Define the term drift velocity.
On the basis of electron drift, derive an
expression for resistivity of a conductor in
terms of number density of free electrons
and relaxation time. On what factors does
resistivity of a conductor depend?
Why alloys like constantan and manganin
are used for making standard resistors?
[Delhi 2016
Answers
Answer:
Explanation:
drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an average velocity of zero.
ρ = m/ ne^2τ
Resistivity of the materiel of a conductor depends on the relaxation time.
Explanation:
We have:
E = - V / l
Where V is the potential across the length l of the conductor.
Vd = eVτ / ml
Current flowing = neAVd
I = neAVd eVr/ml = ne^2AVτ /ml
I/V = ne^2Aτ /ml = 1/R ---- (1)
R = ρ l/A
Comparing (1) and (2)
ρ = m/ ne^2τ
Resistivity of the materiel of a conductor depends on the relaxation time.
Alloys like constantan and manganin are used to make standard resistance coils because: 1. They have a very high value of resistivity. ... Thus, their value of resistivity does not change even for very high values of temperature.
They have a very small temperature coefficient of resistivity. Thus, their value of resistivity does not change even for very high values of temperature.
3. They are least affected by atmospheric conditions like, temperature, moisture, air, etc..
4. The contact potential of alloys with copper is very small.