iGCSE Physics (0625) 4.4 Electrical safety-Exam Style Questions- New Syllabus
▶️ Answer/Explanation
Detailed solution:
A fuse is a safety device designed to break the circuit by melting when the current exceeds a specific limit. For a fuse to cause all components in a circuit to turn off, it must be placed in the main branch where the total current $I_{total}$ flows. In circuits A, B, and C, the fuse is placed in parallel branches; if it blows, current can still flow through the other available paths to the remaining lamps. In circuit D, the fuse is connected in series with the battery before the parallel junction. If this fuse blows, it creates an open circuit, meaning $I = 0~A$ throughout the entire system, causing both lamps to go out immediately.
▶️ Answer/Explanation
Detailed solution:
First, calculate the operating current $I$ using the power formula $P = IV$, rearranged as $I = \frac{P}{V}$.
Substituting the given values: $I = \frac{2700\text{ W}}{240\text{ V}} = 11.25\text{ A}$.
A fuse must have a rating slightly higher than the normal operating current to allow the device to work without blowing during standard use.
Options A, B, and C ($3\text{ A}$, $5\text{ A}$, and $10\text{ A}$) are all lower than $11.25\text{ A}$ and would blow immediately upon switching the device on.
The only suitable rating that is higher than the operating current is $13\text{ A}$.
Therefore, Option D is the correct choice.
▶️ Answer/Explanation
Detailed solution:
Double-insulated appliances are designed with non-conducting outer casings, such as plastic, which prevent the user from touching any live metal parts. Since the casing cannot become live, an earth wire is not required for safety. However, the live wire and neutral wire are still essential to provide a complete circuit for the $P = IV$ power delivery. Additionally, a fuse is still necessary to protect the internal circuitry and cabling from excess current. Therefore, only the earth wire is redundant in this specific setup.