The operation of an electric safety fuse in a domestic circuit depends mainly on which of the following features of the current flowing through it? 1. Magnetic effect of the current 2. Chemical effect of the current 3. Magnitude of the current 4. Heating effect of the current. Select the correct answer using the code given below.

Introduction / Context:

Safety fuses are protective devices used in electrical circuits to prevent damage from excessive current. When current exceeds a certain safe value, the fuse element melts and breaks the circuit, thereby guarding appliances and wiring. This question asks which properties or effects of the current are essential for the correct functioning of a fuse.

Given Data / Assumptions:

• A fuse consists of a thin wire made of a material with relatively low melting point.
• The fuse is connected in series with the circuit it protects.
• If current exceeds a rated value, the fuse wire heats up and melts.
• We consider magnetic, chemical, magnitude and heating effects of current.

Concept / Approach:

The basic principle of a fuse is the heating effect of electric current. According to Joule heating, the heat produced per unit time in a conductor is proportional to I^2 * R, where I is current and R is resistance. If the current magnitude is too high, the heat generated increases sharply, causing the fuse wire to melt when it reaches its melting point. Thus, both the magnitude of the current and the heating effect are crucial. Magnetic and chemical effects are not directly relevant to the simple melting operation of a fuse wire in domestic wiring.

Step-by-Step Solution:

Verification / Alternative check:

Notice that fuses are rated in amperes, such as 5 A or 15 A, indicating the maximum safe current magnitude. If current remains below this value, the heating effect is not sufficient to melt the fuse. When a fault occurs and current rises sharply, the I^2 dependence ensures rapid temperature rise and melting. This clearly shows that magnitude and heating effect of current are the critical factors for fuse operation.

Why Other Options Are Wrong:

Option A: 1, 2, 3 and 4 incorrectly includes magnetic and chemical effects, which are not basic to fuse operation.

Option B: 1, 2 and 3 only omits heating effect, which is the central principle behind melting of the fuse element.

Option D: 4 only acknowledges heating effect but ignores the explicit role of current magnitude; fuse ratings are defined in terms of maximum current.

Common Pitfalls:

Students sometimes focus solely on the heating effect and forget that it depends on the square of current, making the magnitude of current essential. Others may see sparks and think of magnetic effects, but these are secondary phenomena. Always connect the idea of fuse operation to Joule heating and the current rating marked on the fuse.

Final Answer:

The working of safety fuses depends on 3 and 4 only, that is, magnitude of the current and the heating effect of the current.