Question:

In a cathode ray experiment hydrogen gas was subjected to very low pressure and high voltage. The $\frac{e}{m}$ of cathode rays liberated was $-x~C~kg^{-1}$. In the second experiment helium gas was used under the same experimental conditions. What is the $\frac{e}{m}$ $(in~C~kg^{-1})$ of cathode rays liberated in the second experiment?}

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Cathode rays are electrons. Their charge-to-mass ratio is always the same regardless of the gas inside the discharge tube.
Updated On: Jun 17, 2026
  • $-2x$
  • $-x$
  • $\frac{x}{2}$
  • $\frac{x^2}{2}$
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The Correct Option is B

Solution and Explanation

Concept: Cathode rays are streams of electrons. The value of \[ \frac{e}{m} \] for electrons is independent of the gas present in the discharge tube.

Step 1:
Identify the nature of cathode rays.
Cathode rays always consist of electrons. Their properties do not depend on the gas used.

Step 2:
Apply Thomson's conclusion.
Whether hydrogen or helium is used, \[ \frac{e}{m} \] remains unchanged. Hence, \[ \left(\frac{e}{m}\right)_{\text{He}} = -x \]

Step 3:
State the answer.
\[ \boxed{-x} \]
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