Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Magnetic Field

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Magnetic Field

Suppose E and B represent the electric and magnetic fields in a...

Suppose E⃗\vec{E} and B⃗\vec{B} represent the electric and magnetic fields in a reference frame SS, while E⃗′\vec{E}' and B⃗′\vec{B}' represent them in another frame S′S' that moves with a velocity v⃗\vec{v} relative to SS. Which two of the following equations are incorrect?

Options

  1. AEy′=Ey−vBzE_y' = E_y - v B_z
  2. BEy′=Ey−vBzc2E_y' = E_y - \dfrac{v B_z}{c^2}
  3. CBy′=By+vEzB_y' = B_y + v E_z
  4. DBy′=By+vEzc2B_y' = B_y + \dfrac{v E_z}{c^2}

Correct answer

C. By′=By+vEzB_y' = B_y + v E_z

Step-by-step solution

The Lorentz transformations for the transverse components of electric and magnetic fields in SI units, for a frame S′S' moving with velocity vv along the x-axis relative to SS, are given by:

Ey′=γ(Ey−vBz)E_y' = \gamma (E_y - v B_z)

B_y' = \gamma \left(B_y +\dfrac{v}{c^2}E_z\right)

where γ=\gamma =\dfrac{1}{\sqrt{1 - v^2/c^2}}.

For non-relativistic speeds (v≪cv \ll c), γ≈1\gamma \approx 1, and the equations approximate to:

Ey′≈Ey−vBzE_y' \approx E_y - v B_z

By′≈By+B_y' \approx B_y +\dfrac{v}{c^2}EzE_z

This can also be verified using dimensional analysis. In SI units, the dimensions of electric field EE and magnetic field BB are related by [E]=[vB][E] = [vB].

Thus, the term vBzv B_z has the dimensions of an electric field, making equation (1) dimensionally correct.

Similarly, the term \dfrac{v E_z}{c^2} has the dimensions of \dfrac{[v][vB]}{[v^2]} = [B]B], making equation (4) dimensionally correct.

Equations (2) and (3) are dimensionally inconsistent because \dfrac{v B_z}{c^2} does not have the dimensions of an electric field, and vEzdoesnothavethedimensionsofamagneticfield.<br><br>Therefore,equations(2)and(3)areincorrect.v E_z does not have the dimensions of a magnetic field.<br><br>Therefore, equations (2) and (3) are incorrect.
Practice Magnetic Field on Quantrex Academy →

More from Magnetic Field

An electron is projected horizontally with a kinetic energy of 10 keV . A magnetic field… Concepts Of Physics MCQ Edition [Volume 2]In a region where a 1.0 T magnetic field points from south to north, an alpha particle is… Concepts Of Physics MCQ Edition [Volume 2]A particle carrying a charge of 1.0 × 10^ -9 C travels in the x – y plane. A magnetic… Concepts Of Physics MCQ Edition [Volume 2]An experimenter's logbook contains the following entry: "a charged particle is projected… Concepts Of Physics MCQ Edition [Volume 2]A 10 g bullet carrying a charge of 4.00 is fired horizontally at a speed of 270 m s^ -1 .… Concepts Of Physics MCQ Edition [Volume 2]A proton released from rest in a room experiences an initial acceleration a_0 directed… Concepts Of Physics MCQ Edition [Volume 2]A 10 -cm long portion of a straight wire carrying a 10 A current is situated in a 0.1 T… Concepts Of Physics MCQ Edition [Volume 2]An electron is projected horizontally with a kinetic energy of 10 keV . A magnetic field… Concepts Of Physics MCQ Edition [Volume 2] Full Magnetic Field list All Concepts Of Physics MCQ Edition [Volume 2] PYQs