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Magnetism question

2017 · Shift 1 · Q49
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Magnetism question

2017 · Shift 1 · Q49

JEE AdvancedPhysicsMagnetismMCQ+3 / −0.75
A charged particle (electron or proton) is introduced at the origin (x=0,y=0,z=0) with a given initial velocity v→.\overrightarrow v .v. A uniform electric field E→\overrightarrow EE and a uniform magnetic field B→\overrightarrow BB exist everywhere. The velocity v→,\overrightarrow v ,v, electric field E→\overrightarrow EE and magnetic field B→\overrightarrow BB are given in column 1,21,21,2 and 3,3,3, respectively. The quantities E0,B0{E_0},{B_0}E0​,B0​ are positive in magnitude.

Column 1 Column 2 Column 3
(I) Electron with v→=2E0B0x^\overrightarrow v = 2{{{E_0}} \over {{B_0}}}\widehat xv=2B0​E0​​x (i) E→=E0z^\overrightarrow E = {E_0}\widehat zE=E0​z (P) B→=−B0x^\overrightarrow B = - {B_0}\widehat xB=−B0​x
(II) Electron with v→=E0B0y^\overrightarrow v = {{{E_0}} \over {{B_0}}}\widehat yv=B0​E0​​y​ (ii) E→=−E0y^\overrightarrow E = - {E_0}\widehat yE=−E0​y​ (Q) B→=B0x^\overrightarrow B = {B_0}\widehat xB=B0​x
(III) Proton with v→=0\overrightarrow v = 0v=0 (iii) E→=−E0x^\overrightarrow E = - {E_0}\widehat xE=−E0​x (R) B→=B0y^\overrightarrow B = {B_0}\widehat yB=B0​y​
(IV) Proton with v→=2E0B0x^\overrightarrow v = 2{{{E_0}} \over {{B_0}}}\widehat xv=2B0​E0​​x (iv) E→=E0x^\overrightarrow E = {E_0}\widehat xE=E0​x (S) B→=B0z^\overrightarrow B = {B_0}\widehat zB=B0​z
In which case will the particle describe a helical path with axis along the positive zzz direction?
  1. A
    (IV)(i)(S)\left( {{\rm I}V} \right)\left( i \right)\left( S \right)(IV)(i)(S)
  2. B
    (II)(ii)(R)\left( {{\rm I}{\rm I}} \right)\left( {ii} \right)\left( R \right)(II)(ii)(R)
  3. C
    (III)(iii)(P)\left( {{\rm I}{\rm I}{\rm I}} \right)\left( {iii} \right)\left( P \right)(III)(iii)(P)
  4. D
    (IV)(ii)(R)\left( {{\rm I}V} \right)\left( {ii} \right)\left( R \right)(IV)(ii)(R)
View written solutionFree

Correct answer: A

Step-by-step Solution:

  1. Analyze the condition for a helical path: A charged particle follows a helical path when it moves in a uniform magnetic field, and its velocity vector has components both parallel (v→∥\overrightarrow v_\parallelv∥​) and perpendicular (v→⊥\overrightarrow v_\perpv⊥​) to the magnetic field (B→\overrightarrow BB). The axis of the helix is always parallel to the direction of the magnetic field vector B→\overrightarrow BB.

  2. Identify the required direction of the magnetic field: The question states that the axis of the helical path must be along the positive z-direction. This implies that the magnetic field B→\overrightarrow BB must be directed along the z-axis. Looking at the options in Column 3:

    • (P) B→=−B0x^\overrightarrow B = - {B_0}\widehat xB=−B0​x (along negative x-axis)
    • (Q) B→=B0x^\overrightarrow B = {B_0}\widehat xB=B0​x (along positive x-axis)
    • (R) B→=B0y^\overrightarrow B = {B_0}\widehat yB=B0​y​ (along positive y-axis)
    • (S) B→=B0z^\overrightarrow B = {B_0}\widehat zB=B0​z (along positive z-axis) Only option (S) has the magnetic field along the z-axis. Therefore, the correct combination must include (S) from Column 3.
  3. Evaluate the given options: Now, let's check which of the multiple-choice options includes (S) from Column 3.

    • A: (IV)(i)(S)\left( {{\rm I}V} \right)\left( i \right)\left( S \right)(IV)(i)(S) - This option includes (S).
    • B: (II)(ii)(R)\left( {{\rm I}{\rm I}} \right)\left( {ii} \right)\left( R \right)(II)(ii)(R) - This option includes (R), so the axis would be along the y-direction. Incorrect.
    • C: (III)(iii)(P)\left( {{\rm I}{\rm I}{\rm I}} \right)\left( {iii} \right)\left( P \right)(III)(iii)(P) - This option includes (P), so the axis would be along the x-direction. Incorrect.
    • D: (IV)(ii)(R)\left( {{\rm I}V} \right)\left( {ii} \right)\left( R \right)(IV)(ii)(R) - This option includes (R), so the axis would be along the y-direction. Incorrect.
  4. Conclusion from elimination: Based on the direction of the magnetic field, only option A is plausible. Let's verify that this combination indeed results in the described motion.

  5. Detailed analysis of Option A: (IV)(i)(S):

    • Particle and Initial Velocity (IV): Proton with charge q=+eq = +eq=+e and initial velocity v→=2E0B0x^\overrightarrow v = 2{{{E_0}} \over {{B_0}}}\widehat xv=2B0​E0​​x. The initial velocity is purely in the xy-plane (perpendicular to the z-axis).
    • Electric Field (i): E→=E0z^\overrightarrow E = {E_0}\widehat zE=E0​z. The electric field is along the positive z-axis.
    • Magnetic Field (S): B→=B0z^\overrightarrow B = {B_0}\widehat zB=B0​z. The magnetic field is along the positive z-axis.
  6. Calculate the Lorentz Force: The total force on the proton is given by the Lorentz force law: F→=q(E→+v→×B→)\overrightarrow F = q(\overrightarrow E + \overrightarrow v \times \overrightarrow B)F=q(E+v×B)

  7. Analyze the components of motion:

    • Motion parallel to B (z-direction): The electric field E→\overrightarrow EE is parallel to the magnetic field B→\overrightarrow BB. The electric force on the proton is F→E=qE→=eE0z^\overrightarrow F_E = q\overrightarrow E = eE_0\widehat zFE​=qE=eE0​z. This force is constant and directed along the positive z-axis. The magnetic force, F→B=q(v→×B→)\overrightarrow F_B = q(\overrightarrow v \times \overrightarrow B)FB​=q(v×B), is always perpendicular to B→\overrightarrow BB, so it has no z-component. Therefore, the net force in the z-direction is Fz=eE0F_z = eE_0Fz​=eE0​. This causes a constant acceleration az=eE0/ma_z = eE_0/maz​=eE0​/m in the positive z-direction. Since the proton starts at the origin with vz(0)=0v_z(0)=0vz​(0)=0, it will move along the positive z-axis.

    • Motion perpendicular to B (xy-plane): The initial velocity of the proton, v→=2E0B0x^\overrightarrow v = 2{{{E_0}} \over {{B_0}}}\widehat xv=2B0​E0​​x, is entirely perpendicular to B→\overrightarrow BB. The magnetic force component in the xy-plane, q(v→⊥×B→)q(\overrightarrow v_\perp \times \overrightarrow B)q(v⊥​×B), acts as a centripetal force, causing the proton to execute circular motion in the xy-plane. The electric field has no component in this plane.

  8. Combine the motions: The resulting trajectory is a superposition of circular motion in the xy-plane and accelerated linear motion along the positive z-axis. This combined motion describes a helical path whose axis is the z-axis. Since the particle accelerates in the +z direction, it moves along the positive z-axis. The pitch of the helix increases with time, but it is still a helical path.

  9. Final Conclusion: The combination (IV)(i)(S) correctly describes a particle moving in a helical path with its axis along the positive z-direction. Therefore, option A is the correct answer.

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