current ELECTRICITY Flashcards
(41 cards)
OHMS law
current flowing thru a conductor is directly proportional to the potential difference V across the ends of th conductor, provided temp is kept constant
V= IR
RESISTIVITY DEPENDS ON:
open EYES WIDELY
𝝆
material
TEMP
RELAXATION TIME 𝜏
number density n
RESISTANCE DEPEAND ON
- material
- lenght (directly)
- area (inversely)
resistance
SI
dimension
SI: ohm
R= 𝝆 l/A
[ M^1 L^2 T^-3 A^-2]
resistivity 𝝆
SI :ohm m
𝝆 = RA/l
microscopic form of OHMS LAW
OR
VECTOR FORM OF ohms law
or
relation between J, σ , E
- from ohms law =V= IR
- V = I 𝝆 l/A
- V/l =J 𝝆
- V/I 1/𝝆 = J
- E σ = J
- J = σ E
current density** J ** and its direction ?
used in microscopic fom of OHMS LAW
LETITFLOW
it is the current flowing per unit AREA
SI: A m^-2
direction along electric current
DRIFT VELOCITY
THE average velocity with which the free e- get drifted under the INFLUENCE OF ELECTRIC FIELD
RELAXATION TIME 𝜏
THE time taken between two successive collision when e- are accelerated under the influence of E. F
mobility μ
magnitude of drift velocity of charge per unit electric field
SI : m^2 s^-1 V^-1
temperature coefficient α
SI UNIT
α = 𝝆t -𝝆o/ 𝝆o (Tf - To)
K^-1 or c^-1
1Ω
R= V/ I
the resistance of a conductor is said to be 1 Ω if 1A of current flow when potential difference of 1 volt is applied throug across the conductor
DEFINE resistivity
the resistance offered by a conductor of per unit lenght per unit area
HOW DOES RESISTIVITY VARIES WITH TEMP IN A CONDUCTOR
no time to relax
- WITH rise in temp: K.E increses
- collisions of no. of free e-‘s increases
- relaxation time decreases
- resistivity increase
HOW DOES RESISTIVITY VARIES WITH TEMP IN A SEMICONDUCTOR
Si, Ge
- when temp rises: n increases
- no. of free e- increases
- collison of free e- also increases
- relaxation time decreases
- BUT impact of increase in n is> than decrease in relaxation time
- so RESISTIVITY decreases
HOW DOES RESISTIVITY VARIES WITH TEMP IN NICHROME AND MAGNANIN
resistance bery high but weak dependence on temp
- when tempurature increases n increasa and realaxation time decreases
- but they compensate each other such a way that there is only slight INCREASE in RESISTIVITY OR RESISTANCE
LIMITATION OF OHMS LAW
draw graphs as well
- the relation between V AND I is not linear i.e. potential difference vary non linearly with current
e.g semiconductor - the variation of I with V MAY DEPEND UPON THE sign of V applied
on reversing the direction but keeping the magnitude fixed produce a current of same magnitude
e.g : junction diode
- the relation between I and V is not unique i.e. there is more than one value of V for the same current
cells
a source of energy that maintains continous flow of charge
EMF
SI UNIT
not a force
THE MAX POTENTIAL DIFFERENCE BTWN TWO ELECTRODES of a cell when no current is drawn form the cell or in an open circuit
VOLT
terminal POTENTIAL difference
use terminallll
potential difference between two terminals of the cell in a closed circuit
Internal resistance (r)
the resistance offered by the material of electrolyte to the flow of current inside the cell
Ω
factors depending on internal resistance
- nature of electrolyte
- conc of electrolyte (directly prop)
- temperature (inversely)
- distance btwn electrodes (directly)
- area of electrode (inversly)
expression of internal resistance
r = (ε/V - 1)R
relation between pd and r for discharging (V< ε)
V = ε- Ir
