Final answer:
A short circuit is a dangerous condition where electricity follows a low-resistance path across a voltage source, causing a high amount of current to flow, which could lead to excessive heat and potential fire hazard.
Step-by-step explanation:
A short circuit occurs when electricity follows an unintended, low-resistance path across a voltage source, causing a potentially dangerous amount of current to flow through this path. This situation can arise, for instance, if the insulation on the wires of a toaster wears out, allowing the wires to come into contact and form a short circuit with very low resistance, denoted as 'r'. According to the equation P = V2/r, where P is the power, V is the voltage, and r is the resistance, the thermal power generated in the event of a short circuit can be very high, causing the wire to melt or burn. This can happen, for example, in a parallel circuit: if one branch shorts out, all the current will divert through that branch due to its low resistance, potentially causing excessive thermal power to be released, thereby creating a fire hazard.