Final answer:
The most practical source of ignition for starting a fire is friction. It is a reliable and accessible method that results in a combustion reaction, producing the necessary heat and light to start a fire.
Step-by-step explanation:
The most practical source of ignition for starting a fire is friction. When a match head is slid along a rough surface, as shown in Figure 5.1, it creates friction that initiates a combustion reaction. This reaction produces energy in the form of heat and light, both critical for igniting a fire. Although other sources like electricity and heat can also start fires, friction is readily accessible and reliable for this purpose. Other methods include the photoelectric effect, which can convert electrical energy into light energy, but this is not typically used for starting a fire.
Regarding the various forms of energy and how we can use them, it is important to note that in a postindustrial society, electricity is one of the most valuable commodities. Additionally, we utilize heat to make objects glow, as seen in incandescent lights, and electricity to illuminate gases in fluorescent lights. These processes, however, are usually not practical for initiating a campfire, where the simple act of creating friction provides both heat and an ignition point.