Page 127 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

9. Why does the fuse wire have a low melting point?

Ans: A fuse wire has a low melting point so it can quickly melt & break the circuit if the
current exceeds a safe level, preventing damage & fire hazards.

F. Long-answer questions:
1. Explain the working of an electric bell. Draw a labelled diagram for the same.
Ans: Working of an electric bell–

When the switch
is pressed, electric

current flows through
the coil. The iron

core acts as an
electromagnet. The
iron core attracts the

hammer towards itself
and the hammer hits

the bell producing
a sound. The circuit

breaks at the screw contact when the hammer moves towards the iron core. At this
point, the iron core ceases to be an electromagnet. The hammer is pulled back to its
original position due to the spring action of the iron strip, which then touches the

contact again to complete the circuit. The circuit is once again completed and the
current flows through the coil once more. The hammer again strikes the bell. The

process keeps repeating itself and we hear the ringing sound of an electric bell.
2. Write a short note on an MCB.
Ans: Nowadays, fuses in our houses

are being replaced with
miniature circuit breakers

(MCBs). MCB is a switch which
automatically trips off when

it detects excess current in
the circuit. When the MCB

is reset, the circuit becomes
complete and the flow of current in the circuit resumes. This is more convenient than
an electric fuse because in a fuse, once the wire melts, the circuit is permanently

broken and the fuse has to be replaced to resume the current in the circuit.
3. Explain the creation of an electromagnet through an experiment.

Ans: Wind a wire around an iron nail. Now, make a circuit by connecting a cell and a switch

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