Final answer:
In a parallel circuit, each device operates independently, connected to the voltage source, allowing the use of individual appliances even if another fails. If the total current exceeds the circuit capacity, a fuse may blow. Overloading the voltage source can also lead to reduced device efficiency due to voltage drop.
Step-by-step explanation:
The reason why you can use one light or appliance at a time in a parallel circuit even if a load fails is that each load is independently connected to the voltage source. In a parallel circuit, each resistor, such as a lightbulb or appliance, draws the same current it would if it alone were connected to the voltage source. This implies that each device operates independently, and the failure of one will not affect the operation of others. This is similar to how household appliances and car headlights operate. The current drawn by each device in a parallel circuit depends on the power rating of the device and the voltage of the source. For example, an 1800-W toaster, a 1400-W electric frying pan, and a 75-W lamp would draw different current amounts in a 15-A, 120-V parallel circuit setup. If the total current drawn exceeds the capacity of the circuit (in this case a 15-A fuse), then the fuse will blow. However, if the voltage source becomes overloaded, such as if too many devices are connected and drawing power beyond what the source can supply, this can lead to a drop in output voltage. Consequently, even being in a parallel configuration, the devices might not function at full efficiency, as they will not receive the full voltage they are designed for.