All electrical appliances like a bulb or a heater's coil are based on the movement of charges. Like flowing water constitute water current in rivers, similarly electric charge flowing through a conductor or a metallic wire constitutes electric current - the quantity of charge flowing per unit time.
Electric current is the charge flowing through any cross section of the conductor in a unit time.
i = charge (Q) / time (t)
where Q is the charge in coulomb flowing through the conductor in t seconds. If 1 coulomb (C) of charge flows through any cross section of a conductor in 1 second (s), the current flowing it will be 1 ampere (A).
Ampere is the SI unit of current given in the honour of the French scientist, Andre Marie Ampere (1775-1836). However, small currents are more conveniently expressed in milliampere (mA), and microampere (μA). Current is a scalar quantity.
1 mA = 10-3 A
1 μA = 10-6 A
An ammeter is an instrument which on connecting in series in an electrical circuit indicates how many amperes of current is flowing in the electric circuit.
All metals contain large number of free electrons which act as charge carriers. In a metallic conductor or wire, these free electrons move with a sufficiently high velocity in all possible directions between the atoms of the conductor or wire and even then there is no net flow of electrons. But when battery is connected across the ends of the wire, the electrons drift in one direction i.e., current flows along the wire in one direction from positive terminal of the battery to the negative terminal of the battery along the wire with a very small velocity called drift velocity of the electrons.
All materials can be divided into two categories on the basis of movement of charges through them: conductors and insulators.
Conductors are the materials which allow the electric current to flow through them quite freely. For example, metals like silver, copper, aluminum.
Insulators are the materials which do not allow electricity to flow through them freely. For example, rubber, glass, bakelite.
Example 1: A current of 0.5 A is drawn by a filament of an electric bulb for 5th part of an hour. Find the amount of electric charge that flows through the circuit.
Given, i = 0.5 A
t = 1/5 of an hour = 12 minutes = 720 s
Q = it
= 0.5 A x 720 s
= 360 C