Unit of Charge
The charge on an electron is so small that
it is not convenient to select it as the unit
of
charge.
In practice, coulomb is used as the unit of
charge.
One coulomb of charge is equal to
the charge on 625 x 10 electrons ie.
1 coulomb = Charge on 625 x 10 electrons
Thus when we say that a body has a positive
charge of I coulomb (IC), it means that it
has a deficit of 625 x 101 electrons from
the normal due share.
3.
Free Electrons
We know that electrons move around the nucleus
of an atom in different orbits.
The electrons
in the last orbit are called valence electrons.
In certain substances, especially metals (e.g.
copper,
aluminium etc), the valence electrons are
so weakly attached to their nuclei that they
can be
easily removed or detached.
Such electrons are called free electrons It
may be noted here that
all valence electrons in a metal are not free
electrons.
It has been found that one atom of metal
can provide at the most one free electron.
Since a small piece of metal has billions
of atoms,
one can expect a very large number of free
electrons in metals.
For example, 1 cm' of copper
has about 8.5 x 10 free electrons at room
temperature.
4.
Electric Current
The flow of free electrons (or charge) in
a definite direction is called electric current
The
flow of electric current is shown in Fig.
1.1.
The copper strip has a large number of free
electrons.
When electric pressure or voltage is applied,
the free electrons being negatively charged
start
moving towards the positive terminal around
the circuit as shown in Fig. 1.1.
This directed flow
of electrons is called electric current.
Conventionally, the direction of electric
current is taken along the direction of motion
of
positive charges.
When current is caused by electrons (e.g.
in metals), the direction of current
is opposite to the direction of electron flow
On a relative scale, the spacing between the
nucleus and valence electrons is vast.
If a copper atom could
be magnified until the electrons were as large
as coins, the valence electrons would be several
kilometres
away from the nucleus.
This relatively large distance dictates that
valence electron is only weakly attached
to the nucleus.
5.
Measurement of Current
The flow of charge in a definite direction
is called electric current.
It is measured by the
time rate of flow of charge through the conductor.
If q is the charge flowing through any cross-
section of the conductor in time 1, then,
Electric current, I =
is given by :
If the rate of flow of charge varies with
time, then current at any time (instantaneous
current)
dq i
di
where dq is the small charge passing through
any cross-section of the conductor in small
time
dt.
The SI unit of electric current is ampere.
If q 1C and t 1s, then I 1/1 1 ampere.
One ampere of current is said to flow through
a wire if at any cross-section, one coulomb
of charge flows in one second.
If n electrons are passing through any cross-section
of the wire in time 1, then,
19
_ ne
where e = -1.6 1019 C
