Concept: Positive rays (canal rays) are ionized gas molecules. Their charge-to-mass ratio depends entirely on the mass of the gas used in the tube.
Formula: $$ \frac{e}{m} = \frac{\text{Charge}}{\text{Mass}} $$
Solution: - To maximize the fraction \( e/m \), the denominator (mass) must be as small as possible.
- Let's compare the atomic/molecular masses of the gases:
- Helium (\( \text{He} \)) \( \approx 4 \text{ amu} \)
- Nitrogen (\( \text{N}_2 \)) \( \approx 28 \text{ amu} \)
- Neon (\( \text{Ne} \)) \( \approx 20 \text{ amu} \)
- Hydrogen (\( \text{H}_2 \)) \( \approx 2 \text{ amu} \) (and a single ionized H atom is \( \approx 1 \text{ amu} \)).
- Hydrogen has the lowest mass of any element, so it yields the absolute maximum \( e/m \) ratio.
Why other options are incorrect: They are much heavier gases, which increases the denominator and significantly lowers the \( e/m \) ratio.
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