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JEE Advanced · Physics · 20. Magnetism & Current

An electron and a proton are moving on straight parallel paths with same velocity. They enter a semi-infinite region of uniform magnetic field perpendicular to the velocity. Which of the following statement(s) is/are true?

  1. A They will never come out of the magnetic field region
  2. B They will come out travelling along parallel paths
  3. C They will come out at the same time
  4. D They will come out at different times
Verified Solution

Answer & Solution

Correct Answer

(D) They will come out at different times

Step-by-step Solution

Detailed explanation

\(r=\frac{m v}{B q}\)
or \(\quad r \propto m\)
\(\therefore r_e < r_p\) as \(m_e < m_p\)
Further, \(\quad T=\frac{2 \pi m}{B q}\)
or \(\quad T \propto m\)

\(\begin{aligned}
\therefore T_e & < T_p \\
t_e & =\frac{T_e}{2}
\end{aligned}\)
and
or \(\quad t_e < t_p\)
\(\therefore\) Correct options are (b) and (d).
Note In JEE 2011, official answer key, correct options were given bc, bd, bcd, here student should realise that both the electron and proton enter the magnetic field at the same time or at different times.
Analysis of Question
(i) Question is simple.
(ii) In the situation given above particle will never complete the circle unless magnetic field is all around.
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