Electric
current – Physics Notes-
Electricity
The flow of
charged particles in a definite direction is called electricity .
* Those substances which permit the flow of charges
through them are called conductors , e.g. , metals .
* Those substances which do not permit the flow of
charges are called insulators , e.g. , glass , mica , etc.
Electric
Current:
Amount of
charge passing through any cross section of conductor in one second is called
electric current.
* If Q coulomb charge passes through cross
section of conductor in time ‘ t’ second
Then
, charge passing in 1 second = Q
/ t
Hence electric current , I = Q /
t
Thus, The rate of flow of charge passing through any cross section of conductor is
called electric current.
*
SI unit of current = Coulomb / sec or
Ampere ( A)
*
Current is a Scalar quantity.
* Current
will only flow through a conductor if p.d. is maintained across its ends .
*
Electric current is measured by Ammeter
and Galvanometer
*
Electric current produce magnetic field.
* Electric current is two types , AC
current and DC current
Direction Of Electric current
* In conductor Electric current is produced due to flow
of electrons .
* Direction of current is opposite to that of
moving electrons.
* Electric current flow in circuit from positive
terminal to negative terminal of battery .
* In conductor, Electric current flow from high
potential to low potential.
Q- What is difference between ammeter and
Galvanometer?
Ans-
Ammeter
* It is a device used to determine the magnitude of
current flowing in a circuit.
* It is used in an electrical circuit
* It measures both the Alternate and Direct current.
*It is less sensitive.
* It is more accurate.
* It works with or without the presence of the magnetic
field.
* An ideal ammeter should have zero resistance.
Galvanometer
* It is a device used to detect the strength and
direction of small current present in the circuit
* It is used in bridge and potentiometer.
* It measures only the direct current.
* It is more sensitive
* It is less accurate
* It works due to the presence of the magnetic field
* Galvanometer has moderate resistance (10-100 ohms).
Q. How a galvanometer can be used as an
ammeter?
Ans-
* A galvanometer is used for measuring small currents.
* By converting a galvanometer into an ammeter large
currents can be detected.
* To convert a galvanometer into an ammeter, a low
resistance known as shunt resistance is connected parallel to the galvanometer.
CURRENT CARRIERS
( i ) Current carriers in solid conductors
* In solid conductors ( e.g. , metals ) , there are a
large number of free electrons . When . electric field ( i.e. , p.d ) is
applied to the conductor , the free electrons start drifting in a direction opposite to the field to constitute electric current .
* Hence , free electrons are the current carriers in
solid conductors .
( ii ) Current carriers in liquids
* Some liquids are conductors of electricity . A
conducting liquid is called an electrolyte ( e.g. , solution of CuSO4
) .
* The electrolytic solution provides positive ions (
e.g. , Cu2+ ) and negative
ions ( e.g. , SO42- ) .
* When external electric field ( i.e. , p.d. ) is
applied , the positive ions move in one direction and negative ions in the
opposite direction to constitute electric current .
* Hence , in conducting liquids , ions ( positive and
negative ) are the current carriers .
( iii ) Current carriers in gases
* Under ordinary conditions , gases are insulators .
* When a gas under low pressure is subjected to high
electric field ( i.e. , high p.d. ) , ionisation of gas molecules takes place ,
i.e. , electrons and positive ions are formed .
* Hence , current carriers in gases are free electrons
and positive ions .
CURRENT CONDUCTION IN METALLIC CONDUCTORS
* Metals have a large number of free electrons , about
1028 per m³ .
* In the absence of electric field ( Potential
difference ) , these electrons are in a state of random motion due to thermal
energy . The average speed of these electrons is sufficiently high ( = 105
m/s ) at room temperature .
* These velocities are distributed randomly in all
directions so that there is no net movement of charge in any particular
direction . Consequently , no current is established in the conductor in the
absence of electric field .
* When potential difference is applied across the ends
of a conductor , electric field is
applied at every point of the conductor wire
. The electric field exerts force on the free electrons which start
accelerating towards the positive terminal .
Relaxation time ( t ) :
* As the free electrons move in conductor in presence
of electric field , they collide again
and again with positive ions of the metal . Each collision destroys the extra
velocity gained by the free electrons .
The
average time that an electron spends between two collisions is called the
relaxation time ( t ) . Its value is of the order of 10-14 second
.
Drift Velocity
* In Conductor Free electrons are continuously
accelerated in presence of the electric field . Collisions between electrons
and cations prevent electrons velocity from becoming large . The
result is that electric field provides a small constant velocity towards
positive terminal which is superimposed on the random motion of the electrons .
This constant velocity is called the drift velocity .
* The average velocity with which free
electrons get drifted in a metallic conductor under the influence of electric
field is called drift velocity ( vd ) .
* The drift velocity of free electrons is of the order
of 10-5 m/s .
Drift
Velocity, vd = (eEt) /m
Where, e = magnitude of Charge on electron
E = Magnitude of electric field
t = Relaxation time of free electron
Relation Between Current and Drift Velocity
I = n e A vd
Where, n = electron density ( no. of free electrons
per unit volume )
e = Magnitude
of charge on electron
A =
cross section area of conductor
Vd = Drift velocity
For
Given conductor, A , n and e are constant,
So
, I
∝ vd
* Current flowing
through conductor is directly proportional to the drift velocity
* Yet drift
velocity is very small. But small drift velocity of large number of free
electrons give sufficient current.
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