Electric Current and Ohm's law Test - 7

 Answer :- b

Solution :- The temperature co-efficient of resistance of manganin is very low. Hence its resistance is almost independent of temperature. Hence it is used in making standard resistances.

Drift velocity is directly proportional to potential difference.
Drift velocity is defined as the average velocity with which free electrons get drifted towards the positive end of the conductor under the influence of an external electric field.
Drift velocity is given by
v_{d ​} = eE τ​/ m
But, E=V/l ​
(if l is length of the conductor and V is constant potential difference applied across the ends of the conductor)
∴v_d ​= eV τ​/ml
⇒v_d ​∝V
So, when the potential difference is doubled the drift velocity will be doubled.
Note - Current flowing through a conductor is directly proportional to the drift velocity.

The resistance of a conductor depends on thickness (cross sectional area of the wire), length and temperature.
Resistivity is defined as the measure of the resisting power of a specified material to the flow of an electric current.
R=ρ(I/A)​
where R = Resistance of the conductor
ρ= Resistivity of the conductor
l = length of the conductor
A = Area of cross section
i.e., ρ=R (A ​/l)

the first colour shows the unit digit
the second colour shows the tens digit
the third colour shows the power of 10
the fourth colour shows the tolerance
green=5
red=2
orange=3
no colour=20%
therefore approximate resistance=52x10 ​³ Ωand tolerance of 20%

When we use a heating element that has
1. High specific resistance: a small quantity of wire is used to generate the required amount of heat or else the quantity of wire would be required higher in order to heat.
2. High melting point: a higher melting point is selected so that higher temperature is attained. Nickel and chromium alloys are ideal as a heating element.

Semiconductors are materials which have a conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material.

Relaxation time is the average time between two successive collisions between the free electrons and the atoms(kernel) of atoms .
It is denoted by τ.

Mobility of charge carriers in a conductor is defined as the magnitude of their drift velocity per unit applied electric field.
Mobility,
μ=Drift of electric field
μ=V_d E
S.I. unit of mobility is m² Vs or ms NC.

 

Constantan is an alloy of copper (55%) and nickel (45%) where Manganin is an alloy of copper (84%) manganese(12%) and nickel (4%).
When all metals show increase in resistivity with increase in temperature and semiconductors show the reverse, Constantan and Manganin show a completely different property.
Their main feature is their low thermal variation of their resistivity, which is constant over a wide range of temperatures.
Metals, including alloys, have free electrons as charge carriers. Their movement controls by defects. One of defects of usual metal is oscillations of atoms due to temperature. More temperature - more oscillations - more collisions of electrons with atoms - less mobility - more resistivity. In alloys, like constantan and Manganin, atoms are in disorder. So they have big resistivity. Their additional disorder due to temperature increase is insignificant. That is why they have no temperature dependence on resistivity.