Current Electricity 1 Flashcards
define current
Current is the rate of flow of charge per unit time.
I= Q/t
Unit: ampere
it is a scalar quanitty
state ohm’s law
The potential difference across the ends of a conductor is directly proportional to the potential difference through it, provided the physicl conditions like pressure temp remain a constant
V ∝ I
V=IR
R is resistance
what is resistance
Resistnance is the opposition to the flow fo current
It depends on length of conductor, area of c.s, temperature and nature of material of conductor,
R ∝ l
R∝A
R->nature
R=ρl/A
what is resistivity
It is the resistance offered by a conductor of unit length and unit area of cross section.
Unit: Ωm
It depends only on the nature of material of conductor and temperature
1/ρ=σ, conductivity
what is drift velocity
It is the average velocity gained by free electrns of a conductor in the influence of an external electric field.
It is noted that even though the electric field accelerated electrons between 2 collisions, yet it does not produce any net acceleration because electrons keep colliding with the ions and the velocity gained by the electrons is lost in the next collision, hence it moves with a constant avg velocity in opp direction of electric field.
effect of temp of drift velocity
when temperature increases, the kinetic energy of electrons increases, hence they move faster
the relaxation time decreases, drift velocity decreases
define relaxation itime
Relaxation time is the time interval between two successive collisions of electrons in a conductor, when current flows.
what is current density
Current per unit area (taken normal to the current), I/A, is called
current density and is denoted by j.
directed along E
define mobility
The mobility fo a charge carrier is defined as the the drift velocity per unit electric field.
μ=e𝜏/m
Unit: m²/Vs
sq. metre per volt per second
Mobility is positive
What are ohmic and non ohmic conductors
cnductors which obey ohms law are called ohmic ocnductors while those which do not follow ohm’s law are called non ohmic conductors
i) limitation of ohms law (graph)
V ceases to be proportional to I
For a small current, a metallic conductor obeys OHm’s law and the V-I graph is a straight line but if a large currnt is passed through the conductor or if potential is applied for a long time, heat is developed and it deviates from Ohm’s Law
ii) Deviation from Ohm’s Law
Diodes
It consists of a p and n type semiconductor, a voltage is applied at the junction. The resulting current is not directly proprotional to the potential.
The relation between V and I depends on the sign of V. In
other words, if I is the current for a certain V, then reversing
the direction of V keeping its magnitude fixed, does not
produce a current of the same magnitude as I in the opposite direction
iii) limitation o fohm’s law
GaAs ( gallium arsenide)
The relation between V and I is not unique, i.e., there is more than
one value of V for the same current I (Fig. 3.7). A material exhibiting
such behaviour is GaAs.
comment on the resistivity of insulators, conductors and semiconductors
Metals have low resistivities in the range of 10–8 Wm to 10–6 Wm. At the
other end are insulators like ceramic, rubber and plastics having
resistivities 1018 times greater than metals or more. In between the two
are the semiconductors. These, however, have resistivities
characteristically decreasing with a rise in temperature. The resistivities
of semiconductors can be decreased by adding small amount of suitable
impurities. This last feature is exploited in use of semiconductors for
electronic devices.
resisitivity of alloys are greater than that of metals
In an alloy like nichrome (made of nickel and chromium), the free electron finds a disordered arrangment of nickel ions and cromium ions. Due to it, the electron is scattered by them randomly and very frequently. As a result of it, the value of relaxation time of electorn decreases and hence resistivity increases because
ρ
∝
1
/
τ
.
