Final answer:
Excessive current flow in a circuit typically occurs due to a short circuit, where worn insulation leads to direct contact between wires, creating a low resistance path that allows for a high current flow.
Step-by-step explanation:
Reasons for Excessive Current Flow in a Circuit
The reasons for an excessive flow of current in a circuit can be attributed to a variety of factors. A common reason is a short circuit, which is an undesired low-resistance path across a voltage source. For instance, if the insulation on the wires of a toaster wears out, the wires can come into contact, creating a low resistance path. This would result in a large amount of electrical current flowing through this path because of the reduced resistance, according to the formula P = V²/r (power equals voltage squared divided by resistance). As the current increases, the thermal power generated can cause the cord to melt or burn, as represented in Figure 20.18(a). Additionally, if a significant amount of current flows through a resistor, it can heat up and potentially change state, contradicting Ohm's law that states the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance, as mentioned in Figure 20.18(b).
Factors like worn insulation leading to direct contact between conductors cause an increase in current flow. Additionally, the resistance of a short circuit may decrease further as it heats up, compounding the problem. Resistance is the measure of how much material opposes the flow of charge and varies with temperature, hence materials with low resistance lead to higher currents.