Current Electricity 2 Flashcards
what is the dependence of resistivity of metals on temperature.
At temperatures much lower than 0°C, the graph,
however, deviates considerably from a straight line.
The graph should be a straight line but it is curved upward in the tb.
rT= r0[1 + a (T–T0)]
where rT
is the resistivity at a temperature T and r0
is the same at a
reference temperature T0
. a is called the temperature co-efficient of
resistivity,
define temperatuee coefficient of resistivity
It is the fractional change in resistivity per unit rise in temperature.
Unit: K^-1
Metals have a large and +ve value of alpha
Alloys have a small +ve value
Semiconductors have -ve value of alpha
explain dependence of alloys on temperature and resisitiytty
Some materials like Nichrome (which is an alloy of nickel, iron and
chromium) exhibit a very weak dependence of resistivity with temperature
(Fig. 3.9). Manganin and constantan have similar properties. These
materials are thus widely used in wire bound standard resistors since
their resistance values would change very little with temperatures.
resitivity-temperature-relaxation time
ρ=1/σ=m/τ n e²
WIth the increase in temperature, avg speed of the electrons increases reuslting in more frequent collisions, hence relaxation time decreases. n is not much dependent on temperature in the case for conductors so with tthe increase in temperature, relaxation time decreases, so resitivity increases and conductivity decreases
resistivity-temperature-semiconductor
In the case of semiconductors, n increases with temperature. the decrease in relaxation time is negligible as compared to the increase in number density with temperature. so in semiconductors with the increase in temperature, resistivity decreases and conductivity increases
what is internal resistance
The resistance offered by the chemical material in a cell the flow of current between the 2 terminals is called internal resistance. it is the resistance offered by the cell itself to the flow of charges
the resistance offered by the electrodes and electrolyte of a cell to the flow of current through it.
difference between emf anf terminal velcoity
emf:
-Work done by the source in taking a charge once around the complete circuit
-it is equl ot the maximum potential difference btween the 2 terminals of the source in open circuit
-it is larger than pd and independent of external resistance
terminal velocity
-it is the work done in moving a charge between one point to another in the circuit
- it may exist between any 2 points in a closed circuit
-it is always less than emf
when is terminal potential of a cell greater than emf
when the device is charging
Vt= emf+Ir
Define EMF of a cell
Maximum potential difference between 2 electrodes of aces, when no current is drawn.
Define terminal potential difference
P.d between 2 electrodes of a cell in a closed circuit (ie when current is drawn from the cell)
