Today, In this video, We will know about,
Electric Potential, Potential Difference,
Concept of EMF and Potential Difference, Drift
Velocity of Free Electrons, and Relation between
Current and Drift Velocity.
Let's start with Electrical Potential
Just as, a body raised above the ground has
gravitational potential energy, similarly,
a charged body has electric potential energy.
When a body is charged, work is done in charging
the body.
This work done is stored in the body in the
form of electric potential energy.
The charged body has the capacity to do work
by moving other charges either by attraction
or repulsion.
Quantitatively, electric potential is defined
as
The electric potential at a point is the electric
potential energy per unit charge.
Electric potential, V = Electric potential
energy upon Charge = W upon Q
The SI unit of energy or work is 1 Joule and
that of charge is 1 Coulomb so that SI unit
of electric potential is 1 Joule upon Coulomb,
which is also called 1 volt.
Thus when we say that electric potential at
a point is 10 Volt, it means that if we place
a charge of 1 Coulomb at that point, the charge
will have electric potential energy of 10
Joule.
Similarly, if we place a charge of 2 Coulomb
at that point, the charge will have electric
potential energy of 20 Joule.
Note, that potential energy per unit charge,
that is electric potential, is 10 Volt.
Potential Difference
The difference in the potentials of two charged
bodies is called potential difference.
Consider two bodies A and B having potentials
of 5 Volt and 3 Volt respectively as shown
in Figure.
Each coulomb of charge on body A has an energy
of 5 joules while each coulomb of charge on
body B has an energy of 3 joules.
Clearly, the body A is at higher potential
than body B.
If the two bodies are joined through a conductor,
then electrons will flow from body B to body
A.
Here, You should note that, The conventional
current flow will be in the opposite direction,
that is, from body A to body B.
When the two bodies attain the same potential,
the flow of current stops.
Therefore, we arrive at a very important conclusion,
that, current will flow in a circuit if potential
difference exists.
No potential difference, no current flow.
It may be noted that potential difference
is sometimes called voltage.
Unit.
Since the unit of electric potential is volt,
one can expect that the unit of potential
difference will also be volt. it is defined
as
The potential difference between two points
is 1 volt if one joule of work is done in
transferring 1 Coulomb of charge from the
point of lower potential to the point of higher
potential.
Concept of EMF and Potential Difference.
There is a distinct difference between EMF
and potential difference.
The EMF of a device, say a battery, is a measure
of the energy the battery gives to each coulomb
of charge.
Thus if a battery supplies 4 joules of energy
per coulomb, we say that it has an EMF of
4 volts.
The energy given to each coulomb in a battery
is due to the chemical action.
The potential difference between two points,
say A and B, is a measure of the energy used
by one coulomb in moving from A to B.
Thus if potential difference between points
A and B is 2 volts, it means that each coulomb
will give up an energy of 2 joules in moving
from A to B.
Drift Velocity of Free Electrons
When potential difference or voltage, is applied
across the ends of a metallic wire, the free
electrons start drifting towards the positive
terminal of the source.
The average velocity with which free electrons
get drifted in a metallic conductor under
the influence of potential difference is called
drift-velocity of the free electrons.
The drift velocity of free electrons is very
small, of the order of 10 raised to minus
5 meter per second.
Here, You may wonder that if electrons drift
so slowly, how room light turns on quickly
when switch is closed ?
The answer is that propagation of electric
field takes place with the speed of light.
When we apply electric field, That is potential
difference, to a wire, the free electrons
everywhere in the wire begin drifting almost
at once.
Relation between Current and Drift Velocity
Consider a portion of a copper wire through
which current I is flowing as shown in Figure.
Clearly, copper wire is under the influence
of electric field.
Let A = area of cross-section, of the wire.
n = electron density, that is number of free
electrons per unit volume.
e = charge on each electron.
Vd = drift velocity of free electrons.
In one second, all those free electrons within
a distance vd, to the right of cross section,
at P, that is in a volume, A Vd, will flow
through the cross-section at P, as shown in
Figure.
This volume contains n A Vd electrons and,
hence, a charge , n A Vd e
Therefore, a charge of n e A Vd per second
passes the cross-section at P.
Therefore, I = n e A Vd
Since A, n and e are constant, I is directly
proportional to Vd.
Hence, current flowing through a conductor,
is directly proportional, to the drift velocity,
of free electrons.
