Final answer:
You should not connect the negative end of a battery directly to its positive end as it causes a short circuit, allowing electrons to flow without resistance which generates heat and can be dangerous.
Step-by-step explanation:
The reason not to connect the negative end of a battery directly to the positive end is because doing so creates a short circuit. When a wire connects the positive and negative terminals, as shown in Figure 19.2, it allows electrons to move from the negative terminal to the positive terminal without any resistance. This flow of electrons generates a large current that causes the battery to heat up rapidly, potentially leading to a hazardous situation, including a fire or explosion.
Detail related to why it's not advisable can be found in the description of Figure 19.3, explaining how a battery is supposed to function by moving negative charge through a device, like a headlight, and not directly across the terminals. This controlled movement of charge is what allows batteries to do work, like lighting the headlight, without overheating.