Question 20

   Question: The reciprocal of resistance is:

   Options: 

    A. conductance

    B. reactance

    C. mobility

    D. conductivity

   Correct Answer: A

   Year: NEET Re-2022

   Solution: (No specific detailed solution provided in the source; answer stated as A)

   Step Solution:

    1.  Define resistance ($R$) as the opposition offered to the flow of electric current.

    2.  Recall the physical definition of the reciprocal of resistance.

    3.  Mathematically, this reciprocal is expressed as $1/R$.

    4.  The term for $1/R$ is defined in physics as conductance ($G$).

    5.  Therefore, the reciprocal of resistance is conductance.

   Difficulty Level: Easy

   The Concept Name: Electrical Conductance

   Short cut solution: Direct conceptual definition: Resistance opposes flow; Conductance is its inverse.

Question 51

   Question: Across a metallic conductor of non-uniform cross section a constant potential difference is applied. The quantity which remains constant along the conductor is

   Options: 

    A. drift velocity

    B. electric field

    C. current density

    D. current

   Correct Answer: D

   Year: 2015

   Solution: The area of cross section of conductor is non uniform so current density will be different but the flow of electrons will be uniform so current will be constant.

   Step Solution:

    1.  Identify that the conductor has a non-uniform cross-section, meaning area $A$ varies at different points.

    2.  Recall the principle of Conservation of Charge for steady-state flow.

    3.  Understand that the amount of charge entering one end per second must equal the amount leaving the other end per second.

    4.  This rate of flow of charge is defined as the current ($I$).

    5.  Since $I$ is independent of area in a single-path conductor, current remains constant while $J=I/A$, $v_d=I/neA$, and $E$ change.

   Difficulty Level: Medium

   The Concept Name: Steady Current and Conservation of Charge

   Short cut solution: In a series circuit or single conductor, current ($I$) always remains constant due to the continuity equation.

Question 116

   Question: The resistance of a discharge tube is

   Options: 

    A. non-ohmic

    B. ohmic

    C. zero

    D. both (b) and (c)

   Correct Answer: A

   Year: 1999

   Solution: (No specific detailed solution provided in the source; answer stated as A)

   Step Solution:

    1.  Define an Ohmic conductor as one where the ratio of voltage to current ($V/I$) is constant.

    2.  Identify a discharge tube as a device where current flows through an ionized gas.

    3.  Recognize that the ionization process in gases does not result in a linear relationship between $V$ and $I$.

    4.  Because the resistance changes depending on the applied voltage and state of ionization, it does not follow Ohm's Law.

    5.  Therefore, its resistance is classified as non-ohmic.

   Difficulty Level: Easy

   The Concept Name: Ohmic vs. Non-Ohmic Conductors

   Short cut solution: Devices like discharge tubes, vacuum tubes, and semiconductors are inherently non-ohmic as they don't have a linear $V-I$ graph.

Question 122

   Question: From the graph between current (I) and voltage (V) shown, identify the portion corresponding to negative resistance.

   Options: 

    A. CD

    B. DE

    C. AB

    D. BC

   Correct Answer: A

   Year: 1997

   Solution: For the negative resistance, when we increase the voltage, the current will decrease. Therefore from the graph, we find that the current in CD is decreased when voltage is increased.

   Step Solution:

    1.  Recall Ohm’s Law where resistance $R = \frac{V}{I}$.

    2.  For a graph of $I$ vs $V$, the slope represents conductance ($\frac{1}{R}$).

    3.  Negative resistance occurs in a region where an increase in voltage ($V$) leads to a decrease in current ($I$).

    4.  Mathematically, this means the slope $\frac{dI}{dV}$ is negative.

    5.  Observing the provided segments, the current value drops as voltage increases only in the CD portion.

   Difficulty Level: Easy

   The Concept Name: Negative Differential Resistance

   Short cut solution: Look for the downward slope on the $I-V$ graph; segment CD is the only one where current decreases as voltage increases.

Question 124

   Question: One kilowatt hour is equal to.

   Options: 

    A. $36 \times 10^{-5}\text{ J}$

    B. $36 \times 10^{-4}\text{ J}$

    C. $36 \times 10^5\text{ J}$

    D. $36 \times 10^3\text{ J}$

   Correct Answer: C

   Year: 1997

   Solution: $1\text{ kWh} = 1000\text{ Wh} = (1000\text{ W}) \times (3600\text{ s}) = 36 \times 10^5\text{ J}$.

   Step Solution:

    1.  Convert the power unit from kilowatts to watts: $1\text{ kW} = 10^3\text{ W}$.

    2.  Convert the time unit from hours to seconds: $1\text{ hour} = 60 \times 60 = 3600\text{ s}$.

    3.  Use the formula for energy: $\text{Energy} = \text{Power} \times \text{Time}$.

    4.  Calculate the total Joules: $1000\text{ W} \times 3600\text{ s} = 3,600,000\text{ J}$.

    5.  Express the result in scientific notation as $36 \times 10^5\text{ J}$.

   Difficulty Level: Easy

   The Concept Name: Conversion of Electrical Energy Units

   Short cut solution: Remember that $1\text{ unit} (1\text{ kWh}) = 3.6 \times 10^6\text{ J}$, which is the same as $36 \times 10^5\text{ J}$.

Question 129

   Question: In good conductors of electricity, the type of bonding that exists is.

   Options: 

    A. metallic

    B. vander Waals

    C. ionic

    D. covalent

   Correct Answer: A

   Year: 1995

   Solution: (No detailed solution provided in the source; answer stated as A).

   Step Solution:

    1.  Identify the physical characteristics of good conductors (e.g., Copper, Silver, Gold).

    2.  Recall that good conductors have a high density of free electrons available for charge transport.

    3.  Analyze the chemical bonding where valence electrons are not bound to individual atoms but form a "sea of electrons."

    4.  Identify this specific type of electrostatic attraction as metallic bonding.

    5.  Conclude that metallic bonding is responsible for the high conductivity in these materials.

   Difficulty Level: Easy

   The Concept Name: Metallic Bonding in Conductors

   Short cut solution: Good electrical conductors are typically metals, which are held together by metallic bonds.

Question 134

   Question: A flow of $10^{7}$ electrons per second in a conducting wire constitutes a current of

   Options: 

    A. $1.6 \times 10^{-12}$ A

    B. $1.6 \times 10^{26}$ A

    C. $1.6 \times 10^{-26}$ A

    D. $1.6 \times 10^{12}$ A

   Correct Answer: A

   Year: 1994

   Solution: Flow of electrons, $\frac{n}{t} = 10^{7} / \text{sec}$. Therefore, current $(I) = \frac{q}{t} = \frac{ne}{t} = \frac{n}{t} \times e = 10^{7} \times (1.6 \times 10^{-19}) = 1.6 \times 10^{-12}$ A.

   Step Solution:

    1.  Identify the rate of flow of electrons ($n/t$) as $10^{7} \text{ s}^{-1}$ and the elementary charge ($e$) as $1.6 \times 10^{-19}$ C.

    2.  Use the fundamental definition of current: $I = \frac{q}{t}$.

    3.  Apply the quantization of charge: $q = ne$, leading to $I = (\frac{n}{t}) \times e$.

    4.  Substitute the values: $I = 10^{7} \times (1.6 \times 10^{-19})$.

    5.  Perform the calculation: $1.6 \times 10^{7-19} = 1.6 \times 10^{-12}$ A.

   Difficulty Level: Easy

   The Concept Name: Quantization of Charge and Definition of Electric Current

   Short cut solution: Directly multiply the number of electrons per second by the charge of a single electron ($1.6 \times 10^{-19}$ C).

Question 135

   Question: Identify the set in which all the three materials are good conductors of electricity

   Options: 

    A. Cu, Hg and NaCl

    B. Cu, Ge and Hg

    C. Cu, Ag and Au

    D. Cu, Si and diamond

   Correct Answer: C

   Year: 1994

   Solution: (Source provides the answer as C without further elaboration).

   Step Solution:

    1.  Evaluate Option A: Cu and Hg are conductors, but NaCl is an insulator in solid form.

    2.  Evaluate Option B: Cu and Hg are conductors, but Ge is a semiconductor.

    3.  Evaluate Option C: Copper (Cu), Silver (Ag), and Gold (Au) are all highly conductive metals.

    4.  Evaluate Option D: Cu is a conductor, but Si is a semiconductor and diamond is an insulator.

    5.  Conclude that Set C is the only set where every material is a good conductor.

   Difficulty Level: Easy

   The Concept Name: Classification of Materials based on Conductivity

   Short cut solution: Identify the set that contains only known transition metals (Copper, Silver, Gold).