1.    Electric  Current  :
Electric charges in motion constitute an electric current. Any medium having practically free electric charges , free to migrate is a conductor of electricity. The electric charge flows from higher potential energy state to lower potential energy state. Positive charge flows from higher to lower potential and negative charge flows from lower to higher. Metals such as gold, silver, copper, aluminium etc. are good conductors.

2.    Electric  Current  In  A  Conductor  :
In absence of  potential  difference across a conductor no net current flows through a corss section. When a potential  difference is applied across a conductor the charge carriers (electrons in case of metallic conductors) flow in a definite direction which constitutes a net current in it . These electrons are not accelerated by electric field in the conductor produced by potential difference across the conductor. They move with a constant drift velocity . The direction of current is along the flow of positive charge (or opposite to flow of negative charge). i = nv_deA,  where  V_d = drift velocity .

 

5.    Sources  Of  Potential  Difference  &  Electromotive  Force :
Dry cells , secondary cells , generator and thermo couple are the devices used for producing potential difference in an electric circuit. The potential difference between the two terminals of a source  when  no  energy  is  drawn from it is called the " Electromotive  force" or " EMF " of the source.  The unit of potential difference is volt.

1 volt = 1 Amphere × 1 Ohm.

6.    Electrical  Resistance  :
The property of a substance which opposes the flow of electric current through it is termed as electrical resistance. Electrical resistance depends on the size, geometery, temperature and internal structure of the conductor.

Where r is the resistivity of the material of the conductor at the given temperature . It is also known as  specific resistance  of the material .
8.    Dependence  Of  Resistance  On  Temperature  :
The resistance of most conductors and all pure metals increases with temperature , but there are a few in which resistance decreases with temperature . If  R_o & R be the resistance of a conductor at 0º C and qº C , then it is found that  R = R_o (1 + a q) .
Here we assume that the dimensions of resistance does not change with temperature if expansion coefficient of material is considerable. Then instead of resistance we use same property for resistivity as    

r = r₀ (1 + aq)

The materials for which resistance decreases with temperature, the temperature coefficient of resistance is negative.

Where a is called the temperature co-efficient of resistance . The unit of a is K^{- 1} of ºC ^-1 reciprocal of resistivity is called conductivity and reciprocal of resistance is called conductance (G) . S.I. unit of G is ohm.

9.    Ohm's  Law  :
Ohm's law is the most fundamental of all the laws in electricity . It says that the current through the cross section or the conductor is proportional to the applied potential difference under the given physical condition . V = R I . Ohm's law is applicable to only metalic conductors .

I - Law (Junction law  or  Nodal Analysis) :This law is based on law of conservation of charge . It states that " The algebric sum of the currents meeting at a point is zero " or total currents entering a junction equals total current leaving the junction .

S I_in = S I_out. It is also known as KCL (Kirchhoff's current law) .

A number of resistances can be connected and all the complecated combinations can be reduced to two different types,namely series and parallel .   

(i)    Resistance  In  Series  :
 When  the  resistances  are connected end toend  then they  are  said to be in series . The current through each resistor  is same . The effective resistance appearing  across  the battery .

R = R₁ + R₂ + R₃ + ................ + R_n     and
V = V₁ + V₂ + V₃ + ................ + V_n  .

A parallel  circuit  of  resistors is one in which the  same  voltage  is  applied  across  all  the components in a parallel grouping of resistors R₁, R₂, R₃, ........, R_n .

Conclusions :
(a)    Potential difference across each resistor is same .
(b)    I = I₁ + I₂ + I₃ + .......... I_n  .

If a  cell  of  emf E and internal  resistance r be connected with a  resistance  R  the  total  resistance  of  the circuit is (R + r) .

(ii)    Cells  In  Parallel  :   
If m  cells  each  of emf  E  &  internal resistance r  be connected  in  parallel and  if  thiscombination  be connected  to  an external resistance then the emf of the circuit = E .

14.    Potentiometer  :
A potentiometer is a linear conductor of uniform cross-section with a steady current set up in it. This maintains a uniform potential gradient along the length of the wire . Any potential difference which is less then the potential difference maintained across the potentiometer wire can be measured using this . The potentiometer equation is

16.    Voltmeter  :
A high resistance is put in series with galvanometer . It is used to measure potential difference .   

17.    Relative  Potential  :
While solving an electric circuit it is convinient to chose a reference point and assigning its voltage as zero. Then all other potential are measured with respect to this point . This point is also called the common point .

18.    Electrical  Power  :
The energy liberated per second in a device is called its power . The electrical power P delivered by an electrical device is given by  P = VI  ,  where  V = potential difference across device  & I = current. If the current enters the higher potential point of the device then power is consumed by it (i.e. acts as load) . If the current enters the lower potential point then the device supplies power (i.e. acts as source) .

19.    Heating  Effect  Of  Electric  Current  :
When a current is passed through a resistor energy is wested in over coming the resistances of the wire . This energy is converted into heat .

20.    Joules  Law  Of  Electrical  Heating  :
The  heat  generated (in joules) when a current of I ampere  flows  through  a  resistance  of R ohm for T second is given by :

If current is variable passing through the conductor then we use for heat produced in resistance  in time 0 to t is:

21.    Unit  Of  Electrical  Energy  Consumption  :
1 unit of electrical energy = Kilowatt hour = 1 KWh = 3.6 × 10⁶ Joules.