Conductivity Archives

1. What is the correct sequence of conductivity when arranged in ascendin

What is the correct sequence of conductivity when arranged in ascending order?

Metals, semiconductors, insulators

Insulators, metals, semi-conductors

Semiconductors, metals, insulators

Insulators, semiconductors, metals

This question was previously asked in

UPSC CAPF – 2013

Download PDF Attempt Online

D) Insulators, semiconductors, metals

Electrical conductivity is a measure of a material’s ability to conduct electric current. Materials are classified based on their conductivity:
– Insulators have very low conductivity, resisting the flow of charge (e.g., rubber, plastic, glass).
– Semiconductors have conductivity between that of insulators and conductors, which can be significantly altered by temperature, doping, or external fields (e.g., silicon, germanium).
– Metals (conductors) have high conductivity, allowing charge to flow easily (e.g., copper, silver, gold).
Arranging these in ascending order of conductivity means starting from the lowest conductivity to the highest. Therefore, the correct sequence is Insulators < Semiconductors < Metals.

The conductivity of materials is related to the availability of free charge carriers (electrons or ions) within their structure and how easily these carriers can move. In metals, there is a large number of free electrons. In semiconductors, the number of free carriers is much smaller but can be increased. Insulators have very few free charge carriers.

2. Vehicles carrying inflammable materials usually have metallic ropes to

Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion in order to

control the speed of the vehicle

conduct the charge produced by friction

conduct the current produced by inflammable material

provide earthing for lightning

This question was previously asked in

UPSC CAPF – 2011

Download PDF Attempt Online

The correct option is B) conduct the charge produced by friction.

Vehicles carrying inflammable materials can build up static electric charge due to the friction between the vehicle’s tyres and the road, the movement of the liquid/gas within the tank, or friction with air. This static charge can accumulate on the vehicle body. The metallic rope touching the ground provides a path for this charge to be safely conducted away into the earth, preventing the build-up of static electricity and eliminating the risk of a spark that could ignite the inflammable material.

Static electricity is a significant hazard when transporting inflammable substances. Dissipating the charge through earthing (grounding) prevents electrostatic discharge sparks, which could cause a fire or explosion. The metallic rope serves as a conductor connected to the vehicle chassis, ensuring continuous contact with the ground during motion.

3. Which of the following statements are correct about the electrical res

Which of the following statements are correct about the electrical resistance and resistivity of a wire?

1. Both quantities depend on the area of cross-section of the wire
2. Both depend on the temperature
3. Resistance of the wire is directly proportional to the resistivity of the wire
4. Resistivity of the wire is directly proportional to the length of the wire

Select the correct answer using the code given below :

1 and 2

1 and 3

2 and 3

2 and 4

This question was previously asked in

UPSC NDA-2 – 2023

Download PDF Attempt Online

C) 2 and 3 are the correct statements.

– The resistance (R) of a wire is given by the formula R = ρ * (L/A), where ρ is the resistivity of the material, L is the length of the wire, and A is the area of its cross-section.
– Resistivity (ρ) is an intrinsic property of the material itself, depending on the material type and temperature, but not on the dimensions (length or area) of the specific wire.

– Statement 1: Resistance depends on the area of cross-section (inversely proportional, R ∝ 1/A), but resistivity does NOT depend on the area of cross-section. So, statement 1 is incorrect.
– Statement 2: Both resistance and resistivity of most metallic conductors are dependent on temperature. For metals, both generally increase with increasing temperature. So, statement 2 is correct.
– Statement 3: From the formula R = ρ * (L/A), Resistance (R) is directly proportional to resistivity (ρ) for a wire of fixed length and area. So, statement 3 is correct.
– Statement 4: Resistivity (ρ) is a material property and does not depend on the length of the wire. So, statement 4 is incorrect.
– Thus, only statements 2 and 3 are correct.

4. Which one of the following graphs correctly represents the current (I)

Which one of the following graphs correctly represents the current (I) – voltage (V) variation for a rectangular piece of a semiconductor wafer ?

Graph (a)

Graph (b)

Graph (c)

Graph (d)

This question was previously asked in

UPSC NDA-2 – 2023

Download PDF Attempt Online

Graph (a) correctly represents the current (I) – voltage (V) variation for a rectangular piece of a semiconductor wafer under conditions where it behaves ohmically.

While semiconductors are known for non-linear behavior in devices like diodes and transistors, a simple rectangular piece of bulk semiconductor material with ohmic contacts behaves like a resistor. According to Ohm’s Law, the current (I) flowing through a resistor is directly proportional to the applied voltage (V) (V = IR or I = V/R), provided the temperature is constant. This relationship is represented by a straight line passing through the origin on an I-V graph. Graph (a) shows this linear relationship, indicating constant resistance.

Graphs (b), (c), and (d) represent non-linear current-voltage characteristics typical of semiconductor devices (like diodes or transistors) or bulk materials under specific conditions (e.g., high electric fields causing velocity saturation). However, for a basic piece of material operating within its ohmic region, the linear relationship is the expected behavior.

5. The majority charge carriers in a p-type semiconductor are

The majority charge carriers in a p-type semiconductor are

free electrons

conduction electrons

ions

holes

This question was previously asked in

UPSC NDA-2 – 2017

Download PDF Attempt Online

A p-type semiconductor is created by doping an intrinsic semiconductor (like silicon or germanium) with trivalent impurity atoms (like boron, gallium, or indium). Trivalent atoms have three valence electrons. When a trivalent atom substitutes a semiconductor atom (which has four valence electrons), there is a deficiency of one electron to form a complete covalent bond with the surrounding semiconductor atoms. This deficiency is called a “hole.” These holes can accept electrons from neighboring bonds and effectively move through the crystal lattice, acting as positive charge carriers. In a p-type semiconductor, the number of holes is much greater than the number of free electrons (which are present due to thermal generation), making holes the majority charge carriers.

– P-type semiconductors are created by doping with trivalent impurities.
– Trivalent impurities create electron deficiencies called holes.
– Holes act as positive charge carriers.
– In p-type semiconductors, holes are the majority charge carriers.

In contrast, n-type semiconductors are created by doping with pentavalent impurity atoms (like phosphorus, arsenic, or antimony). Pentavalent atoms have five valence electrons, one more than needed for covalent bonding with four neighbors. This extra electron is loosely bound and easily becomes a free electron, which acts as a negative charge carrier. In n-type semiconductors, free electrons are the majority charge carriers. Ions (the doping atoms fixed in the lattice) are not mobile charge carriers.

6. Which one of the following is primarily responsible for conduction of

Which one of the following is primarily responsible for conduction of current in a metal ?

Bound electrons

Free electrons

Both bound and free electrons

Ions

This question was previously asked in

UPSC NDA-1 – 2024

Download PDF Attempt Online

In metals, the atoms are arranged in a lattice structure. The outermost electrons of metal atoms are not bound to specific atoms but are delocalized and form a “sea” of electrons that can move freely throughout the material. These are called free electrons or conduction electrons. When a voltage is applied across a metal conductor, these free electrons drift in a directed manner, constituting an electric current. Bound electrons are tightly held within the atoms and do not contribute to conduction. Ions (the positively charged atomic remnants in the lattice) are essentially fixed in position and do not move to carry current in solid metals.

– Electrical current in conductors is the flow of charge carriers.
– In metals, the charge carriers are free electrons.
– Free electrons are valence electrons that are not bound to individual atoms and can move throughout the material.

In other materials, different charge carriers are responsible for conduction. For example, in electrolytes, both positive and negative ions carry current. In semiconductors, both electrons and holes (absence of an electron) contribute to conduction. However, in typical solid metals, the primary carriers are free electrons.

7. Why are the tyres of aircrafts made of conducting rubber? 1. So that

Why are the tyres of aircrafts made of conducting rubber?

1. So that the charge accumulated on the aircraft in flight, by rubbing the air, can easily be transferred to ground on landing.
2. So that the charge accumulated due to the operation of various electronic equipments in the aircraft in flight can easily be transferred to ground on landing.

Select the correct answer using the code given below.

1 only

2 only

Both 1 and 2

Neither 1 nor 2

This question was previously asked in

UPSC NDA-1 – 2022

Download PDF Attempt Online

The correct option is C.

Aircraft accumulate static electric charge during flight due to friction with air (triboelectric effect) and the operation of onboard electronic equipment.

Statement 1 is correct: Friction with air (rubbing) during flight causes the aircraft to accumulate static charge, a phenomenon known as triboelectric charging. This is a significant source of charge buildup, especially in dry air.
Statement 2 is correct: The operation of various electronic systems within the aircraft can also contribute to the accumulation of static charge on the aircraft’s structure.
Conducting rubber tyres provide a path for this accumulated static charge to safely discharge to the ground upon landing, preventing the build-up of a large potential difference between the aircraft and the ground, which could otherwise lead to a spark. Such a spark could pose a fire hazard, especially during refueling, or cause damage to sensitive electronic components. Therefore, both reasons contribute to the necessity of conducting tyres.

8. Which one of the following correctly represents the SI unit of

Which one of the following correctly represents the SI unit of resistivity?

$Omega$

$Omega$/m

$Omega$ cm

$Omega$ m

This question was previously asked in

UPSC NDA-1 – 2022

Download PDF Attempt Online

The correct SI unit of resistivity is Ohm meter ($\Omega$ m).

Resistivity ($\rho$) is an intrinsic property of a material that quantifies how strongly it resists electric current. It is related to resistance ($R$), length ($L$), and cross-sectional area ($A$) of a conductor by the formula $R = \rho \frac{L}{A}$.

Rearranging the formula, $\rho = \frac{R \times A}{L}$. Substituting the SI units: $R$ is in Ohms ($\Omega$), $A$ is in square meters (m²), and $L$ is in meters (m). Therefore, the unit of resistivity is $\frac{\Omega \times \text{m}^2}{\text{m}} = \Omega \text{ m}$.

9. Which one of the following statements is correct with regard to the ma

Which one of the following statements is correct with regard to the material of electrical insulators ?

They contain no electrons

Electrons do not flow easily through them

They are crystals

They have more number of electrons than the protons on their surface

This question was previously asked in

UPSC NDA-1 – 2017

Download PDF Attempt Online

The correct statement is that electrons do not flow easily through electrical insulators.

Electrical insulators are materials characterized by their high resistance to the flow of electric current. This property arises because the electrons in these materials are tightly bound to the atoms and are not free to move readily under the influence of an electric field.

Option A is incorrect; all matter contains electrons. Option C is incorrect; insulators can be crystalline (like quartz), amorphous (like glass), or polymeric (like rubber, plastic). Option D is incorrect; insulators are generally electrically neutral, meaning the total number of electrons is equal to the total number of protons. The key characteristic of insulators is the low mobility of charge carriers, specifically electrons in most common insulators.

10. The product of conductivity and resistivity of a conductor

The product of conductivity and resistivity of a conductor

depends on pressure applied

depends on current flowing through conductor

is the same for all conductors

varies from conductor to conductor

This question was previously asked in

UPSC NDA-1 – 2015

Download PDF Attempt Online

Resistivity ($\rho$) and conductivity ($\sigma$) are inverse properties of a material concerning its ability to conduct electricity.

By definition, conductivity is the reciprocal of resistivity ($\sigma = 1/\rho$). Therefore, the product of conductivity and resistivity is always equal to 1 ($\sigma \times \rho = (1/\rho) \times \rho = 1$). This relationship holds true for all materials, regardless of whether they are conductors, insulators, or semiconductors, and it is independent of the physical conditions like pressure or the current flowing through the conductor (assuming the material obeys Ohm’s Law).

Resistivity is a fundamental property of the material itself, like density or melting point. Conductivity is simply another way of expressing this property. Their product being 1 is a mathematical consequence of their definition relative to each other.