Physics MCQs for Class 12 with Answers Chapter 3 Current Electricity

Q.1. In a current carrying conductor the net charge is
(a) 1.6 x 10–19 coulomb
(b) 6.25 x 10–18 coulomb
(c) zero
(d) infinite

Answer Answer:
(c)
Explanation:
In a current carrying conductor, the net charge is zero.

Q.2. The current which is assumed to be flowing in a circuit from positive terminal to negative, is called
(a) direct current
(b) pulsating current
(c) conventional current
(d) alternating current

Answer Answer:
(c)

Q.3. When no current is passed through a conductor,
(a) the free electrons do not move
(b) the average speed of a free electron over a large period of time is not zero
(c) the average velocity of a free electron over a large period of time is zero
(d) the average of the velocities of all the free electrons at an instant is non zero

Answer Answer:
(d)

Q.4. A current passes through a wire of nonuniform cross-section. Which of the following quantities are independent of the cross-section?
(a) The charge crossing
(b) Drift velocity
(c) Current density
(d) Free-electron density

Answer Answer:
(d)

Q.5. 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 Answer:
(a)
Explanation:
J =σE ⇒ Jρ=E
J is current density, E is electric field
so B = ρ = resistivity.

Q.6. Drift velocity of electrons is due to
(a) motion of conduction electrons due to random collisions.
(b) motion of conduction electrons due to electric field E
(c) repulsion to the conduction electrons due to inner electrons of ions.
(d) collision of conduction electrons with each other.

Answer Answer:
(b)
Explanation:
Motion of conduction electrons due to random collisions has no preffered direction and average to zero. Drift velocity is caused due to motion of conduction electrons due to applied electric field E.

Q.7. The speed at which the current travels, in conductor, is nearly equal to
(a) 3 × 104 m/s
(b) 3 × 105 m/s
(c) 4 × 106 m/s
(d) 3 × 108 m/s

Answer Answer:
(d)

Q.8. In the absence of an electric field, the mean velocity of free electrons in a conductor at absolute temperature (T) is
(a) zero
(b) independent of T
(c) proportional to T
(d) proportional to T2

Answer Answer:
(a)

Q.9. When a potential difference V is applied across a conductor at a temperature T, the drift velocity of electrons is proportional to
(a) V
(b) V
(c) T
(d) T

Answer Answer:
(b)

Q.10. For which of the following dependence of drift velocity vd on electric field E, is Ohm’s law obeyed?
(a) vd α E2
(b) vd = E1/2
(c) vd = constant
(d) vd = E

Answer Answer:
(d)

Q.11. The current density (number of free electrons per m3) in metallic conductor is of the order of
(a) 1022
(b) 1024
(c) 1026
(d) 1028

Answer Answer:
(d)

Q.12. 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 Answer:
(c)

Q.13. 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) increases, thermal velocity of the electron increases
(b) decreases, thermal velocity of the electron increases
(c) increases, thermal velocity of the electron decreases
(d) decreases, thermal velocity of the electron decreases

Answer Answer:
(b)
Explanation:
When the temperature increases, resistance increases. As the e.m.f. applied is the same, the current density decreases the drift velocity decreases. But the rms velocity of the electron due to thermal motion is proportional to √T . The Thermal velocity increases.

Q.14. The electric field intensity E, current density J and specific resistance k are related to each other through the relation
(a) E = J/k
(b) E = J k
(c) E = k/J
(d) k = J E

Answer Answer:
(b)

Q.15. The relaxation time in conductors
(a) increases with the increases of temperature
(b) decreases with the increases of temperature
(c) it does not depends on temperature
(d) all of sudden changes at 400 K

Answer Answer:
(b)
Explanation:
Because as temperature increases, the resistivity increases and hence the relaxation time decreases for conductors

Q.16. We are able to obtain fairly large currents in a conductor because
(a) the electron drift speed is usually very large
(b) 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
(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 Answer:
(b)

Q.17. In conductor when electrons move between two collisions, their paths are … A… when external fields are absent and … B…when external filed is present. Here, A and B refer to
(a) straight lines, straight lines
(b) straight lines, curved lines
(c) curved lines, straight lines
(d) curved lines, curved lines

Answer Answer:
(b)
Explanation:
In conductor when electrons move between two collisions, their paths are straight lines when external fields are absent and paths are curved in general when external field is present.

Q.18.

Answer Answer:
(a)

Q.19. Two or more resistors are said to be in …A… if one end of all resistors is joined together and similarly the other ends joined together, Here, A refers to
(a) series
(b) parallel
(c) either (a) or (b)
(d) None of these

Answer Answer:
(b)

Q.20. The example of non-ohmic resistance is
(a) diode
(b) copper wire
(c) filament lamp
(d) carbon resistor

Answer Answer:
(a)

Q.21. Constantan wire is used for making standard resistance, because it has
(a) high melting point
(b) low specific resistance
(c) high specific resistance
(d) negligible temperature coefficient of resistance

Answer Answer:
(d)

Q.22. At temperature 0K, the germanium behaves as a / an
(a) conductor
(b) insulator
(c) super-conductor
(d) ferromagnetic

Answer Answer:
(b)

Q.23. Which of the following is used for the formation of thermistor?
(a) Copper oxide
(b) Nickel oxide
(c) Iron oxide
(d) All of the above

Answer Answer:
(d)

Q.24. What is the suitable material for electric fuse?
(a) Cu
(b) Constantan
(c) Tin-lead alloy
(d) Nichrome

Answer Answer:
(c)

Q.25. A strip of copper and another of germanium are cooled from room temperature to 80 K. The resistance of
(a) each of these increases
(b) each of these decreases
(c) copper strip increases and that of germanium decreases
(d) copper strip decreases and that of germanium increases

Answer Answer:
(d)

Q.26. The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then
(a) the resistance and the specific resistance, will both remain unchanged
(b) the resistance will be doubled and the specific resistance will be halved
(c) the resistance will be halved and the specific resistance will remain unchanged
(d) the resistance will be halved and the specific resistance will be doubled

Answer Answer:
(c)

Q.27. Nichrome or Manganin is widely used in wire bound standard resistors because of their
(a) temperature independent resistivity
(b) very weak temperature dependent resistivity.
(c) strong dependence of resistivity with temperature.
(d) mechanical strength.

Answer Answer:
(b) These materials exhibit a very weak dependence of resistivity on temperature. Their resistance values would be changed very little with temperature as shown in figure. Hence these materials are widely used as heating element.

Q.28. With increase in temperature the conductivity of
(a) metals increases and of semiconductor decreases.
(b) semiconductors increases and metals decreases.
(c) in both metals and semiconductors increases.
(d) in both metal and semiconductor decreases.

Answer Answer:
(b) Semiconductors having negative temperature coefficient of resistivity whereas metals are having positive temperature coefficient of resistivity with increase in temperature the resistivity of metal increases where a resistivity of semiconductor decreases.

Q.29. 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 Answer:
(a) decreases.

Q.30. To minimise the power loss in the transmission cables connecting the power stations to homes and factories, the transmission cables carry current
(a) at a very low voltage.
(b) at a very high voltage
(c) at 220 volt
(d) neither at a very high voltage nor at a very low voltage.

Answer Answer:
(b) The power dissipated in the transmission cables is inversely proportional to the square of voltage at which current is transmitted through the cables. Therefore to minimize the power loss the transmission cables carry current at a very high voltage.

Q.31. Appliances based on heating effect of current work on
(a) only a.c.
(b) only d.c.
(c) both a.c. and d.c.
(d) None of these

Answer Answer:
(c)

Q.32. In the series combination of two or more than two resistances
(a) the current through each resistance is same
(b) the voltage through each resistance is same
(c) neither current nor voltage through each resistance is same
(d) both current and voltage through each resistance are same.

Answer Answer:
(a) In series combination, current across its circuit components is always constant and in parallel combination the voltage across the circuit components in constant.

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