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An ionization chamber with parallel conducting plates as anode and cathode has 5 × 10⁷ electrons and the same number of singly charged positive ions per cm³. The electrons are moving towards the anode with velocity 0.4 m/s. The current density from anode to cathode is 4 μA/m². The velocity of positive ions moving towards cathode is

1. 0.4 m/s
2. 1.6 m/s
3. zero
4. 0.1 m/s

Correct answer: 0.1 m/s

Solution

The current density is the sum of contributions from both electrons and positive ions. Using the formula for current density, J = nqv, and equating the contributions from electrons and ions, we find the velocity of positive ions to be 0.1 m/s.

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