Displacement Current, Electromagnetic Waves and Their Characteristics – Displacement current: NEET MCQs with Answers

15 free questions on Displacement Current, Electromagnetic Waves and Their Characteristics – Displacement current for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Displacement current. Electromagnetic waves and their characteristics, Transverse nature of electromagnetic waves

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Question 1

Previous-year question
An em wave is propagating in a medium with a velocity v̄ = vî. The instantaneous oscillating electric field of this em wave is along the +y axis. Then the direction of oscillating magnetic field of the em wave will be along [NEET 2018]
  1. −z direction
  2. +z direction
  3. −x direction
  4. −y direction
Show answer
Answer: (B) +z direction
Explanation: Propagation is along Ē × B̄. With Ē along +ĵ and travel along +î, we need ĵ × B̂ = î, which is satisfied by B̄ along +k̂, i.e. the +z direction.

Question 2

Previous-year question
In an electromagnetic wave in free space the root mean square value of the electric field is E_rms = 6 V/m. The peak value of the magnetic field is [NEET 2017]
  1. 2.83 × 10⁻⁸ T
  2. 0.70 × 10⁻⁸ T
  3. 4.23 × 10⁻⁸ T
  4. 1.41 × 10⁻⁸ T
Show answer
Answer: (A) 2.83 × 10⁻⁸ T
Explanation: B_rms = E_rms/c = 6/(3 × 10⁸) = 2 × 10⁻⁸ T, and the peak value is √2 times this, i.e. 2.83 × 10⁻⁸ T.

Question 3

Previous-year question
Out of the following options, which one can be used to produce a propagating electromagnetic wave? [NEET 2016]
  1. A charge moving at constant velocity
  2. A stationary charge
  3. A chargeless particle
  4. An accelerating charge
Show answer
Answer: (D) An accelerating charge
Explanation: Only an accelerating charge radiates. A stationary charge has a static field, a uniformly moving charge carries its fields along with it, and a chargeless particle produces no electromagnetic field at all.

Question 4

Previous-year question
Light with an energy flux of 25 × 10⁴ W m⁻² falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is [AIPMT 2014]
  1. 1.25 × 10⁻⁶ N
  2. 2.50 × 10⁻⁶ N
  3. 1.20 × 10⁻⁶ N
  4. 3.0 × 10⁻⁶ N
Show answer
Answer: (B) 2.50 × 10⁻⁶ N
Explanation: For a perfect reflector F = 2IA/c = 2(25 × 10⁴)(15 × 10⁻⁴)/(3 × 10⁸) = 750/(3 × 10⁸) = 2.50 × 10⁻⁶ N.

Question 5

Previous-year question
For protecting a sensitive equipment from an external magnetic field, it should be [AIPMT 1998]
  1. placed inside an aluminium can
  2. placed inside an iron can
  3. wrapped with insulation around it when passing current through it
  4. surrounded with fine copper sheet
Show answer
Answer: (B) placed inside an iron can
Explanation: Iron is ferromagnetic and has very high permeability, so magnetic field lines are drawn into the iron walls and are effectively excluded from the enclosed cavity. This is the principle of magnetic shielding. Aluminium and copper, being essentially non-magnetic, provide no such protection against a steady magnetic field.

Question 6

Previous-year question
When a diamagnetic substance is brought near a strong magnet, it is [AIPMT 1999]
  1. attracted by the magnet
  2. repelled by the magnet
  3. repelled by the North pole and attracted by the South pole
  4. attracted by the North pole and repelled by the South pole
Show answer
Answer: (B) repelled by the magnet
Explanation: A diamagnetic substance develops a feeble magnetisation opposite in direction to the applied field, so it is repelled by a strong magnet irrespective of which pole is presented.

Question 7

Previous-year question
In which type of material does the magnetic susceptibility not depend on temperature? [AIPMT 2001]
  1. Diamagnetic
  2. Paramagnetic
  3. Ferromagnetic
  4. Ferrite
Show answer
Answer: (A) Diamagnetic
Explanation: Diamagnetism arises from the field-induced modification of orbital electron motion, a mechanism that does not involve the alignment of permanent dipoles. It is therefore not disturbed by thermal agitation, and the susceptibility of a diamagnetic substance is essentially independent of temperature. Paramagnetic and ferromagnetic susceptibilities both vary with T.
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Question 8

Previous-year question
A diamagnetic material placed in a magnetic field moves [AIPMT 2003]
  1. perpendicular to the field
  2. from weaker to stronger parts of the field
  3. from stronger to weaker parts of the field
  4. none of the above
Show answer
Answer: (C) from stronger to weaker parts of the field
Explanation: A diamagnetic material is magnetised opposite to the applied field, so in a non-uniform field the resultant force drives it towards the region of lower field strength — that is, from the stronger to the weaker parts of the field.

Question 9

Previous-year question
According to Curie's law, the magnetic susceptibility of a paramagnetic substance at an absolute temperature T is proportional to [AIPMT 2003]
  1. 1/T²
  2. T²
  3. 1/T
  4. T
Show answer
Answer: (C) 1/T
Explanation: Curie's law states χ_m = C/T, i.e. the susceptibility of a paramagnetic substance varies inversely with the absolute temperature. Rising temperature increases thermal randomisation of the atomic dipoles, which reduces the net magnetisation.

Question 10

Previous-year question
If the magnetic dipole moment of an atom of a diamagnetic material, a paramagnetic material and a ferromagnetic material are denoted by μ_d, μ_p and μ_f respectively, then [AIPMT 2005]
  1. μ_d ≠ 0 and μ_f ≠ 0
  2. μ_p = 0 and μ_f ≠ 0
  3. μ_d = 0 and μ_p ≠ 0
  4. μ_d ≠ 0 and μ_p = 0
Show answer
Answer: (C) μ_d = 0 and μ_p ≠ 0
Explanation: In a diamagnetic atom the electrons are all paired with oppositely directed spins, so the individual moments cancel completely and μ_d = 0. Paramagnetic atoms have unpaired electrons and hence a permanent moment, μ_p ≠ 0; the same holds for ferromagnetic atoms, μ_f ≠ 0.

Question 11

Previous-year question
Above the Curie temperature, [AIPMT 2006]
  1. a ferromagnetic substance becomes paramagnetic
  2. a paramagnetic substance becomes diamagnetic
  3. a diamagnetic substance becomes paramagnetic
  4. a paramagnetic substance becomes ferromagnetic
Show answer
Answer: (A) a ferromagnetic substance becomes paramagnetic
Explanation: The Curie temperature marks the point at which thermal energy becomes sufficient to disrupt the alignment of magnetic dipoles within the domains of a ferromagnet. The substance thereby loses its strong magnetic behaviour and turns paramagnetic.

Question 12

Previous-year question
Nickel shows ferromagnetic properties at room temperature. If the temperature is increased beyond the Curie temperature, then it will show [AIPMT 2007]
  1. paramagnetism
  2. anti-ferromagnetism
  3. no magnetic property
  4. diamagnetism
Show answer
Answer: (A) paramagnetism
Explanation: Above the Curie temperature the increased thermal agitation destroys the ordered alignment of magnetic domains. The material loses its ferromagnetic character and behaves as a paramagnetic substance, with susceptibility then following the Curie–Weiss law.

Question 13

Previous-year question
If a diamagnetic substance is brought near the North or the South pole of a bar magnet, it is [AIPMT 2009]
  1. repelled by both the poles
  2. repelled by the North pole and attracted by the South pole
  3. attracted by the North pole and repelled by the South pole
  4. attracted by both the poles
Show answer
Answer: (A) repelled by both the poles
Explanation: A diamagnetic substance acquires a weak magnetisation directed opposite to the applied field, whichever pole is brought near. It therefore tends to move from a stronger to a weaker field region and is repelled by both the North and the South pole.

Question 14

Previous-year question
There are four lightweight rod samples A, B, C and D, separately suspended by threads. A bar magnet is slowly brought near each sample and the following observations are noted: [AIPMT 2011] (i) A is feebly repelled (ii) B is feebly attracted (iii) C is strongly attracted (iv) D remains unaffected Which one of the following is true?
  1. C is of a diamagnetic material
  2. D is of a ferromagnetic material
  3. A is of a non-magnetic material
  4. B is of a paramagnetic material
Show answer
Answer: (D) B is of a paramagnetic material
Explanation: Feeble repulsion identifies A as diamagnetic; feeble attraction identifies B as paramagnetic; strong attraction identifies C as ferromagnetic; and no effect identifies D as non-magnetic. Hence only the statement that B is paramagnetic is correct.

Question 15

Previous-year question
The magnetic susceptibility is negative for [NEET 2016]
  1. paramagnetic material only
  2. ferromagnetic material only
  3. paramagnetic and ferromagnetic materials
  4. diamagnetic material only
Show answer
Answer: (D) diamagnetic material only
Explanation: Since μ_r = 1 + χ, a negative susceptibility means μ_r < 1, which is characteristic of diamagnetic substances — they are magnetised in a direction opposite to the applied field. Paramagnetic and ferromagnetic materials both have positive susceptibility.

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