Question

Order of e/m ratio of proton, a-particle and electron is:

• A. $ {{\left( \frac{e}{m} \right)}_{p}}>{{\left( \frac{e}{m} \right)}_{\alpha }}>{{\left( \frac{e}{m} \right)}_{e}} $
• B. $ {{\left( \frac{e}{m} \right)}_{e}}>{{\left( \frac{e}{m} \right)}_{p}}>{{\left( \frac{e}{m} \right)}_{\alpha }} $
• C. $ {{\left( \frac{e}{m} \right)}_{\alpha }}>{{\left( \frac{e}{m} \right)}_{e}}>{{\left( \frac{e}{m} \right)}_{p}} $
• D. none of the above

Answer 1

Answer: (B)

Key Idea: Lighest particle will have highest e/m ratio. The mass of electron $ =9.1\times {{10}^{-31}}\,\text{kg} $ Mass of proton $ =1.67\times {{10}^{-27}}\text{kg} $ Mass of a-particle $ =6.6\times {{10}^{-27}}\,\text{kg} $ Since, $ \frac{e}{m} $ ratio is required, e will be same in case of electron and proton but is 2e for $ \alpha - $ particle. Hence, that ratio will he highest whose mass (m) is lowest irrespective of charge on $ \alpha - $ particle as it does not give greater value to this ratio. As is observed from masses, mass of electron is lowest, therefore e/m ratio of electron is highest. Similarly, $ \alpha - $ particle is heaviest, it will have lowest, $ \frac{e}{m} $ ration