NEET Physics Electric Current Drift of Electrons Ohms Law Resistance and Resistivity MCQs

Question. When no current is passed through a conductor
(a) the average of the velocities of all the free electrons at an instant is non zero
(b) the average velocity of a free electron over a large period of time is zero
(c) the average speed of a free electron over a large period of time is not zero
(d) the free electrons do not move

Answer: A

Question: A current passes through a wire of nonuniform crosssection. Which of the following quantities are independent of the cross-section?
(a) Free-electron density
(b) Drift velocity
(c) The charge crossing
(d) Current density

Answer: A

Question: In the equation AB = C, A is the current density, C is the electric field, Then B is
(a) resistivity
(b) conductivity
(c) potential difference
(d) resistance

Answer: A

Question: For which of the following dependence of drift velocity vd on electric field E, is Ohm’s law obeyed?
(a) v_d = E
(b) v_d = E^1/2
(c) v_d = constant
(d) None of these

Answer: A

Question: Constantan wire is used for making standard resistance, because it has
(a) negligible temperature coefficient of resistance
(b) high specific resistance
(c) low specific resistance
(d) high melting point

Answer: A

Question: A strip of copper and another of germanium are cooled from room temperature to 80 K. The resistance of
(a) copper strip decreases and that of germanium increases
(b) copper strip increases and that of germanium decreases
(c) each of these decreases
(d) each of these increases

Answer: A

Question: The resistance of a metal increases with increasing temperature because
(a) the collisions of the conducting electrons with the electrons increase
(b) the collisions of the conducting electrons with the lattice consisting of the ions of the metal increase
(c) the number of conduction electrons decreases
(d) the number of conduction electrons increases

Answer: A

Question: Consider the following statements and select the correct statement(s).
I. Current is the time rate of flow of charge through anycross-section
II. For a given conductor current does not change withchange in cross-sectional area
III. The net charge in a current carrying conductor is infinite
(a) I and II
(b) I and III
(c) II and III
(d) I, II and III

Answer: A

Question: The relaxation time in conductors
(a) decreases with the increases of temperature
(b) increases with the increases of temperature
(c) it does not depends on temperature
(d) all of sudden changes at 400 K

Answer: A

Question: A metal wire is subjected to a constant potential difference. When the temperature of the metal wire increases, the drift velocity of the electron in it
(a) decreases, thermal velocity of the electron increases
(b) increases, thermal velocity of the electron increases
(c) increases, thermal velocity of the electron decreases
(d) decreases, thermal velocity of the electron decreases

Answer: A

Question: A current passes through a resistor. If K₁ and K₂ represent the average kinetic energy of the conduction electrons and the metal ions respectively then
(a) K₁ > K₂
(b) K₁ = K₂
(c) K₁ < K₂
(d) Any of these three may occur

Answer: A

Question: The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then
(a) the resistance will be halved and the specific resistance will remain unchanged
(b) the resistance will be halved and the specific resistance will be doubled
(c) the resistance will be doubled and the specific resistance will be halved
(d) the resistance and the specific resistance, will both remain unchanged

Answer: A

Question: We are able to obtain fairly large currents in a conductor because
(a) the number density of free electrons is very high and this can compensate for the low values of the electron drift speed and the very small magnitude of the electron charge
(b) the electron drift speed is usually very large
(c) the number density of free electrons as well as the electron drift speeds are very large and these compensate for the very small magnitude of the electron charge
(d) the very small magnitude of the electron charge has to be divided by the still smaller product of the number density and drift speed to get the electric current

Answer: A

Question: Two wires A and B of the same material, having radii in the ratio 1 : 2 and carry currents in the ratio 4 : 1. The ratio of drift speed of electrons in A and B is
(a) 16 : 1
(b) 1 : 16
(c) 1 : 4
(d) 4 : 1

Answer: A

Question: When a current I is set up in a wire of radius r, the drift velocity is v_d. If the same current is set up through a wire of radius 2 r, the drift velocity will be
(a) v_d/4
(b) v_d/2
(c) 2v_d
(d) 4v_d

Answer: A

Question. When no current is passed through a conductor
(a) the average of the velocities of all the free electrons at an instant is non zero
(b) the average velocity of a free electron over a large period of time is zero
(c) the average speed of a free electron over a large period of time is not zero
(d) the free electrons do not move

Answer: A

Question. A current passes through a wire of nonuniform crosssection. Which of the following quantities are independent of the cross-section?
(a) Free-electron density
(b) Drift velocity
(c) The charge crossing
(d) Current density

Answer: A

Question. In the equation AB = C, A is the current density, C is the electric field, Then B is
(a) resistivity
(b) conductivity
(c) potential difference
(d) resistance

Answer: A

Question. For which of the following dependence of drift velocity vd on electric field E, is Ohm’s law obeyed?
(a) v_d = E
(b) v_d = E^1/2
(c) v_d = constant
(d) None of these

Answer: A

Question. Constantan wire is used for making standard resistance, because it has
(a) negligible temperature coefficient of resistance
(b) high specific resistance
(c) low specific resistance
(d) high melting point

Answer: A

Question. A strip of copper and another of germanium are cooled from room temperature to 80 K. The resistance of
(a) copper strip decreases and that of germanium increases
(b) copper strip increases and that of germanium decreases
(c) each of these decreases
(d) each of these increases

Answer: A

Question. The resistance of a metal increases with increasing temperature because
(a) the collisions of the conducting electrons with the electrons increase
(b) the collisions of the conducting electrons with the lattice consisting of the ions of the metal increase
(c) the number of conduction electrons decreases
(d) the number of conduction electrons increases

Answer: A

Question. Consider the following statements and select the correct statement(s).
I. Current is the time rate of flow of charge through anycross-section
II. For a given conductor current does not change withchange in cross-sectional area
III. The net charge in a current carrying conductor is infinite
(a) I and II
(b) I and III
(c) II and III
(d) I, II and III

Answer: A

Question. The relaxation time in conductors
(a) decreases with the increases of temperature
(b) increases with the increases of temperature
(c) it does not depends on temperature
(d) all of sudden changes at 400 K

Answer: A

Question. A metal wire is subjected to a constant potential difference. When the temperature of the metal wire increases, the drift velocity of the electron in it
(a) decreases, thermal velocity of the electron increases
(b) increases, thermal velocity of the electron increases
(c) increases, thermal velocity of the electron decreases
(d) decreases, thermal velocity of the electron decreases

Answer: A

Question. A current passes through a resistor. If K₁ and K₂ represent the average kinetic energy of the conduction electrons and the metal ions respectively then
(a) K₁ > K₂
(b) K₁ = K₂
(c) K₁ < K₂
(d) Any of these three may occur

Answer: B

Question. The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then
(a) the resistance will be halved and the specific resistance will remain unchanged
(b) the resistance will be halved and the specific resistance will be doubled
(c) the resistance will be doubled and the specific resistance will be halved
(d) the resistance and the specific resistance, will both remain unchanged

Answer: A

Question. We are able to obtain fairly large currents in a conductor because
(a) the number density of free electrons is very high and this can compensate for the low values of the electron drift speed and the very small magnitude of the electron charge
(b) the electron drift speed is usually very large
(c) the number density of free electrons as well as the electron drift speeds are very large and these compensate for the very small magnitude of the electron charge
(d) the very small magnitude of the electron charge has to be divided by the still smaller product of the number density and drift speed to get the electric current

Answer: A

Question. Two wires A and B of the same material, having radii in the ratio 1 : 2 and carry currents in the ratio 4 : 1. The ratio of drift speed of electrons in A and B is
(a) 16 : 1
(b) 1 : 16
(c) 1 : 4
(d) 4 : 1

Answer: A

Question. When a current I is set up in a wire of radius r, the drift velocity is v_d. If the same current is set up through a wire of radius 2 r, the drift velocity will be
(a) v_d/4
(b) v_d/2
(c) 2v_d
(d) 4v_d

Answer: A

Question. A straight conductor of uniform cross-section carries a current I. If s is the specific charge of an electron, the momentum of all the free electrons per unit length of the conductor, due to their drift velocity only is
(a) I/s
(b) I s
(c) (I / s)²
(d) None of these

Answer: A

Question: A wire X is half the diameter and half the length of a wire Y of similar material. The ratio of resistance of X to that of Y is
(a) 2 : 1
(b) 1 : 1
(c) 4 : 1
(d) 8 : 1

Answer: A

Question: The amount of charge Q passed in time t through a crosssection of a wire is Q = 5 t² + 3 t + 1. The value of current at time t = 5 s is
(a) 53 A
(b) 9 A
(c) 49 A
(d) None of these

Answer: A

Question: The resistance of a wire is R. It is bent at the middle by 180° and both the ends are twisted together to make a shorter wire. The resistance of the new wire is
(a) R/4
(b) R/8
(c) R/2
(d) 2 R

Answer: A

Question: At what temperature will the resistance of a copper wire becomes three times its value at 0°C? (Temperature coefficient of resistance of copper is 4 × 10^–3/°C)
(a) 500°C
(b) 450°C
(c) 400°C
(d) 550°C

Answer: A

Question: A wire of a certain material is stretched slowly by ten per cent. Its new resistance and specific resistance become respectively:
(a) 1.21 times, same
(b) 1.2 times, 1.3 times
(c) 1.1 times, 1.1 times
(d) both remain the same

Answer: A

Question: The length of a given cylindrical wire is increased by 100%. Due to the consequent decrease in diameter the change in the resistance of the wire will be
(a) 300%
(b) 50%
(c) 100%
(d) 200%

Answer: A

Question: The masses of the three wires of copper are in the ratio of 1 : 3 : 5 and their lengths are in the ratio of 5 : 3 : 1. The ratio of their electrical resistance is
(a) 125 : 45 : 3
(b) 1 : 25 : 125
(c) 5 : 3 : 1
(d) 1 : 3 : 5

Answer: A

Question: The number of free electrons per 100 mm of ordinary copper wire is 2 × 10²¹. Average drift speed of electrons is 0.25 mm/s. The current flowing is
(a) 0.8 A
(b) 8 A
(c) 80 A
(d) 5 A

Answer: A

Question: An electron beam has an aperture 1.0 mm². A total of 6×10¹⁶ electrons go through any perpendicular cross section per second. Find the current density in the beam. (in A/m²)
(a) 9.6 × 10³
(b) 8.6 × 10¹¹
(c) 6.6 × 10⁵
(d) 9.1 × 10¹³

Answer: A

Question. A straight conductor of uniform cross-section carries a current I. If s is the specific charge of an electron, the momentum of all the free electrons per unit length of the conductor, due to their drift velocity only is
(a) I/s
(b) I s
(c) (I / s)²
(d) None of these

Answer: A

Question. A wire X is half the diameter and half the length of a wire Y of similar material. The ratio of resistance of X to that of Y is
(a) 2 : 1
(b) 1 : 1
(c) 4 : 1
(d) 8 : 1

Answer: A

Question. The amount of charge Q passed in time t through a crosssection of a wire is Q = 5 t² + 3 t + 1. The value of current at time t = 5 s is
(a) 53 A
(b) 9 A
(c) 49 A
(d) None of these

Answer: A

Question. The resistance of a wire is R. It is bent at the middle by 180° and both the ends are twisted together to make a shorter wire. The resistance of the new wire is
(a) R/4
(b) R/8
(c) R/2
(d) 2 R

Answer: A

Question. At what temperature will the resistance of a copper wire becomes three times its value at 0°C? (Temperature coefficient of resistance of copper is 4 × 10^–3/°C)
(a) 500°C
(b) 450°C
(c) 400°C
(d) 550°C

Answer: A

Question. A wire of a certain material is stretched slowly by ten per cent. Its new resistance and specific resistance become respectively:
(a) 1.21 times, same
(b) 1.2 times, 1.3 times
(c) 1.1 times, 1.1 times
(d) both remain the same

Answer: A

Question. The length of a given cylindrical wire is increased by 100%. Due to the consequent decrease in diameter the change in the resistance of the wire will be
(a) 300%
(b) 50%
(c) 100%
(d) 200%

Answer: A

Question. The masses of the three wires of copper are in the ratio of 1 : 3 : 5 and their lengths are in the ratio of 5 : 3 : 1. The ratio of their electrical resistance is
(a) 125 : 45 : 3
(b) 1 : 25 : 125
(c) 5 : 3 : 1
(d) 1 : 3 : 5

Answer: A

Question. The number of free electrons per 100 mm of ordinary copper wire is 2 × 10²¹. Average drift speed of electrons is 0.25 mm/s. The current flowing is
(a) 0.8 A
(b) 8 A
(c) 80 A
(d) 5 A

Answer: A

Question. An electron beam has an aperture 1.0 mm². A total of 6×10¹⁶ electrons go through any perpendicular cross section per second. Find the current density in the beam. (in A/m²)
(a) 9.6 × 10³
(b) 8.6 × 10¹¹
(c) 6.6 × 10⁵
(d) 9.1 × 10¹³

Answer: A