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Class VII - science: Electric Current And Its Effects
One Word Answer Questions:
Q) The combination of two or more cells is called?
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Q) Name the safety device based on the heating effect of electric current?
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Q) Give an example of a device which uses electromagnet?
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Q) The coil of wire present in an electric heater is called?
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Q) In a electric cell, the thicker, shorter line represents which terminal?
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Q) Name the device which is used to break the electric circuit or to complete it?
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Q) Materials that allow electricity to pass through them are called?
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Q) Give an example of an insulator?
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Short Answer Questions:
Q) What is an Electrical circuit?
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Q) What is a circuit diagram?
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Q) Define heating effect of electric current?
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Q) What is an electromagnet? State some uses of it?
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Q) Explain briefly about the working of an electrical circuit?
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Q) What are the components of an electric circuit?
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Q) Define conductors?
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Q) What is a filament?
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Q) Why are conductors and insulators equally important? Give reasons?
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Q) What is a fused bulb? What happens to the electric cell when a bulb is fused?
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Long Answer Questions:
Q) Explain briefly the working of an electric bell?
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Q) Explain briefly the working of a torch bulb?
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Q) Explain briefly the working of an electric cell?
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Electric Current And Its Effects
electric current

"The flow of electric charge is called electric current".
In an electric circuit, the charge is often carried by moving electrons. The SI unit of electric current is Ampere (A). The SI unit of electric charge is Coulomb (C).

electric circuit

“The path through which electric charge moves is called electric circuit”.

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An Electric Circuit
Symbols Of Electric Components

Different components of electric circuit are shown by certain symbols. Use of symbol makes it easy to represent an electric circuit. Use of standard symbol makes it easy to understand by anybody.
Example: A TV mechanic can identify different components of a TV by looking at its circuit diagram.

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Symbol Of Electric Component
Electric Cell

It is a device which produces electric charge because of some chemical reactions. The cell which is used in a torch is called dry cell. The wet cells are used in car batteries. A normal dry cell gives an output of 1.5 Volt (V).

Battery

A group of cells is called battery. More than one cell is used in most of the devices; because the device needs more than 1.5 V of power.

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Connecting Two Cells Together To Make A Battery
Closed Circuit

When the circuit is complete, it is called closed circuit. Current flows only in a closed circuit.

Open Circuit

When the circuit is incomplete, it is called open circuit. Current does not flow in an open circuit.

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An Open And Closed Circuit
Heating Effects

This is “When electric current flows through a conducting wire, the temperature of wire increases”. called heating effect of electric current.

  • Devices Which Work on Heating Effect of Electric Current:

Incandescent bulb is an example of a device which works because of the heating effect of electric current. The filament of bulb is made of tungsten. Tungsten has very high melting point and that is the reason it is used in electric bulb. Moreover, argon gas is filled inside the bulb. Argon prevents the filament from catching fire.

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Incandescent Bulb
Elements of Heating Appliances

A heating appliance has a highly coiled wire or a metallic rod. The highly coiled structure increases the surface area and thus provides more heat. The elements are usually made of constantan which is a metal with very high melting point.

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Electric Heater
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.

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Electric Fuse Used In Buildings
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Electric Fuse Used In Electrical Appliances
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.

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Miniature Circuit Breaker (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.

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Electromagnetic

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

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An Electromagnet
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. 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

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An Electric Bell Circuit
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