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Moving Charges and Magnetism · Question 29

JEE Main 2025 (Online) 2nd April Morning ShiftMCQMedium📈 Topic in 23% papers+4−1
1

A monochromatic light of wavelength is incident on a metallic plate having work function . An electron, emitted normally to the plate from a point A with maximum kinetic energy, enters a constant magnetic field, perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point B. The distance between A and B is : (Given : The magnitude of charge of an electron is e and mass is m, h is Planck's constant and c is velocity of light. Take the magnetic field exists throughout the path of electron)

  1. 2m ( hc - )/eB
  2. 8 m ( hc - )/eB
  3. m(hc/ - )/eB
  4. 2 m(hc/ - )/eB

Answer

(2) 8 m ( hc - )/eB

Solution

Einstein's photoelectric equation: K = hc -

Momentum of the electron: p = mv = 2mK

In the magnetic field ⊥ velocity it moves on a circle: r = mveB = 2mK eB

Its initial velocity is normal to the plate, so the plate contains a diameter of the circle; the electron describes a semicircle and hits the plate at the other end of that diameter:

AB = 2r = 2 2mK eB = 8m ( hc - )eB

Answer: (2)

JEE Main 2025 (Online) 2nd April Morning Shift

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2

Let B1 be the magnitude of magnetic field at center of a circular coil of radius R carrying current I. Let B2 be the magnitude of magnetic field at an axial distance 'x' from the center. For x : R = 3 : 4, B2B1 is :

  1. 64 : 125
  2. 25 : 16
  3. 4 : 5
  4. 16 : 25
JEE Main 2025 (Online) 2nd April Morning Shift
3

In a moving coil galvanometer, two moving coils M1 and M2 have the following particulars : R1 = 5 , N1 = 15, A1 = 3.6 10-3 m2, B1 = 0.25 T R2 = 7 , N2 = 21, A2 = 1.8 10-3 m2, B2 = 0.50 T Assuming that torsional constant of the springs are same for both coils, what will be the ratio of voltage sensitivity of M1 and M2 ?

  1. 1 : 1
  2. 1 : 3
  3. 1 : 2
  4. 1 : 4
JEE Main 2025 (Online) 2nd April Evening Shift
4

An electron with mass 'm' with an initial velocity (t = 0) v = v0 i (v0 > 0) enters a magnetic field B = B0 j. If the initial de Broglie wavelength at t = 0 is 0 then its value after time 't' would be :

  1. 0 1 - e2 B02 t2m2
  2. 0
  3. 0 1 + e2 B02 t2m2
  4. 0 1 + e2 B02 t2m2
JEE Main 2025 (Online) 2nd April Evening Shift
5

Consider a long straight wire of a circular cross-section (radius a) carrying a steady current I. The current is uniformly distributed across this cross-section. The distances from the centre of the wire’s cross-section at which the magnetic field [inside the wire, outside the wire] is half of the maximum possible magnetic field, any where due to the wire, will be :

  1. [a/2, 3a]
  2. [a/4, 3a/2]
  3. [a/2, 2a]
  4. [a/4, 2a]
JEE Main 2025 (Online) 29th January Morning Shift
6

Consider a long thin conducting wire carrying a uniform current I. A particle having mass "m" and charge "q" is released at a distance "a" from the wire with a speed v0 along the direction of current in the wire. The particle gets attracted to the wire due to magnetic force. The particle turns round when it is at distance x from the wire. The value of x is [ 0 is vacuum permeability]

  1. a [1 - mv02q 0 I ]
  2. ae- 4 m v0q 0 I
  3. a [1 - mv0q 0 I ]
  4. a2
JEE Main 2025 (Online) 28th January Morning Shift
7

[figure] An infinite wire has a circular bend of radius a, and carrying a current I as shown in the figure. The magnitude of magnetic field at the origin O of the arc is given by:

  1. 02 Ia [ 2+2 ]
  2. 04 Ia [ 3 2+1 ]
  3. 04 Ia [ 2+1 ]
  4. 04 Ia [ 3 2+2 ]
JEE Main 2025 (Online) 28th January Evening Shift

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