Samacheer Class 7 Science - Electricity

Cell – A device which converts chemical energy into electrical energy
Switch – used to open or close a circuit
Circuit – A complete path for the flow of an electric current
Miniature circuit Breaker – Reset by hand, circuit becomes complete once again
Fuse – safety device used in electric circuit
V.Analogy:

An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is I.

Parallel circuits:
It is connected by branches.
Voltage remains the same across each component of the circuit
Each bulb is fully powered.
All bulbs stay lit if one goes out.
Serial circuits:
It consist of single loop connection.
Current remains same in all parts of the circuit.
All the bulbs share power.
All bulbs go out if one goes out.

In telephones, a changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate. The diaphragm is made up a metal that can be attracted to magnets.
The diaphragm is attached to spring that is fixed to the earpiece.
When a current flows through the wires, the soft – iron bar becomes an electromagnet.
The diaphragm becomes attracted to the electromagnet.
As the person on the other end of the line speaks, his voice cause the current in the circuit to change. This causes the diaphragm in the earpiece to vibrate, producing sound.

When an electric current passes through a wire, the electrical energy is converted to heat.
In heating appliances, the heating element is made up of materials with high melting point. An example of such a material is nichrome (an alloy of nickel, iron and chromium).
The heating effect of electric current has many practical applications.
The electric bulb, geyser, iron box, immersible water heater are based on this effect.
These appliances have heating coils of high resistance.
Generation of heat due to electric current is known as the heating effect of electricity.

A dry cell is a portable form of a leclanche cell.
It consists of zinc vessel which acts as a negative electrode or anode.
The vessel contains a moist paste of saw dust saturated with a solution of ammonium chloride an chloride.
The ammonium chloride acts as an electrolyte.
The purpose of zinc pide is to maintain the moistness of the paste being highly gyroscopic.
The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as positive electrode or cathode.
It is surrounded by a closely packed mixture of charcoal and manganese dioxide (Mn0 2 ) in a muslin bag.
Here Mn0 2 acts as depolarizer. The zinc vessel is sealed at the top with pitch or shellac.
A small hole is provided in it to allow the gases formed by the chemical action to escape.
The chemical action inside the cell is the same as in avalanche cell.

The possible reasons so that even at switch ‘ON’ position the bulb is not glowing are:
The cell may be discharge
The bulb may be fused.
The wire may be broken from inside
The connection may be loose.