Test: Conductors & Dielectrics - Electrical Engineering (EE) MCQ

Answer: c
Explanation: Normally, metals are said to be good conductors. Here mercury is the only metal (which is in liquid form). The other options are insulators.

Answer: b
Explanation: Conductors are materials with least band gap energy. The smallest range in this group is 0.2-0.4 eV.

Answer: a
Explanation: Conduction in metals is only due to majority carriers, which are electrons. Electrons and holes are responsible for conduction in a semiconductor.

Answer: c
Explanation: The instantaneous current flowing in a transmission line, when measured using an ammeter, will give RMS current value. This value is 70.7% of the peak value. This is because, due to oscillations in AC, it is not possible to measure peak value. Hence to normalise, we consider current at any time in a line will be the RMS current.

Answer: a
Explanation: The band gap energy in electron volt when wavelength is given is, Eg = 1.24(μm)/λ = 1.24 x 10^-6/1240 x 10^-9 = 1eV.

Answer: a
Explanation: At any frequency, the current through the metal will be due to conduction current. Only at high frequencies and when medium is air, the conduction is due to displacement current. Thus in general the current in metal is due to conduction current, which depends on the mobility of the carriers.

Answer: d
Explanation: In conductors, the free electrons exist in the conduction band. Since the band gap energy is very low, less energy is required to transport the free electrons to the conduction band, as they are readily available to conduct.

Answer: b
Explanation: A beam of electrons in a vacuum tube is called convection current. It occurs when current flows through an insulating medium like liquid, vacuum etc.

Answer: c
Explanation: The conduction current density is given by, J = σE
J = 500 X 2 = 1000 units.

Answer: b
Explanation: The convection current density is given by, J = ρeV
J = 200 X 12= 2400 units.

Answer: a
Explanation: For a material to be dielectric, its permittivity should be very high. This is seen in insulators. For a material to be insulator, the condition is to have large band gap energy. However, this is not necessary for a dielectric.

Answer: b
Explanation: Iron and magnesium are metals. Hence they need not be considered. Both ceramics and plastic are insulators. But dielectric constant is more for ceramics always. Hence ceramics is the best dielectric.

Answer: b
Explanation: On increasing the temperature, the free electrons in an insulator can be promoted from valence to conduction band. Gradually, it can act as a conductor through heating process. This condition is called dielectric breakdown, wherein the insulator loses its dielectric property and starts to conduct.

Answer: b
Explanation: The electric susceptibility is given by χe = εr – 1. For a susceptibility of 4, the dielectric constant will be 5. It has no unit.

Answer: b
Explanation: Superconductors are characterised by diamagnetism behaviour and zero resistivity, which true for a dielectric. They occur only at low temperature. Thus a dielectric can become a superconductor at low temperatures with very high dielectric breakdown voltage.

Answer: d
Explanation: Critical field is that strong magnetic field which can destroy the superconductivity of a material. The temperature at which this occurs is called transition temperature.

Answer: b
Explanation: Due to perfect diamagnetism in a superconductor, its magnetic susceptibility will be negative. This phenomenon is called Meissner effect.

Answer: b
Explanation: Superconducting materials depends only on the applied magnetic field, resultant magnetization at the temperature considered. It is independent of the applied electric field and the corresponding polarization.

Answer: b
Explanation: The mean free path is defined as the average distance travelled by an electron before collision takes place. It is given by, d = v x τc, where v is the velocity and τc is the collision time. Thus d = 18 x 2 = 36m.

Answer: c
Explanation: When the kinetic energy and one electron volt are equal, we can equate mv²/2 = eV. Put e and m in the equation to get velocity v = 5.9 x 10⁵ m/s.