The ratio of potential difference (V) between any two points on a conductor to the current (I) flowing between them, is constant, provided the temperature of the conductor does not change.

V/I = Constant

V/I = R

Put in another way, it simply means that provided R is kept constant, current is directly proportional to potential difference across the ends of the conductor. For a constant value of R, if the value of V is increased, the value of I increases; if V is decreased, then I decreases.  Also notice that if V is constant and R is increased, I decreases.  Similarly, if V is constant and R is decreased, I increases.

The Voltage is the difference in electric potential between those two points. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B. The 'Volt' (V) is the unit of voltage. Instrument used for measuring voltage is Voltmeter.

To have flow of electrons or electricity between the two ends of a conductor, the difference of electric pressure is necessary. The difference of electric pressure is known as potential difference. Rate of flow of electricity or current strength depends upon the potential difference. Greater the potential difference, the greater will be the current strength, lessor the potential difference, lesser the current strength. No potential difference, no current.

The potential difference is that which causes the current to flow in a closed circuit.

The electromotive force (e.m.f.) of a source is the potential difference between the terminals of the source when no current is drawn from it i.e. when the source is in the open or infinite resistance circuit.

The potential difference is the difference of voltage between the terminals when the load is connected with the source. The unit of potential difference in practical as well as in mks (or SI) system of units is volt.

The current flowing in a conductor always causes the conductor to become hot.  This is known as thermal effect as in an electric heater or electric iron.

When the temperature of the conductor is increased sufficiently, then light is emitted.  This is known as the luminous effect as in an electric bulb (i.e. incandescent lamp).  Light can also be obtained by ionizing the mercury or sodium vapor as in as fluorescent tube and sodium vapor lamp respectively.

When current flows through a certain liquid chemical known as electrolyte, Chemical changes occur in the liquid and in metals immersed in it and connected to the circuit.  This is known as the chemical effect as in battery charging.

The current flowing in a conductor always produces a magnetic field around the conductor.  This is known as the magnetic effect as in an electric bell or electric motors.

Electricity is the modern friend of man.  It has become a part and parcel of our daily life.  Where ever we may cast our eyes, we find the achievements and attainment of electricity.  It has really made the life of a person quite comfortable.

Talking on telephone with friends and relatives, watching football match on T.V. effective & efficient cooling of Air conditioners, cooking the food by cooking range, heating the food by microwave oven, quick ironing of clothes with electric iron etc have been possible only due to our modern friend electricity.

An electric bulb, an electric bell and a room heater, all require electric current for their operation but current produces different effects in each case. Therefore before numerating the various applications of electricity, it is essential that we should consider the various effects of electric current.The electrons in the outermost orbit are known as valence electrons.  In some elements these valence electrons are loosely bounded to their nucleus and can he dislodged by some means or the other.  Such electrons are also known as free electrons. Certain materials such as Copper, Aluminium have many free electrons.  These free electrons used to move haphazardly in all directions from atom to atom in the material. If, however the free electrons can be caused to move through the material in the same general direction, the net electrons movement is the flow of electricity and is called an electric current.