Class 7 Science

# Electric Current

## Electric Fuse

Electric fuse is a safety device which is used in household wirings and in many appliances. Electric fuse has body made of ceramic and two points for attaching the fuse wire. The fuse wire melts whenever there is overload in the wiring. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring. In electrical devices a glass fuse is often used. This is a small glass tube, in which lies the fuse wire.

## MCBs (Miniature Circuit Breaker)

MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built in mechanism by which the electric supply is automatically resumed.

## Review Questions

Question 1: The electric bulb works on which property of electric current?

Answer: Heating effect of electric current.

Question 2: What is the full form of MCB?

## Magnetic Effect of Electric Current

Hans Christian Oersted (1777 – 1851) was the first to show that electric current also produces magnetic effect. He kept a magnetic compass near a current carrying conductor. It was observed that the magnetic compass shows deflection, when the current passes through the conductor.

### Electromagnet

Magnetic effect of electric current has been used in making powerful electromagnets. For this, wire is wrapped around an iron rod in many turns and electric current is supplied to the wire. As long as the current is supplied the iron rod behaves like a magnet. More number of turns makes more powerful magnet. Electromagnet is used in electric bells and also in powerful cranes.

### Electric Bell

#### Structure of Electric Bell

• Electric Bell is composed of two rods of cast iron. Coil is made around the iron rods.
• A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end.
• Another end of the strip is connected to the circuit.
• A gong is placed in a position so that it can be hit by the hammer.

#### Working of Electric Bell

When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the point and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.