Class 9 Science Note

Current Electricity and Magnetism

Electric circuit: Electric circuit is the combination of source of current, conducting wires, and loads. They are of two types;


Closed circuit:

The circuit in which switch is closed and the electric current passes in the circuit is called closed circuit.



Open circuit:

The circuit in which switch is open and the electric current does not flow in the circuit is called open circuit.


Direction of flow of current

Flow of current is flow of positive charge from the positive terminal of the battery to the        negative terminal of it. This is the conventional method.


Conductor and insulator

The materials which allow electric current to pass through it are called conductor.

The materials which do not allow the electric current to pass through it are called insulator.


Potential difference

Potential difference between two points in a circuit may be defined as amount of work done in moving 1 Coulomb charge from a point of lower potential to the point of higher potential.

 Potential difference (V) = $\frac{{{\rm{work\: \: done\: }}\left( {\rm{W}} \right){\rm{\: }}}}{{{\rm{charges\: \: transferred}}\left( {\rm{Q}} \right){\rm{\: }}}}$

  V   = W/Q



The ability of material to resist the flow of current through it is called resistance. The resistance of the wire depends upon:

Length of wire: Resistance of wire is directly proportional to the length of wire


Area of wire: Resistance of wire is inversely proportional to the area of wire


Temperature of wire: Resistance of wire is directly proportional to the temperature of wire


Nature of material: Resistance of wire depends on the materials of which it is made. Copper, silver has low resistance because of the free electron present in them.


Relation of current, potential differences and resistances

Ohms law states that voltage developed across wire is directly to the current flowing through the wire.

V${\rm{\: }} \propto $  I

V = RI, where, R = resistance of wire.

If V=1volt,   I= 1 ampere   then, R= 1 ohm.

 So the resistance of the wire is 1 ohm if   1 ampere current flows when the potential difference of 1 volt is applied between two ends.

Where, V= potential difference across the two different points

  I = current in the wire







Properties of magnets

A magnet attracts the magnetic materials.

An isolated magnet doesn’t exist.

Like poles of magnet repel whereas unlike poles attract.


The region around the magnet where magnetic force of influence can be felt is called magnetic field.


Magnetism is physical phenomenon that is mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field. Every material is influenced to some extent by a magnetic field.


Terrestrial magnetism

 The magnetism related to the earth is called terrestrial magnetism. Earth behaves like huge magnet. If we suspend the magnet or needle of compass, it sets itself pointing the direction of north and South Pole of the earth.


Elements of the terrestrial magnetism

Angle of declination: The angle made by the freely suspended magnet and earth’s geographical north –south axis is called angle of declination. This angle is always less than 900


Angle of dip: The angle made by the direction of the earth’s magnetic field at a place and the horizontal line is called the angle of dip. The value of angle at dip at equator is 00 whereas the at pole it is 900

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