First slide

electric current

Question

An ionization chamber with parallel conducting plates as anode and cathode has $5 \times 10^{7}$ electrons and the same number of singly-charged positive ions per $c m^{3}$ . The electrons are moving at $4 μ A / m^{2} .$ The current density from anode to cathode is The velocity of positive ions moving towards cathode is

Easy

A

0.4 m/s
B

16 m/s
C

zero
D

0.1 m/s

Solution

Current density of drifting electrons j = nev
$\begin{array}{l} n = 5 \times 10^{7} c m^{- 3} = 5 \times 10^{7} \times 10^{6} m^{- 3} \\ v = 0.4 m^{- 1}, e = 1.6 \times 10^{- 19} C \Rightarrow j = 3.2 \times 10^{- 6} A m^{- 2} \end{array}$
Current density of ions
$= (4 - 3.2) \times 10^{- 6} = 0.8 \times 10^{- 6} \frac{A}{m^{2}}$
This gives v for ions = $0.1 m s^{- 1}$

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