Direct Current:- 
Steady current whose magnitude and polarity doesn't change with time is called direct current(D.C). It is generated by cell and DC Generators.

Potential Difference:-
The amount of workdone on a moving a unit positive charge from one point to another point within the electric circuit is called potential difference. It's S.I.unit is Volt.

Electromotive Force (emf):-
The amount of workdone by a source to convert non-electrical form of energy into electrical energy is called emf. Emf is measured in open circuit and S.I. Unit is Volt.

1V Potential Difference(P.d):-
The potential difference between two points in an electric circuit is said to be 1 V if 1J of work is done on moving a unit positive charge from the point to another.

Emf P.d

The amount of work done by a source to convert non-electrical energy into electrical energy is emf.

The amount of workdone on moving a unit positive charge from one point to other in a circuit is p.d.

It is measured in open circuit.

It is measured in the closed circuit.

Except in charging case, emf is greater than p.d. Except is charging case p.d. is less than emf.
It is cause. It is effect.

Drift Velocity:-
When a metallic conductor is connected with a battery potential, it's electrons starts to move towards the end which is connected with the positive terminal of the battery. During the process they collide with the atom of lattice and get deaccelerated. However, they are again accelerated by the battery potential until they get deaccelerated by another collision. When they suffer from the collision and get deaccelerated and again get accelerated by the battery potential. Due to this acceleration and deacceleration electrons on conductor move with slowand steady velocity which is called drift velocity.

Expression for Drift Velocity:-

Let us consider a piece of metallic conductor (AB) connected with a battery. The free electrons of a conductor start to move towards end A of the conductor which is connected ti the positive terminial of the battery. Let the electron cross the small segment (PQ) of the conductor of length 'dr' in small time 'dt' with the drift velocity"v". If  "A" be the cross sectional area of the conductor,
The volume of sgement PQ is given by (V)=A. dx
If the conductor contains 'n' number of frree electrons per unit volume, the number of free electrons on segment PQ=n.A.dx     
The change that flows through segment PQ=n.e.A.dx and the current flowing through it,

=Charge÷ time 
=n.e.A.dx÷ dt
Or, i=v.e.n.A-----------equation i
i.e v=i÷e.n.A-----------equation ii

QUESTION] In which of the metal the drift velocity of elctrons is higher iron or copper, Why?
We know that,
Copper is a good conductor of electricity than the iron. Hence the number of free elctrons per unit volume in copper is greater than the number of the free elctrons per unit volume in the iron. Hence, from the above equatiion, the drift velocity of electrons is higher in iron than in copper

Electrical Resistance:-
The property of a material to oppose the flow of current through it is called  resistance. It's S.I.Unit is ohm ().

Conduction of Electricity In Metals
A conductor consists of large number of free elctrons. These free electrons are in random motion within the conductor such that the elecric field generated by one electron is cancelled by the electric field generated by the another electron. Hence, there is no electric field generated in the conductor.


When the conductor is connected to the battery the electrons starts to drift towards the enf of the conductor connected to the anode of the battery. As the electrons reach to the end, it leaves the conducor that reaches to the positive terminal of the battery and neutralizes the positive charge. At the same time, an electron leaves the negative terminal of the battery, that reaches to the conductor to fulfill the electron defficiency on the conductor. In this way, the electric charge flows in the circuit.

Direction of Flow of Electricity 

  1. Conventional Direction Of Flow of Electricity:-
    Accordinf to this direction, electric current flows from the positive terrminal to the negative terminal of cell in an electrical circuit.



      2. Electron Direction Of Flow of Electricity:-
 According to this direction, electric current flows from   the negative terminal to the positive terminal of the cell in an electric circuit.