Question

The e/m ratio for cathode rays:
A. is constant
B. varies as the atomic number of the element forming the cathode in the discharge tube changes
C. varies as the atomic number of the gas in the discharge tube varies
D. has the smallest value when the discharge tube is filled with hydrogen

Answer 1

Hint: Cathode rays are made up of negatively charged particles called electrons. These rays produce ionization of gases through which they travel.

Complete step-by-step solution:
The ratio of electromagnetic for a cathode ray is fixed at \[1.758820 \times {10^{11}}C/kg.\] It is equal to the ratio of e/m for an electron. This is because cathode rays consist of electrons.
\[\dfrac{e}{m} = \left( {\dfrac{{{\text{Charge present on the electron}}}}{{mass{\text{ of the electron}}}}} \right)\]
\[ = \dfrac{{\left( {1.6 \times {{10}^{ - 19}}C} \right)}}{{\left( {9.1 \times {{10}^{ - 31}}kg} \right)}}\]
\[ = 1.758820 \times {10^{11}}C/kg\]
The e/m ratio does not depend on the type of gas filled in the discharge tube.
For only the cathode rays, the constituent particle always remains the same that is why the ratio is the same.

Hence option (A) is correct.

Note: Beam rays inside cathode rays consist of \[\left( {{e^ - }} \right)\] electrons moving with a very high velocity, these electrons are identical. Hence, the charge by mass ratio (e/m) of cathode rays is the same for all the gases. But for the positively charged ions changes (e/m) ratio as the number of nucleons changes in the nucleus of each gas can be different.