Question

The highest value of e/m of anode rays has been observed when the discharge tube is filled with:
(A) Nitrogen
(B) Oxygen
(C) Hydrogen
(D) Helium

Answer 1

Hint: Try to recall that the ratio e/m of anode rays depends upon the nature of gas taken inside the discharge tube and the value of m is equal to the atomic mass of the gas taken. Now, by using this you can easily find the correct option from the given ones.

Complete answer:

• It is known to you that the value of charge (e) on the particles constituting the anode rays is found to be the whole number multiple of the charge present on the electron.
• Also, the mass (m) of the particles constituting the anode rays is found to be nearly equal to that of atomic or molecular mass of the gas.
• Now, coming to the question, calculate the value of e/m in each of the options given in order to find out the highest value of e/m. Let mass of one proton is equal to mass of one neutron = m and charge on one electron be $e$:
  A. Nitrogen: Total number of electrons in nitrogen gas = 14.
  So, total charges on particles = $14e$
  Atomic mass of nitrogen = 28 m
  For nitrogen, $\dfrac{e}{m} = \dfrac{{14e}}{{28m}} = 0.5e/m$
  B. Oxygen: Total number of electrons in oxygen gas = 16.
  So, total charges on particles = $16e$
  Atomic mass of oxygen = 32 m
  For oxygen, $\dfrac{e}{m} = \dfrac{{16e}}{{32m}} = 0.5e/m$.

C. Hydrogen: Total number of electrons in hydrogen gas = 2.
So, total charges on particles = $2e$
Atomic mass of hydrogen = 2 m
For hydrogen, $\dfrac{e}{m} = \dfrac{{2e}}{{2m}} = 1e/m$
D. Helium: Total number of electrons in helium gas = 2.
So, total charges on particles = $2e$
Atomic mass of helium = 4 m
For helium, $\dfrac{e}{m} = \dfrac{{2e}}{{4m}} = 0.5e/m$.
Therefore from above we can easily conclude that option C is the correct option to the given question.

Note:

• It should be remembered to you that the speed of anode rays is much less than that of cathode rays and they are made up of material particles.
• Also, you should remember that they travel in a straight line and they are positively charged as indicated by direction of deflection of rays in electric and magnetic fields.