Electric Circuit 
A continuous and closed path of an electric current is called an electric circuit.
Electric current is expressed by the amount of charge flowing through a particular area in unit time.

Direction of Electric current

Electric current was considered to be the flow of positive charges.Conventionally, in an electric circuit, the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.If a net charge Q flows across any cross-section of a conductor in time t, then the current I, through the cross-section is:

          I = Q/t

The SI unit of electric charge is Coulomb (C).The SI unit of electric current is ampere (A).

One ampere
One ampere is constituted by the flow of one coulomb of charge per second.

Ammeter 
An instrument called ammeter measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured.

Note that the electric current flows in the circuit from the positive terminal of the cell to the negative terminal of the cell through the bulb and ammeter.

Potential Difference
The electrons move only if there is a difference of electric pressure called the Potential Difference along the conductor. The chemical action within a cell generates the potential difference across the terminals of the cell. When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current.

One volt 
One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.

1 volt = 1 joule/1 coulomb

OHM’S LAW
The potential difference, V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same. 

V ∝ I
V/I = constant
V = IR
R =  Resistance

Resistance
It is the property of a conductor to resist the flow of charges through it.Its SI unit is ohm(Ω).
The resistance of the conductor depends (i) on its length, (ii) on its area of cross-section, and (iii) on the nature of its material. 
R ∝ l 
and R ∝ 1/A
R ∝ l/A
or, R = ρl/A
where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor.The SI unit of resistivity is Ω m.

Variable Resistance 
A component used to regulate current without changing the voltage source is called variable resistance.

Rheostat
It is often used to change the resistance in the circuit.

RESISTANCE OF A SYSTEM OF RESISTORS

(i)Series resistor 

Resistors are joined end to end.The current is the same in every part of the circuit or the same current through each resistor.The resistance of the combination is greater than any individual resistance.The disadvantage of a series circuit is that when one component fails the circuit is broken and none of the components works

(ii) Parallel resistor 

Resistors are joined in parallel.The equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.The potential difference across each resistor is also V.The total resistance in a parallel circuit is decreased. This is helpful particularly when each gadget has different resistance and requires different current to operate properly.