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How does fire behave?

On this page, you’ll obtain information to understand how fire behaves and the evidence it may leave behind.

a What is fire?
b How does fire develop?
c How does fire behave?
Safety note

At the Fire Safety Research Institute, part of UL Research Institutes, fire protection engineers and safety scientists study fire in a very controlled and scientific way. Do not attempt to recreate these tests yourself: instead, this pathway will take you through these experiments virtually. Your teachers can provide you with similar experiences that allow you to explore fire in a safe setting. If you are interested in performing similar tests yourself, consider pursuing a career as a safety scientist!

Asset: How does fire behave?
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During a fire, changes in temperature affect the pressure and volume of gases in the room. These changes cause fire to behave in predictable ways.

Let’s take a close look at the vocabulary that will help us communicate about fire behavior. 

Temperature

Temperature measures the average kinetic energy of molecules in a substance.

Pressure is the force exerted on the walls of a container by gases (or liquids).

Volume is the amount of space something takes up.

Asset: How does ventilation affect fire?
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Ventilation is the exchange of hot, oxygen-depleted gases, with cool, oxygen-rich air.

Let’s take a close look at the vocabulary that helps us communicate how ventilation impacts fire.

Exchange

The exchange of hot gas and cool air is caused by unequal pressure between gases inside the room and the air outside. These differences drive a circulation that provides the fire with oxygen it needs to sustain.

The fire uses the oxygen in the room during combustion, so these hot gases are depleted (or lacking) of oxygen.

Hot gases exert more pressure on their surroundings. Hot, higher-pressure gases, inside the room, flow toward the lower pressure area, outside of the window.

They leave behind a low-pressure area near the base of the fire.

The cooler air outside of the window is fresh and full of oxygen.

The cooler air enters the room low to the ground. It fills the lower pressure area left behind as the hot gases rise and exit the room.

Real Lab Footage

Watch the video to observe how fuels impact fire behavior.

Fuels are different.

  • Synthetic, or man-made fuels may have 2-3 times more stored energy per pound than a natural fuel such as wood.
  • Physical properties like density, mass, and orientation can affect how quickly a fuel ignites.

Knowing how fuels burn differently can help investigators make claims about the most likely fuel in a fire. Ready to investigate these differences yourself? 

 

Asset: Organic vs. Synthetic Fuels
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Fire is an example of matter and energy transformations. The chemical energy stored in fuel is transformed into other forms of energy and matter.

 

Potential energy is stored energy

The amount of potential energy stored in an object depends on its composition and/or relative position.

There are many forms of potential energy:

  • Chemical energy – energy stored within bonds between molecules. Examples are food and gasoline
  • Nuclear energy– energy created when nuclei of atoms are split or combined. Examples are the energy that fuels stars like our sun, or nuclear power plants.
  • Elastic energy – energy stored within an object due to being stretched or deformed.  Examples are stretching a rubber band or jumping on a trampoline.
  • Gravitational energy – energy stored in an object because of its height. Examples are a book sitting on a shelf or a rollercoaster at the top of a hill.

Which of these forms of energy can you observe during a fire?

There are many forms of kinetic energy:

  • Radiant energy –  energy related to the movement of electromagnetic waves. Examples: are visible light and infrared radiation, like heat lamps that warm food. 
  • Electrical energy – energy from moving electrons. Examples are electricity that powers your devices or lightning.
  • Thermal energy – energy due to the movement of molecules in a substance. Thermal energy depends on temperature and volume.  Examples are boiling water, or hot steam.

Which of these forms of energy can you observe during a fire?  

Classroom Investigation 03

Energy & Combustion

Watch the video to observe how the energy stored in different fuels affects how it ignites and combusts. 

With proper adult supervision, you can complete this classroom investigation: Energy & Combustion Investigation Guide.

 

Asset: Energy and Combustion
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Challenge

Recall what you’ve learned about fire behavior from this page. What evidence might you observe in the burn scene that can indicate how the fire moved through the room?

As hot gases flow in a room, they can leave behind patterns of soot and ash. These lines of demarcation show what areas were affected by the heat and smoke, and which were not.

These patterns can be used to trace the damage back to the origin.

line of demarcation on wall

line of demarcation

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