CBSE ADDA

Current Electricity

The term ‘current’ in general stands for the continuous flow of any material particles, e.g. water current means continuous flow of water particles through a pipe or a channel. Electric current in general means a continuous flow of electrons, ions or any electrically charged particles through a medium. In our everyday life, we are familiar with the electric current flowing through metallic conductors where flow of electrons only takes place. Positive charges can flow only through gases and nonmetallic solids and liquids.

Units of electrical quantities

I. Charge: The unit of charge in the S.I. system is coulomb.

2. Current strength: Current strength is often simply called current, its symbol is ‘I’ and the SI unit of it is ampere.

Idea of resistance-its influence on regulating current

The term "resist" means to oppose, so the property of opposing or resisting flow of electric charges in a conductor means resistance. The body resisting flow of charge is a resistor.

Resistance is the natural property of every material body by virtue of which the body opposes flow of electric charges through it.

The influence of resistance on a conductor is thus greater the resistance, less is the current through it and vice versa.

Unit of resistance is Ohm.

Ohm's law

The current flowing through a conductor is directly proportional to the potential difference between its ends when temperature and other physical conditions of the conductor remain unaltered.

Ohm's law is not valid for current flowing through gases under low pressure, electrolytes and semiconductors.

Heating effect of current

A potential difference applied across a conductor creates electrons drift from a point of lower potential to another point of higher potential. The electrons collide with the constituent particles of the conductor creating larger rate of vibration of the particles. So, according to the kinetic-theory an increase of heat energy occurs in the conductor causing rise of temperature of the conductor. This is heating effect of electric current. The amount of heat generated in a body depends upon current strength, resistance of the body and the time of flow of current. These factors are formulated in Joule's laws of heating effect of current.

Magnetic effect of current

A current-carrying conductor always generates a magnetic field around it. Oersted performed an experiment to show this, where a conductor was held above and parallel to a freely rotating pivoted magnetic needle. It was found that the magnetic needle deflected due to current flow in the conductor. The deflection increased with increase of current and it reversed when current was reversed. Thus, the current-carrying conductor associates a magnetic field around it due to which the magnetic needle deflects.

Ampere's Swimming rule gives the direction of magnetic field.

The rule is: If a man be imagined swimming along a conductor carrying current and in the direction of current with his face in normal position with respect to his body, turned towards the needle, then the North Pole will deflect towards his left Itcuui.

If the conductor carrying electric current is perpendicular to the needle, no deflection of the needle occurs.

Fleming's left hand rule

We stretch the thumb, forefinger and the middle finger of our left hand so that the three fingers are perpendicular to each other and if the forefinger is directed along the magnetic field (N-pole to S-pole), the middle finger along the direction of current, then the thumb gives the direction of deflection of the conductor.