simplest example of this is when you get a suntan. The sun transmits heat energy to your body by radiation heat transfer.