When the ends of the conductor are connected
to the battery through a bulb, the bulb glows because
energy transfer takes place from battery to the bulb.
The electrons are responsible for this transfer of
energy. If the electrons are responsible for transfer
of energy from battery to bulb, they must have an ordered motion. When
the electrons are in ordered motion, there will be a net charge crossing
through any cross section of the conductor. See figure 2. This ordered
motion of electrons is called electric current.
Thus we can say that electric current is ordered motion of charges.
Let us define electric current quantitatively,
Electric current is defined as the amount of charge crossing any cross
section of the conductor in one second.
Let Q be the charge crossing through any cross section of the conductor
in a time interval t. Then the amount of charge crossing through that cross
section in one second is Q/t. Therefore,
Electric current = electric charge/time interval
I = Q/t
The SI unit of electric current is ampere denoted by A.
1 Ampere = 1 Coulomb/1 Second (1 Coulomb = 6.625 x 1018 electrons)
1 A = 1 C/s
• Why do electrons move in specified direction?
When the conductor is not connected to the circuit through a battery,
the electrons inside the conductor are in random motion but when the
conductor is connected to a battery, the electrons move in a specified
direction. This shows that something causes the electron to move it in a
specified direction. When the ends of the conductor are connected to the
terminals of a battery a uniform electric field is set up throughout the
conductor. This field makes the electrons move in a specified direction.
• In which direction do the electrons move?
• Do the electrons accelerate continuously?
• Do they move with a constant speed?
The free electrons in the conductor are accelerated by the electric
field and move in a direction opposite to the direction of the field. When
electrons are in motion under the influence of the field, they collide with
lattice ions, lose energy and may even come to a halt at every collision.
They are again accelerated by the electric field and make collisions with
other lattice ions. In this way they continue to move along the conductor.