Question

Two identical fuses are rated $5A$If they are joined

• A. in parallel, the combination acts as a fuse of rating $5A$
• B. in parallel, the combination acts as a fuse of rating $10A$
• C. in series, the combination acts as a fuse of rating $5A$
• D. in series, the combination acts as a fuse of rating $10A$

Answer 1

Answer: (B), (C)

in series, the combination acts as a fuse of rating $5A$

1. The fuse is used to break the circuit whenever the current through the circuit exceeds the rated value. A fuse is a piece of metal wire in between the circuit which melts when heat dissipated through the current in the circuit exceeds the rated value.
2. When fuses are connected in parallel, the total current can divide into two paths. And we have each path containing one $5A$rated fuse. So, parallel combination acts as a fuse of rating $5A+5A=10A$ Thus, option (B) is correct.
3. When fuses are connected in series, the rated current does not change. This can be concluded by showing that the length of the fuse wire does not decide the rated current of the fuse. It can be shown as follows.
4. Resistance of fuse is $R=\rho \frac{L}{A}$and $P=I^{2}R$ $P=I^{2}P\frac{L}{A}$ So heat dissipated depends on the length of the fuse.
5. But the amount of heat needed to raise the temperature of the fuse also depend on the length.
6. That’s why the length of the wire does not decide the rated current of the fuse. So, option (C) is also correct.

So, the correct options are (B) and (C).